RFC 1247
OSPF Version 2
13. The Flooding Procedure Link State Update packets provide the mechanism for flooding link state advertisements. A Link State Update packet may contain several distinct advertisements, and floods each advertisement one hop further from its point of origination. To make the flooding procedure reliable, each advertisement must be acknowledged separately. Acknowledgments are transmitted in Link State Acknowledgment packets. Many separate acknowledgments can be grouped together into a single packet. The flooding procedure starts when a Link State Update packet has been received. Many consistency checks have been made on the received packet before being handed to the flooding procedure (see Section 8.2). In particular, the Link State Update packet has been associated with a particular neighbor, and a particular area. If the neighbor is in a lesser state than Exchange, the packet should be dropped without further processing. All types of link state advertisements, other than AS external links, are associated with a specific area. However, link state advertisements do not contain an area field. A link state advertisement's area must be deduced from the Link State Update packet header. For each link state advertisement contained in the packet, the following steps are taken: (1) Validate the advertisement's link state checksum. If the checksum turns out to be invalid, discard the advertisement and get the next one from the Link State Update packet. (2) Examine the link state advertisement's LS type. If the LS type is unknown, discard the advertisement and get the next one from the Link State Update Packet. This specification defines LS Types 1-5 (see Section 4.3). (3) Else if this is a AS external advertisement (LS type = 5), and the area has been configured as a stub area, discard the advertisement and get the next one from the Link State Update Packet. AS external advertisements are not flooded into/throughout stub areas (see Section 3.6). (4) Else if the advertisement's age is equal to MaxAge, and there is currently no instance of the advertisement in the router's link state database, then take the following actions:
(a) Acknowledge the receipt of the advertisement by sending a Link
State Acknowledgment packet back to the sending neighbor (see
Section 13.5).
(b) Purge all outstanding requests for equal or previous instances
of the advertisement from the sending neighbor's Link State
Request list (see Section 10).
(c) If the sending neighbor is in state Exchange or in state
Loading, then install the MaxAge advertisement in the link state
database. Otherwise, simply discard the advertisement. In
either case, examine the next advertisement (if any) listed in
the Link State Update packet.
(5) Otherwise, find the instance of this advertisement that is currently
contained in the router's link state database. If there is no
database copy, or the received advertisement is more recent than the
database copy (see Section 13.1 below for the determination of which
advertisement is more recent) the following steps must be performed:
(a) If there is already a database copy, and if the database copy
was installed less than MinLSInterval seconds ago, discard the
new advertisement (without acknowledging it) and examine the
next advertisement (if any) listed in the Link State Update
packet.
(b) Otherwise immediately flood the new advertisement out some
subset of the router's interfaces (see Section 13.3). In some
cases (e.g., the state of the receiving interface is DR and the
advertisement was received from a router other than the Backup
DR) the advertisement will be flooded back out the receiving
interface. This occurrence should be noted for later use by the
acknowledgment process (Section 13.5).
(c) Remove the current database copy from all neighbors' Link state
retransmission lists.
(d) Install the new advertisement in the link state database
(replacing the current database copy). This may cause the
routing table calculation to be scheduled. In addition,
timestamp the new advertisement with the current time (i.e., the
time it was received). The flooding procedure cannot overwrite
the newly installed advertisement until MinLSInterval seconds
have elapsed. The advertisement installation process is
discussed further in Section 13.2.
(e) Possibly acknowledge the receipt of the advertisement by sending
a Link State Acknowledgment packet back out the receiving
interface. This is explained below in Section 13.5.
(f) If this new link state advertisement indicates that it was
originated by this router itself, the router must advance the
advertisement's link state sequence number, and issue a new
instance of the advertisement (see Section 13.4).
(6) Else, if there is an instance of the advertisement on the sending
neighbor's Link state request list, an error has occurred in the
Database Description process. In this case, restart the Database
Description process by generating the neighbor event BadLSReq for
the sending neighbor and stop processing the Link State Update
packet.
(7) Else, if the received advertisement is the same instance as the
database copy (i.e., neither one is more recent) the following two
steps should be performed:
(a) If the advertisement is listed in the Link state retransmission
list for the receiving adjacency, the router itself is expecting
an acknowledgment for this advertisement. The router should
treat the received advertisement as an acknowledgment, by
removing the advertisement from the Link state retransmission
list. This is termed an "implied acknowledgment". Its
occurrence should be noted for later use by the acknowledgment
process (Section 13.5).
(b) Possibly acknowledge the receipt of the advertisement by sending
a Link State Acknowledgment packet back out the receiving
interface. This is explained below in Section 13.5.
(8) Else, the database copy is more recent. Note an unusual event to
network management, discard the advertisement and process the next
link state advertisement contained in the packet.
13.1 Determining which link state is newer
When a router encounters two instances of a link state advertisement, it
must determine which is more recent. This occurred above when comparing
a received advertisement to the database copy. This comparison must
also be done during the database exchange procedure which occurs during
adjacency bring-up.
A link state advertisement is identified by its LS type, Link State ID
and Advertising Router. For two instances of the same advertisement,
the LS sequence number, LS age, and LS checksum fields are used to
determine which instance is more recent:
o The advertisement having the newer LS sequence number is more
recent. See Section 12.1.6 for an explanation of the LS sequence
number space. If both instances have the same LS sequence number,
then:
o If the two instances have different LS checksums, then the instance
having the larger LS checksum (when considered as a 16-bit unsigned
integer) is considered more recent.
o Else, if only one of the instances is of age MaxAge, the instance of
age MaxAge is considered to be more recent.
o Else, if the ages of the two instances differ by more than
MaxAgeDiff, the instance having the smaller (younger) age is
considered to be more recent.
o Else, the two instances are considered to be identical.
13.2 Installing link state advertisements in the database
Installing a new link state advertisement in the database, either as the
result of flooding or a newly self originated advertisement, may cause
the routing table structure to be recalculated. The contents of the new
advertisement should be compared to the old instance, if present. If
there is no difference, there is no need to recalculate the routing
table. (Note that even if the contents are the same, the LS checksum
will probably be different, since the checksum covers the LS sequence
number.)
If the contents are different, the following pieces of the routing table
must be recalculated, depending on the LS type field:
Router links, network links
The entire routing table must be recalculated, starting with the
shortest path calculations for each area (not just the area whose
topological database has changed). The reason that the shortest
path calculation cannot be restricted to the single changed area has
to do with the fact that AS boundary routers may belong to multiple
areas. A change in the area currently providing the best route may
force the router to use an intra-area route provided by a different
area.[15]
Summary link
The best route to the destination described by the summary link
advertisement must be re-examined (see Section 16.5). If this
destination is an AS boundary router, it may also be necessary to
re-examine all the AS external link advertisements.
AS external link
The best route to the destination described by the AS external link
advertisement must be re-examined (see Section 16.6).
Also, any old instance of the advertisement must be removed from the
database when the new advertisement is installed. This old instance
must also be removed from all neighbors' Link state retransmission lists
(see Section 10).
13.3 Next step in the flooding procedure
When a new (and more recent) advertisement has been received, it must be
flooded out some set of the router's interfaces. This section describes
the second part of flooding procedure (the first part being the
processing that occurred in Section 13), namely, selecting the outgoing
interfaces and adding the advertisement to the appropriate neighbors'
Link state retransmission lists. Also included in this part of the
flooding procedure is the maintenance of the neighbors' Link state
request lists.
This section is equally applicable to the flooding of an advertisement
that the router itself has just originated (see Section 12.4). For
these advertisements, this section provides the entirety of the flooding
procedure (i.e., the processing of Section 13 is not performed, since,
for example, the advertisement has not been received from a neighbor and
therefore does not need to be acknowledged).
Depending upon the advertisement's LS type, the advertisement can be
flooded out only certain interfaces. These interfaces, defined by the
following, are called the eligible interfaces:
AS external links (LS Type = 5)
AS external links are flooded throughout the entire AS, with the
exception of stub areas (see Section 3.6). The eligible interfaces
are all the router's interfaces, excluding virtual links and those
interfaces attaching to stub areas.
All other types
All other types are specific to a single area (Area A). The
eligible interfaces are all those interfaces attaching to the Area
A. If Area A is the backbone, this includes all the virtual links.
Link state databases must remain synchronized over all adjacencies
associated with the above eligible interfaces. This is accomplished by
executing the following steps on each eligible interface. It should be
noted that this procedure may decide not to flood a link state
advertisement out a particular interface, if there is a high probability
that the attached neighbors have already received the advertisement.
However, in these cases the flooding procedure must be absolutely sure
that the neighbors eventually do receive the advertisement, so the
advertisement is still added to each adjacency's Link state
retransmission list. For each eligible interface:
(1) Each of the neighbors attached to this interface are examined, to
determine whether they must receive the new advertisement. The
following steps are executed for each neighbor:
(a) If the neighbor is in a lesser state than Exchange, it does not
participate in flooding, and the next neighbor should be
examined.
(b) Else, if the adjacency is not yet full (neighbor state is
Exchange or Loading), examine the Link state request list
associated with this adjacency. If there is an instance of the
new advertisement on the list, it indicates that the neighboring
router has an instance of the advertisement already. Compare
the new advertisement to the neighbor's copy:
o If the new advertisement is less recent, then try the next
neighbor.
o If the two copies are the same instance, then delete the
advertisement from the Link state request list, and try the
next neighbor.[16]
o Else, the new advertisement is more recent. Delete the
advertisement from the Link state request list.
(c) If the new advertisement was received from this neighbor, try
the next neighbor.
(d) At this point we are not positive that the new neighbor has an
up-to-date instance of this new advertisement. Add the new
advertisement to the Link state retransmission list for the
adjacency. This ensures that the flooding procedure is
reliable; the advertisement will be retransmitted at intervals
until an acknowledgment is seen from the neighbor.
(2) The router must now decide whether to flood the new link state
advertisement out this interface. If in the previous step, the link
state advertisement was NOT added to any of the Link state
retransmission lists, there is no need to flood the advertisement
and the next interface should be examined.
(3) If the new advertisement was received on this interface, and it was
received from either the Designated Router or the Backup Designated
Router, chances are all the neighbors have received the
advertisement already. Therefore, examine the next interface.
(4) If the new advertisement was received on this interface, and the
interface state is Backup (i.e., the router itself is the Backup
Designated Router), examine the next interface. The Designated
Router will do the flooding on this interface. If the Designated
Router fails, this router will end up retransmitting the updates.
(5) If this step is reached, the advertisement must be flooded out the
interface. Send a Link State Update packet (with the new
advertisement as contents) out the interface. The advertisement's
LS age must be incremented by InfTransDelay (which must be > 0) when
copied into the outgoing packet (until the LS age field reaches its
maximum value of MaxAge).
On broadcast networks, the Link State Update packets are multicast.
The destination IP address specified for the Link State Update
Packet depends on the state of the interface. If the interface
state is DR or Backup, the address AllSPFRouters should be used.
Otherwise, the address AllDRouters should be used.
On non-broadcast, multi-access networks, separate Link State Update
packets must be sent, as unicasts, to each adjacent neighbor (i.e.,
those in state Exchange or greater). The destination IP addresses
for these packets are the neighbors' IP addresses.
13.4 Receiving self-originated link state
It is a common occurrence to receive a self-originated link state
advertisement via the flooding procedure. If the advertisement received
is a newer instance than the last instance that the router actually
originated, the router must take special action.
The reception of such an advertisement indicates that there are link
state advertisements in the routing domain that were originated before
the last time the router was restarted. In this case, the router must
advance the sequence number for the advertisement one past the received
sequence number, and originate a new instance of the advertisement.
Note also that if the type of the advertisement is Summary link or AS external link, the router may no longer have an (advertisable) route to the destination. In this case, the advertisement should be flushed from the routing domain by incrementing the advertisement's LS age to MaxAge and reflooding (see Section 14.1). 13.5 Sending Link State Acknowledgment packets Each newly received link state advertisement must be acknowledged. This is usually done by sending Link State Acknowledgment packets. However, acknowledgments can also be accomplished implicitly by sending Link State Update packets (see step 7a of Section 13). Many acknowledgments may be grouped together into a single Link State Acknowledgment packet. Such a packet is sent back out the interface that has received the advertisements. The packet can be sent in one of two ways: delayed and sent on an interval timer, or sent directly (as a unicast) to a particular neighbor. The particular acknowledgment strategy used depends on the circumstances surrounding the receipt of the advertisement. Sending delayed acknowledgments accomplishes several things: it facilitates the packaging of multiple acknowledgments in a single packet; it enables a single packet to indicate acknowledgments to several neighbors at once (through multicasting); and it randomizes the acknowledgment packets sent by the various routers attached to a multi- access network. The fixed interval between a router's delayed transmissions must be short (less than RxmtInterval) or needless retransmissions will ensue. Direct acknowledgments are sent to a particular neighbor in response to the receipt of duplicate link state advertisements. These acknowledgments are sent as unicasts, and are sent immediately when the duplicate is received. The precise procedure for sending Link State Acknowledgment packets is described in Table 19. The circumstances surrounding the receipt of the advertisement are listed in the left column. The acknowledgment action then taken is listed in one of the two right columns. This action depends on the state of the concerned interface; interfaces in state Backup behave differently from interfaces in all other states. Action taken in state Circumstances Backup All other states ______________________________________________________________
Action taken in state
Circumstances Backup All other states
______________________________________________________________
Advertisement has No acknowledgment No acknowledgment
been flooded back sent. sent.
out receiving in-
terface (see Sec-
tion 13, step 5b).
______________________________________________________________
Advertisement is Delayed ack- Delayed ack-
more recent than nowledgment sent nowledgment sent.
database copy, but if advertisement
was not flooded received from DR,
back out receiving otherwise do noth-
interface ing
______________________________________________________________
Advertisement is a Delayed ack- No acknowledgment
duplicate, and was nowledgment sent sent.
treated as an im- if advertisement
plied acknowledg- received from DR,
ment (see Section otherwise do noth-
13, step 7a). ing
______________________________________________________________
Advertisement is a Direct acknowledg- Direct acknowledg-
duplicate, and was ment sent. ment sent.
not treated as an
implied ack-
nowledgment.
______________________________________________________________
Advertisement's age Direct acknowledg- Direct acknowledg-
is equal to MaxAge, ment sent. ment sent.
and there is no
current instance of
the advertisement in
the link state
database (see
Section 13, step 4).
Table 19: Sending link state acknowledgements.
Delayed acknowledgments must be delivered to all adjacent routers
associated with the interface. On broadcast networks, this is
accomplished by sending the delayed Link State Acknowledgment packets as
multicasts. The Destination IP address used depends on the state of the
interface. If the state is DR or Backup, the destination AllSPFRouters
is used. In other states, the destination AllDRouters is used. On
non-broadcast networks, delayed acks must be unicast separately over
each adjacency (neighbor whose state is >= Exchange). The reasoning behind sending the above packets as multicasts is best explained by an example. Consider the network configuration depicted in Figure 15. Suppose RT4 has been elected as DR, and RT3 as Backup for the network N3. When router RT4 floods a new advertisement to network N3, it is received by routers RT1, RT2, and RT3. These routers will not flood the advertisement back onto net N3, but they still must ensure that their topological databases remain synchronized with their adjacent neighbors. So RT1, RT2, and RT4 are waiting to see an acknowledgment from RT3. Likewise, RT4 and RT3 are both waiting to see acknowledgments from RT1 and RT2. This is best achieved by sending the acknowledgments as multicasts. The reason that the acknowledgment logic for Backup DRs is slightly different is because they perform differently during the flooding of link state advertisements (see Section 13.3, step 4). 13.6 Retransmitting link state advertisements Advertisements flooded out an adjacency are placed on the adjacency's Link state retransmission list. In order to ensure that flooding is reliable, these advertisements are retransmitted until they are acknowledged. The length of time between retransmissions is a configurable per-interface value, RxmtInterval. If this is set too low for an interface, needless retransmissions will ensue. If the value is set too high, the speed of the flooding, in the face of lost packets, may be affected. Several retransmitted advertisements may fit into a single Link State Update packet. When advertisements are to be retransmitted, only the number fitting in a single Link State Update packet should be transmitted. Another packet of retransmissions can be sent when some of the advertisements are acknowledged, or on the next firing of the retransmission timer. Link State Update Packets carrying retransmissions are always sent as unicasts (directly to the physical address of the neighbor). They are never sent as multicasts. Each advertisement's LS age must be incremented by InfTransDelay (which must be > 0) when copied into the outgoing packet (until the LS age field reaches its maximum value of MaxAge). If the adjacent router goes down, retransmissions may occur until the adjacency is destroyed by OSPF's Hello Protocol. When the adjacency is destroyed, the Link state retransmission list is cleared.
13.7 Receiving link state acknowledgments Many consistency checks have been made on a received Link State Acknowledgment packet before it is handed to the flooding procedure. In particular, it has been associated with a particular neighbor. If this neighbor is in a lesser state than Exchange, the packet is discarded. Otherwise, for each acknowledgment in the packet, the following steps are performed: o Does the advertisement acknowledged have an instance on the Link state retransmission list for the neighbor? If not, examine the next acknowledgment. Otherwise: o If the acknowledgment is for the same instance that is contained on the list, remove the item from the list and examine the next acknowledgment. Otherwise: o Log the questionable acknowledgment, and examine the next one. 14. Aging The Link State Database Each link state advertisement has an age field. The age is expressed in seconds. An advertisement's age field is incremented while it is contained in a router's database. Also, when copied into a Link State Update Packet for flooding out a particular interface, the advertisement's age is incremented by InfTransDelay. An advertisement's age is never incremented past the value MaxAge. Advertisements having age MaxAge are not used in the routing table calculation. As a router ages its link state database, an advertisement's age may reach MaxAge.[17] At this time, the router must attempt to flush the advertisement from the routing domain. This is done simply by reflooding the MaxAge advertisement just as if it was a newly originated advertisement (see Section 13.3). When a Database summary list for a newly adjacent neighbor is formed, any MaxAge advertisements present in the link state database are added to the neighbor's Link state retransmission list instead of the neighbor's Database summary list. See Section 10.3 for more details. A MaxAge advertisement is removed entirely from the router's link state database when a) it is no longer contained on any neighbor Link state retransmission lists and b) none of the router's neighbors are in states Exchange or Loading.
When, in the process of aging the link state database, an advertisement's age hits a multiple of CheckAge, its checksum should be verified. If the checksum is incorrect, a program or memory error has been detected, and at the very least the router itself should be restarted. 14.1 Premature aging of advertisements A link state advertisement can be flushed from the routing domain by setting its age to MaxAge and reflooding the advertisement. This procedure follows the same course as flushing an advertisement whose age has naturally reached the value MaxAge (see Section 14). In particular, the MaxAge advertisement is removed from the router's link state database as soon as a) it is no longer contained on any neighbor Link state retransmission lists and b) none of the router's neighbors are in states Exchange or Loading. We call the setting of an advertisement's age to MaxAge premature aging. Premature aging is used when it is time for a self-originated advertisement's sequence number field to wrap. At this point, the current advertisement instance (having LS sequence number of 0x7fffffff) must be prematurely aged and flushed from the routing domain before a new instance with sequence number 0x80000001 can be originated. See Section 12.1.6 for more information. Premature aging can also be used when, for example, one of the router's previously advertised external routes is no longer reachable. In this circumstance, the router can flush its external advertisement from the routing domain via premature aging. This procedure is preferable to the alternative, which is to originate a new advertisement for the destination specifying a metric of LSInfinity. A router may only prematurely age its own (self-originated) link state advertisements. These are the link state advertisements having the router's own OSPF Router ID in the Advertising Router field. 15. Virtual Links The single backbone area (Area ID = 0) cannot be disconnected, or some areas of the Autonomous System will become unreachable. To establish/maintain connectivity of the backbone, virtual links can be configured through non-backbone areas. Virtual links serve to connect separate components of the backbone. The two endpoints of a virtual link are area border routers. The virtual link must be configured in both routers. The configuration information in each router consists of the other virtual endpoint (the other area border router), and the non-
backbone area the two routers have in common (called the transit area).
Virtual links cannot be configured through stub areas (see Section 3.6).
The virtual link is treated as if it were an unnumbered point-to-point
network (belonging to the backbone) joining the two area border routers.
An attempt is made to establish an adjacency over the virtual link.
When this adjacency is established, the virtual link will be included in
backbone router links advertisements, and OSPF packets pertaining to the
backbone area will flow over the adjacency. Such an adjacency has been
referred to as a "virtual adjacency".
In each endpoint router, the cost and viability of the virtual link is
discovered by examining the routing table entry for the other endpoint
router. (The entry's associated area must be the configured transit
area). Actually, there may be a separate routing table entry for each
Type of Service. These are called the virtual link's corresponding
routing table entries. The Interface Up event occurs for a virtual link
when its corresponding TOS 0 routing table entry becomes reachable.
Conversely, the Interface Down event occurs when its TOS 0 routing table
entry becomes unreachable.[18] In other words, the virtual link's
viability is determined by the existence of an intra-area path, through
the transit area, between the two endpoints. The other details
concerning virtual links are as follows:
o AS external links are NEVER flooded over virtual adjacencies. This
would be duplication of effort, since the same AS external links are
already flooded throughout the virtual link's transit area. For
this same reason, AS external link advertisements are not summarized
over virtual adjacencies during the database exchange process.
o The cost of a virtual link is NOT configured. It is defined to be
the cost of the intra-area path between the two defining area border
routers. This cost appears in the virtual link's corresponding
routing table entry. When the cost of a virtual link changes, a new
router links advertisement should be originated for the backbone
area.
o Just as the virtual link's cost and viability are determined by the
routing table build process (through construction of the routing
table entry for the other endpoint), so are the IP interface address
for the virtual interface and the virtual neighbor's IP address.
These are used when sending protocol packets over the virtual link.
o In each endpoint's router links advertisement for the backbone, the
virtual link is represented as a link having link type 4, Link ID
set to the virtual neighbor's OSPF Router ID and Link Data set to
the virtual interface's IP address. See Section 12.4.1 for more
information. Also, it may be the case that there is a TOS 0 path,
but no non-zero TOS paths to the other endpoint router. In this
case, non-zero TOS costs must be set to LSInfinity in the router
links advertisement.
o When virtual links are configured for the backbone, information
concerning backbone networks should not be condensed before being
summarized for the transit areas. In other words, each backbone
network should be advertised in a separate summary link
advertisement, regardless of the backbone's configured area address
ranges. See Section 12.4.3 for more information.
o The time between link state retransmissions, RxmtInterval, is
configured for a virtual link. This should be well over the
expected round-trip delay between the two routers. This may be hard
to estimate for a virtual link. It is better to err on the side of
making it too large.
16. Calculation Of The Routing Table
This section details the OSPF routing table calculation. Using its
attached areas' link state databases as input, a router runs the
following algorithm, building its routing table step by step. At each
step, the router must access individual pieces of the link state
databases (e.g., a router links advertisement originated by a certain
router). This access is performed by the lookup function discussed in
Section 12.2. The lookup process may return a link state advertisement
whose LS age is equal to MaxAge. Such an advertisement should not be
used in the routing table calculation, and is treated just as if the
lookup process had failed.
The OSPF routing table's organization is explained in Section 11. Two
examples of the routing table build process are presented in Sections
11.2 and 11.3. This process can be broken into the following steps:
(1) The present routing table is invalidated. The routing table is
built again from scratch. The old routing table is saved so that
changes in routing table entries can be identified.
(2) The intra-area routes are calculated by building the shortest path
tree for each attached area. In particular, all routing table
entries whose Destination type is "area border router" are
calculated in this step. This step is described in two parts. At
first the tree is constructed by only considering those links
between routers and transit networks. Then the stub networks are
incorporated into the tree.
(3) The inter-area routes are calculated, through examination of summary
link advertisements. If the router is attached to multiple areas
(i.e., it is an area border router), only backbone summary link
advertisements are examined.
(4) For those routing entries whose next hop is over a virtual link, a
real (physical) next hop is calculated. The real next hop will be
on one of the router's directly attached networks. This step only
concerns routers having configured virtual links.
(5) Routes to external destinations are calculated, through examination
of AS external link advertisements. The location of the AS boundary
routers (which originate the AS external link advertisements) has
been determined in steps 2-4.
Steps 2-5 are explained in further detail below. The explanations
describe the calculations for TOS 0 only. It may also be necessary to
perform each step (separately) for each of the non-zero TOS values.[19]
For more information concerning the building of non-zero TOS routes see
Section 16.9.
Changes made to routing table entries as a result of these calculations
can cause the OSPF protocol to take further actions. For example, a
change to an intra-area route will cause an area border router to
originate new summary link advertisements (see Section 12.4). See
Section 16.7 for a complete list of the OSPF protocol actions resulting
from routing table table changes.
16.1 Calculating the shortest-path tree for an area
This calculation yields the set of intra-area routes associated with an
area (called hereafter Area A). A router calculates the shortest-path
tree using itself as the root.[20] The formation of the shortest path
tree is done here in two stages. In the first stage, only links between
routers and transit networks are considered. Using the Dijkstra
algorithm, a tree is formed from this subset of the link state database.
In the second stage, leaves are added to the tree by considering the
links to stub networks.
The procedure will be explained using the graph terminology that was
introduced in Section 2. The area's link state database is represented
as a directed graph. The graph's vertices are routers, transit networks
and stub networks. The first stage of the procedure concerns only the
transit vertices (routers and transit networks) and their connecting
links. Throughout the shortest path calculation, the following data is
also associated with each transit vertex:
Vertex (node) ID
A 32-bit number uniquely identifying the vertex. For router
vertices this is the OSPF Router ID. For network vertices, this is
the IP address of the network's Designated Router.
A link state advertisement
Each transit vertex has an associated link state advertisement. For
router vertices, this is a router links advertisement. For transit
networks, this is a network links advertisement (which is actually
originated by the network's Designated Router). In any case, the
advertisement's Link State ID is always equal to the above Vertex
ID.
List of next hops
The list of next hops for the current shortest paths from the root
to this vertex. There can be multiple shortest paths due to the
equal-cost multipath capability. Each next hop indicates the
outgoing router interface to use when forwarding traffic to the
destination. On multi-access networks, the next hop also includes
the IP address of the next router (if any) in the path towards the
destination.
Distance from root
The link state cost of the current shortest path(s) from the root to
the vertex. The link state cost of a path is calculated as the sum
of the costs of the path's constituent links (as advertised in
router links and network links advertisements). One path is said to
be "shorter" than another if it has a smaller link state cost.
The first stage of the procedure can now be summarized as follows. At
each iteration of the algorithm, there is a list of candidate vertices.
The shortest paths from the root to these vertices have not
(necessarily) been found. The candidate vertex closest to the root is
added to the shortest-path tree, removed from the candidate list, and
its adjacent vertices are examined for possible addition to/modification
of the candidate list. The algorithm then iterates again. It
terminates when the candidate list becomes empty.
The following steps describe the first stage in detail. Remember that
we are computing the shortest path tree for Area A. All references to
link state database lookup below are from Area A's database.
(1) Initialize the algorithm's data structures. Clear the list of
candidate vertices. Initialize the shortest-path tree to only the
root (which is the router doing the calculation).
(2) Call the vertex just added to the tree vertex V. Examine the link
state advertisement associated with vertex V. This is a lookup in
the area link state database based on the Vertex ID. Each link
described by the advertisement gives the cost to an adjacent vertex.
For each described link, (say it joins vertex V to vertex W):
(a) If this is a link to a stub network, examine the next link in
V's advertisement. Links to stub networks will be considered in
the second stage of the shortest path calculation.
(b) Otherwise, W is a transit vertex (router or transit network).
Look up the vertex W's link state advertisement (router links or
network links) in Area A's link state database. If the
advertisement does not exist, or its age is equal to MaxAge, or
it does not have a link back to vertex V, examine the next link
in V's advertisement. Both ends of a link must advertise the
link before it will be used for data traffic.[21]
(c) If vertex W is already on the shortest-path tree, examine the
next link in the advertisement.
(d) If the cost of the link (from V to W) is LSInfinity, the link
should not be used for data traffic. In this case, examine the
next link in the advertisement.
(e) Calculate the link state cost D of the resulting path from the
root to vertex W. D is equal to the sum of the link state cost
of the (already calculated) shortest path to vertex V and the
advertised cost of the link between vertices V and W. If D is:
o Greater than the value that already appears for vertex W on
the candidate list, then examine the next link.
o Equal to the value that appears for vertex W on the the
candidate list, calculate the set of next hops that result
from using the advertised link. Input to this calculation
is the destination (W), and its parent (V). This
calculation is shown in Section 16.1.1. This set of hops
should be added to the next hop values that appear for W on
the candidate list.
o Less than the value that appears for vertex W on the the
candidate list, or if W does not yet appear on the candidate
list, then set the entry for W on the candidate list to
indicate a distance of D from the root. Also calculate the
list of next hops that result from using the advertised
link, setting the next hop values for W accordingly. The
next hop calculation is described in Section 16.1.1; it
takes as input the destination (W) and its parent (V).
(3) If at this step the candidate list is empty, the shortest-path tree
(of transit vertices) has been completely built and this stage of
the algorithm terminates. Otherwise, choose the vertex belonging to
the candidate list that is closest to the root, and add it to the
shortest-path tree (removing it from the candidate list in the
process).
(4) Possibly modify the routing table. For those routing table entries
modified, the associated area will be set to Area A, the path type
will be set to intra-area, and the cost will be set to the newly
discovered shortest path's calculated distance.
If the newly added vertex is an area border router, a routing table
entry is added whose destination type is "area border router". The
Options field found in the associated router links advertisement is
copied into the routing table entry's Optional capabilities field.
If the newly added vertex is an AS boundary router, the routing
table entry of type "AS boundary router" for the destination is
located. Since routers can belong to more than one area, it is
possible that several sets of intra-area paths exist to the AS
boundary router, each set using a different area. However, the AS
boundary router's routing table entry must indicate a set of paths
which utilize a single area. The area leading to the routing table
entry is selected as follows: A set of intra-area paths having no
virtual next hops is always preferred over a set of intra-area paths
in which some virtual next hops appear[22] ; all other things being
equal the set of paths having lower cost is preferred. Note that
whenever an AS boundary router's routing table entry is
added/modified, the Options found in the associated router links
advertisement is copied into the routing table entry's Optional
capabilities field.
If the newly added vertex is a transit network, the routing table
entry for the network is located. The entry's destination ID is the
IP network number, which can be obtained by masking the Vertex ID
(Link State ID) with its associated subnet mask (found in the
associated network links advertisement). If the routing table entry
already exists (i.e., there is already an intra-area route to the
destination installed in the routing table), multiple vertices have
mapped to the same IP network. For example, this can occur when a
new Designated Router is being established. In this case, the
current routing table entry should be overwritten if and only if the
newly found path is just as short and the current routing table
entry's Link State Origin has a smaller Link State ID than the newly
added vertex' link state advertisement.
If there is no routing table entry for the network (the usual case),
a routing table entry for the IP network should be added. The
routing table entry's Link State origin should be set to the newly
added vertex' link state advertisement.
(5) Iterate the algorithm by returning to Step 2.
The stub networks are added to the tree in the procedure's second stage.
In this stage, all router vertices are again examined. Those that have
been determined to be unreachable in the above first phase are
discarded. For each reachable router vertex (call it V), the associated
router links advertisement is found in the link state database. Each
stub network link appearing in the advertisement is then examined, and
the following steps are executed:
(1) If the cost of the stub network link is LSInfinity, the link should
not be used for data traffic. In this case, go on to examine the
next stub network link in the advertisement.
(2) Otherwise, Calculate the distance D of stub network from the root.
D is equal to the distance from the root to the router vertex
(calculated in stage 1), plus the stub network link's advertised
cost. Compare this distance to the current best cost to the stub
network. This is done by looking up the network's current routing
table entry. If the calculated distance D is larger, go on to
examine the next stub network link in the advertisement.
(3) If this step is reached, the stub network's routing table entry must
be updated. Calculate the set of next hops that would result from
using the stub network link. This calculation is shown in Section
16.1.1; input to this calculation is the destination (the stub
network) and the parent vertex (the router vertex). If the distance
D is the same as the current routing table cost, simply add this set
of next hops to the routing table entry's list of next hops. In
this case, the routing table already has a Link State origin. If
this Link State origin is a router links advertisement whose Link
State ID is smaller than V's Router ID, reset the Link State origin
to V's router links advertisement.
Otherwise D is smaller than the routing table cost. Overwrite the
current routing table entry by setting the routing table entry's
cost to D, and by setting the entry's list of next hops to the newly
calculated set. Set the routing table entry's Link State origin to
V's router links advertisement. Then go on to examine the next stub
network link.
For all routing table entries added/modified in the second stage, the associated area will be set to Area A and the path type will be set to intra-area. When the list of reachable router links is exhausted, the second stage is completed. At this time, all intra-area routes associated with Area A have been determined. The specification does not require that the above two stage method be used to calculate the shortest path tree. However, if another algorithm is used, an identical tree must be produced. For this reason, it is important to note that links between transit vertices must be bidirectional in ordered to be included in the above tree. It should also be mentioned that algorithms exist for incrementally updating the shortest-path tree (see [BBN]). 16.1.1 The next hop calculation This section explains how to calculate the current set of next hops to use for a destination. Each next hop consists of the outgoing interface to use in forwarding packets to the destination together with the next hop router (if any). The next hop calculation is invoked each time a shorter path to the destination is discovered. This can happen in either stage of the shortest-path tree calculation (see Section 16.1). In stage 1 of the shortest-path tree calculation a shorter path is found as the destination is added to the candidate list, or when the destination's entry on the candidate list is modified (Step 2e of Stage 1). In stage 2 a shorter path is discovered each time the destination's routing table entry is modified (Step 3 of Stage 2). The set of next hops to use for the destination may be recalculated several times during the shortest-path tree calculation, as shorter and shorter paths are discovered. In the end, the destination's routing table entry will always reflect the next hops resulting from the absolute shortest path(s). Input to the next hop calculation is a) the destination and b) its parent in the current shortest path between the root (the calculating router) and the destination. The parent is always a transit vertex (i.e., always a router or a transit network). If there is at least one intervening router in the current shortest path between the destination and the root, the destination simply inherits the set of next hops from the parent. Otherwise, there are two cases. In the first case, the parent vertex is the root (the calculating router itself). This means that the destination is either a directly connected network or directly connected router. The next hop in this case is simply the OSPF interface connecting to the network/router; no next hop router is required.
In the second case, the destination is a router, and its parent vertex is a network. The list of next hops is then determined by examining the destination's router links advertisement. For each link in the advertisement that points back to the parent network, the link's Link Data field provides the IP address of a next hop router. The outgoing interface to use can then be derived from the next hop IP address (or it can be inherited from the parent network). 16.2 Calculating the inter-area routes The inter-area routes are calculated by examining summary link advertisements. If the router has active attachments to multiple areas, only backbone summary link advertisements are examined. Routers attached to a single area examine that area's summary links. In either case, the summary links examined below are all part of a single area's link state database (call it Area A). Summary link advertisements are originated by the area border routers. Each summary link advertisement in Area A is considered in turn. Remember that the destination described by a summary link advertisement is either a network (type 3 summary link advertisements) or an AS boundary router (type 4 summary link advertisements). For each summary link advertisement: (1) If the cost specified by the advertisement is LSInfinity, then examine the next advertisement. (2) If the advertisement was originated by the calculating router itself, examine the next advertisement. (3) If the collection of destinations described by the summary link falls into one of the router's configured area address ranges (see Section 3.5) and the particular area address range is active, the summary link should be ignored. Active means that there are one or more reachable (by intra-area paths) networks contained in the area range. In this case, all addresses in the area range are assumed to be either reachable via intra-area paths, or else to be unreachable by any other means. (4) Else, call the destination described by the advertisement N, and the area border originating the advertisement BR. Look up the routing table entry for BR having A as its associated area. If no such entry exists for router BR (i.e., BR is unreachable in Area A), do nothing with this advertisement and consider the next in the list. Else, this advertisement describes an inter-area path to destination N, whose cost is the distance to BR plus the cost specified in the
advertisement. Call the cost of this inter-area path IAC.
(5) Next, look up the routing table entry for the destination N. (The
entry's Destination type is either Network or AS boundary router.)
If no entry exists for N or if the entry's path type is "AS
external", install the inter-area path to N, with associated area A,
cost IAC, next hop equal to the list of next hops to router BR, and
advertising router equal to BR.
(6) Else, if the paths present in the table are intra-area paths, do
nothing with the advertisement (intra-area paths are always
preferred).
(7) Else, the paths present in the routing table are also inter-area
paths. Install the new path through BR if it is cheaper, overriding
the paths in the routing table. Otherwise, if the new path is the
same cost, add it to the list of paths that appear in the routing
table entry.
16.3 Resolving virtual next hops
This step is only necessary in area-border routers having configured
virtual links. In these routers, some of the routing table entries may
have virtual next hops. That is, one or more of the next hops installed
in Sections 16.1 and 16.2 may be over a virtual link. However, when
forwarding data traffic to a destination, the next hops must always be
on a directly attached network.
In this section, each virtual next hop is replaced by a real next hop.
In the process a new routing table distance is calculated that may be
smaller than the previously calculated distance. In this case, the list
of next hops is pruned so that only those giving rise to the new
shortest distance are included, and the routing table entry's distance
is updated accordingly.
______________________________________
(Figure not included in text version.)
Figure 17: Resolving virtual next hops
______________________________________
This resolution of virtual next hops is done only for Destination types
Network or AS Boundary router. Suppose that one of a routing table
entry's next hops is a virtual link. This is determined by the
following combination: the routing table entry's path type is either
intra-area or inter-area, the area associated with the routing table
entry must be the backbone, yet the next hop belongs to a different area
(the virtual link's transit area).
Let N be the above entry's destination, and A the virtual link's transit
area. The real next hop (and new distance) is calculated as follows.
Let D be a distance counter, and set the real next hop NH to null.
Then, look up all the summary link advertisements for N in area A's
database, performing the following steps for each advertisement:[23]
(1) Call the border router that originated the advertisement BR. If
there is no routing table entry for BR having A as associated area
(i.e., BR is unreachable through Area A), examine the next
advertisement.
(2) Else, let X be the distance to BR via Area A. If the cost
advertised by BR (call it Y) to the destination is LSInfinity,
examine the next summary link advertisement. Else, the cost to
destination N through area border router BR is X+Y.
(3) If next hop NH is null or X+Y is smaller is smaller than D, set D to
X+Y and set the next hop NH to the next hop specified in router BR's
routing table entry.
At this point, the real next hop NH should be set, and the distance D
calculated should be less than or equal to the cost originally specified
in destination N's routing table entry. This same calculation should be
done for all of N's virtual next hops, and then N's new cost set to the
minimum calculated distance, with the its new set of next hops that
combination of non-virtual and recalculated next hops that correspond to
this (possibly same as original) distance.
The resolving of virtual next hops may produce unexpected results.
After the virtual next hops are resolved, traffic that was originally
scheduled to go over the virtual link may instead take a different path
through the virtual link's transit area. In other words, virtual links
allow transit traffic to be forwarded through an area, but do not
dictate the precise path that the traffic will take.
As an example, consider the Autonomous System pictured in Figure 17.
There is a single non-backbone area (Area 1) that physically divides the
backbone into two separate pieces. To maintain connectivity of the
backbone, a virtual link has been configured between routers RT1 and RT4. On the right side of the figure, network N1 belongs to the backbone. The dotted lines indicate that there is a much shorter intra-area backbone path between router RT5 and network N1 (cost 20) than there is between router RT4 and network N1 (cost 100). Both router RT4 and router RT5 will inject summary link advertisements for network N1 into Area 1. After the shortest-path tree has been calculated for the backbone, router RT1 (one end of the virtual link) will have selected router RT4 as the virtual next hop for all data traffic destined for network N1. However, since router RT5 is so much closer to network N1, all routers internal to Area 1 (e.g., routers RT2 and RT3) will forward their network N1 traffic towards router RT5, instead of RT4. And indeed, after resolving the virtual next hop by the above calculation, router RT1 will also forward network N1 traffic towards RT5. So, in this example the virtual link enables network N1 traffic to be forwarded through the transit Area 1, but the actual path the data traffic takes does not follow the virtual link. 16.4 Calculating AS external routes AS external routes are calculated by examining AS external link advertisements. Each of the AS external link advertisements is considered in turn. Most AS external advertisements describe routes to specific IP destinations. An AS external advertisement can also describe a default route for the Autonomous System (destination = DefaultDestination). For each AS external link advertisement: (1) If the cost specified by the advertisement is LSInfinity, then examine the next advertisement. (2) If the advertisement was originated by the calculating router itself, examine the next advertisement. (3) Call the destination described by the advertisement N. Look up the routing table entry for the AS boundary router (ASBR) that originated the advertisement. If no entry exists for router ASBR (i.e., ASBR is unreachable), do nothing with this advertisement and consider the next in the list. Else, this advertisement describes an AS external path to destination N. Examine the forwarding address specified in the external advertisement. This indicates the IP address to which packets for the destination should be forwarded. If forwarding address is set to 0.0.0.0, packets should be sent to the ASBR
itself. Otherwise, look up the forwarding address in the routing
table.[24] An intra-area or inter-area path must exist to the
forwarding address. If no such path exists, do nothing with the
advertisement and consider the next in the list.
Call the routing table distance to the forwarding address X (when
the forwarding address is set to 0.0.0.0, this is the distance to
the ASBR itself), and the cost specified in the advertisement Y. X
is in terms of the link state metric, and Y is a Type 1 or 2
external metric.
(4) Next, look up the routing table entry for the destination N. If no
entry exists for N, install the AS external path to N, with next hop
equal to the list of next hops to the forwarding address, and
advertising router equal to ASBR. If the external metric type is 1,
then the path-type is set to type 1 external and the cost is equal
to X+Y. If the external metric type is 2, the the path-type is set
to type 2 external, the link state component of the route's cost is
X, and the Type 2 cost is Y.
(5) Else, if the paths present in the table are not type 1 or type 2
external paths, do nothing (AS external paths have the lowest
priority).
(6) Otherwise, compare the cost of this new AS external path to the ones
present in the table. Type 1 external paths are always shorter than
Type 2 external paths. Type 1 external paths are compared by
looking at the sum of the distance to the forwarding address and the
advertised Type 1 metric (X+Y). Type 2 external paths are compared
by looking at the advertised Type 2 metrics, and then if necessary,
the distance to the forwarding addresses.
If the new path is shorter, it replaces the present paths in the
routing table entry. If the new path is the same cost, it is added
to the routing table entry's list of paths.
16.5 Incremental updates --- summary links
When a new summary link advertisement is received, it is not necessary
to recalculate the entire routing table. Call the destination described
by the summary link advertisement N, and let A be the area to which the
advertisement belongs.
Look up the routing table entry for N. If the next hop to N is a
virtual link through Area A (this means that the entry's associated area
is the backbone, and the listed next hop does not belong to the
backbone, but instead belongs to Area A), the real next hop must again
be resolved. This means running the algorithm in Section 16.3 for destination N only. Else, if there is an intra-area route to destination N nothing need be done (intra-area routes always take precedence). Otherwise, if Area A is the router's sole attached area, or Area A is the backbone, the procedure in Section 16.2 will have to be performed, but only for those summary link advertisements whose destination is N. Before this procedure is performed, the present routing table entry for N should be invalidated (but kept for comparison purposes). If this procedure leads to a virtual next hop, the algorithm in Section 16.3 will again have to be performed in order to calculate the real next hop. If N's routing table entry changes, and N is an AS boundary router, the AS external links will have to be reexamined (Section 16.4). 16.6 Incremental updates --- AS external links When a new AS external link advertisement is received, it is not necessary to recalculate the entire routing table. Call the destination described by the AS external link advertisement N. If there is already an intra-area or inter-area route to the destination, no recalculation is necessary (these routes take precedence). Otherwise, the procedure in Section 16.4 will have to be performed, but only for those AS external link advertisements whose destination is N. Before this procedure is performed, the present routing table entry for N should be invalidated. 16.7 Events generated as a result of routing table changes Changes to routing table entries sometimes cause the OSPF area border routers to take additional actions. These routers need to act on the following routing table changes: o The cost or path type of a routing table entry has changed. If the destination described by this entry is a Network or AS boundary router, and this is not simply a change of AS external routes, new summary link advertisements may have to be generated (potentially one for each attached area, including the backbone). See Section 12.4.3 for more information. If a previously advertised entry has been deleted, or is no longer advertisable to a particular area, the advertisement must be flushed from the routing domain by setting its age to MaxAge and reflooding (see Section 14.1).
o A routing table entry associated with a configured virtual link has
changed. The destination of such a routing table entry is an area
border router. The change indicates a modification to the virtual
link's cost or viability.
If the entry indicates that the area border router is newly
reachable (via TOS 0), the corresponding virtual link is now
operational. An Interface Up event should be generated for the
virtual link, which will cause a virtual adjacency to begin to form
(see Section 10.3). At this time the virtual interface's IP address
and the virtual neighbor's IP address are also calculated.
If the entry indicates that the area border router is no longer
reachable (via TOS 0), the virtual link and its associated adjacency
should be destroyed. This means an Interface Down event should be
generated for the associated virtual link.
If the cost of the entry has changed, and there is a fully
established virtual adjacency, a new router links advertisement for
the backbone must be originated. This in turn may cause further
routing table changes.
16.8 Equal-cost multipath
The OSPF protocol maintains multiple equal-cost routes to all
destinations. This can be seen in the steps used above to calculate the
routing table, and in the definition of the routing table structure.
Each one of the multiple routes will be of the same type (intra-area,
inter-area, type 1 external or type 2 external), cost, and will have the
same associated area. However, each route specifies a separate next hop
and advertising router.
There is no requirement that a router running OSPF keep track of all
possible equal-cost routes to a destination. An implementation may
choose to keep only a fixed number of routes to any given destination.
This does not affect any of the algorithms presented in this
specification.
16.9 Building the non-zero-TOS portion of the routing table
The OSPF protocol can calculate a different set of routes for each IP
TOS (see Section 2.4). Support for TOS-based routing is optional.
TOS-capable and non-TOS-capable routers can be mixed in an OSPF routing
domain. Routers not supporting TOS calculate only the TOS 0 route to
each destination. These routes are then used to forward all data
traffic, regardless of the TOS indications in the data packet's IP
header. A router that does not support TOS indicates this fact to the
other OSPF routers by clearing the T-bit in the Options field of its
router links advertisement.
The above sections detailing the routing table calculations handle the
TOS 0 case only. In general, for routers supporting TOS-based routing,
each piece of the routing table calculation must be rerun separately for
the non-zero TOS values. When calculating routes for TOS X, only TOS X
metrics can be used. Any link state advertisement may specify a
separate cost for each TOS (a cost for TOS 0 must always be specified).
The encoding of TOS in OSPF link state advertisements is described in
Section 12.3.
An advertisement can specify that it is restricted to TOS 0 (i.e., non-
zero TOS is not handled) by clearing the T-bit in the link state
advertisement's Option field. Such advertisements are not used when
calculating routes for non-zero TOS. For this reason, it is possible
that a destination is unreachable for some non-zero TOS. In this case,
the TOS 0 path is used when forwarding packets (see Section 11.1).
The following lists the modifications needed when running the routing
table calculation for a non-zero TOS value (called TOS X). In general,
routers and advertisements that do not support TOS are omitted from the
calculation.
Calculating the shortest-path tree (Section 16.1).
Routers that do not support TOS-based routing should be omitted from
the shortest-path tree calculation. These routers are identified as
those having the T-bit reset in their router links advertisements.
Such routers should never be added to the Dijktra algorithm's
candidate list, nor should their router links advertisements be
examined when adding the stub networks to the tree.
Calculating the inter-area routes (Section 16.2).
Inter-area paths are the concatenation of a path to an area border
router with a summary link. When calculating TOS X routes, both
path components must also specify TOS X. In other words, only TOS X
paths to the area border router are examined, and the area border
router must be advertising a TOS X route to the destination. Note
that this means that summary link advertisements having the T-bit
reset in their Options field are not considered.
Resolving virtual next hops (Section 16.3).
This calculation again considers the concatenation of a path to an
area border router with a summary link. As with inter-area routes,
only TOS X paths to the area border router are examined, and the
area border router must be advertising a TOS X route to the
destination.
Calculating AS external routes (Section 16.4).
This calculation considers the concatenation of a path to a
forwarding address with an AS external link. Only TOS X paths to
the forwarding address are examined, and the AS boundary router must
be advertising a TOS X route to the destination. Note that this
means that AS external link advertisements having the T-bit reset in
their Options field are not considered.
In addition, the advertising AS boundary router must also be
reachable for its advertisements to be considered (see Section
16.4). However, if the advertising router and the forwarding
address are not one in the same, the advertising router need only be
reachable via TOS 0.
[1]The graph's vertices represent either routers, transit networks,
or stub networks. Since routers may belong to multiple areas, it is
not possible to color the graph's vertices.
[2]It is possible for all of a router's interfaces to be unnumbered
point-to-point links. In this case, an IP address must be assigned
to the router. This address will then be advertised in the router's
router links advertisement as a host route.
[3]Note that in these cases both interfaces, the non-virtual and the
virtual, would have the same IP address.
[4]Note that no host route is generated for, and no IP packets can
be addressed to, interfaces to unnumbered point-to-point networks.
This is regardless of such an interface's state.
[5]It is instructive to see what happens when the Designated Router
for the network crashes. Call the Designated Router for the network
RT1, and the the Backup Designated Router RT2. If router RT1
crashes (or maybe its interface to the network dies), the other
routers on the network will detect RT1's absence within
RouterDeadInterval seconds. All routers may not detect this at
precisely the same time; the routers that detect RT1's absence
before RT2 does will, for a time, select RT2 to be both Designated
Router and Backup Designated Router. When RT2 detects that RT1 is
gone it will move itself to Designated Router. At this time, the
remaining router having highest Router Priority will be selected as
Backup Designated Router.
[6]On point-to-point networks, the lower level protocols indicate
whether the neighbor is up and running. Likewise, existence of the
neighbor on virtual links is indicated by the routing table
calculation. However, in both these cases, the Hello Protocol is
still used. This ensures that communication between the neighbors
is bidirectional, and that each of the neighbors has a functioning
routing protocol layer.
[7]When the identity of the Designated Router is changing, it may be
quite common for a neighbor in this state to send the router a
Database Description packet; this means that there is some momentary
disagreement on the Designated Router's identity.
[8]Note that it is possible for a router to resynchronize any of its
fully established adjacencies by setting the adjacency's state back
to ExStart. This will cause the other end of the adjacency to
process a Seq Number Mismatch event, and therefore to also go back
to ExStart state.
[9]The address space of IP networks and the address space of OSPF
Router IDs may overlap. That is, a network may have an IP address
which is identical (when considered as a 32-bit number) to some
router's Router ID.
[10]It is assumed that, for two different address ranges matching
the destination, one range is more specific than the other. Non-
contiguous subnet masks can be configured to violate this
assumption. Such subnet mask configurations cannot be handled by the
OSPF protocol.
[11]MaxAgeDiff is an architectural constant. It indicates the
maximum dispersion of ages, in seconds, that can occur for a single
link state instance as it is flooded throughout the routing domain.
If two advertisements differ by more than this, they are assumed to
be different instances of the same advertisement. This can occur
when a router restarts and loses track of its previous sequence
number. See Section 13.4 for more details.
[12]When two advertisements have different checksums, they are
assumed to be separate instances. This can occur when a router
restarts, and loses track of its previous sequence number. In this
case, since the two advertisements have the same sequence number, it
is not possible to determine which link state is actually newer. If
the wrong advertisement is accepted as newer, the originating router
will originate another instance. See Section 13.4 for further
details.
[13]There is one instance where a lookup must be done based on
partial information. This is during the routing table calculation,
when a network links advertisement must be found based solely on its
Link State ID. The lookup in this case is still well defined, since
no two network advertisements can have the same Link State ID.
[14]This clause covers the case: Inter-area routes are not
summarized to the backbone. This is because inter-area routes are
always associated with the backbone area.
[15]By keeping more information in the routing table, it is possible
for an implementation to recalculate the shortest path tree only for
a single area. In fact, there are incremental algorithms that allow
an implementation to recalculate only a portion of the shortest path
tree [BBN]. These algorithms are beyond the scope of this
specification.
[16]This is how the Link state request list is emptied, which
eventually causes the neighbor state to transition to Full. See
Section 10.9 for more details.
[17]It should be a relatively rare occurrence for an advertisement's
age to reach MaxAge. Usually, the advertisement will be replaced by
a more recent instance before it ages out.
[18]Only the TOS 0 routes are important here. This is because all
routing protocol packets are sent with TOS= 0. See Appendix A.
[19]It may be the case that paths to certain destinations do not
vary based on TOS. For these destinations, the routing calculation
need not be repeated for each TOS value. In addition, there need
only be a single routing table entry for these destinations (instead
of a separate entry for each TOS value).
[20]Strictly speaking, because of equal-cost multipath, the
algorithm does not create a tree. We continue to use the "tree"
terminology because that is what occurs most often in the existing
literature.
[21]This means that before data traffic will flow between a pair of
neighboring routers, their link state databases must be
synchronized. Before synchronization (neighbor state < Full), a
router will not include the connection to its neighbor in its link
state advertisements.
[22]As a result of this clause, when a virtual link exists between
the calculating router and an AS boundary router, the intra-area
path through the virtual link's transit area is always preferred
over the virtual link itself.
[23]Note the similarity between this procedure and the calculation
of inter-area routes by a router internal to Area A.
[24]When the forwarding address is non-zero, it should point to a
router belonging to another Autonomous System. See Section 12.4.4
for more details.
(next page on part 6)
