RFC 2328
OSPF Version 2
Pages: 244
Internet Standard: 54
→ Errata
Obsoletes: 2178
Updated by: 5709 6549 6845 6860 7474 8042 9355 9454
Internet Standard: 54
→ Errata
Obsoletes: 2178
Updated by: 5709 6549 6845 6860 7474 8042 9355 9454
Part 7 of 8 – Pages 168 to 202
16.2. Calculating the inter-area routes The inter-area routes are calculated by examining summary-LSAs. If the router has active attachments to multiple areas, only backbone summary-LSAs are examined. Routers attached to a single area examine that area's summary-LSAs. In either case, the summary-LSAs examined below are all part of a single area's link state database (call it Area A).
Summary-LSAs are originated by the area border routers. Each
summary-LSA in Area A is considered in turn. Remember that the
destination described by a summary-LSA is either a network (Type
3 summary-LSAs) or an AS boundary router (Type 4 summary-LSAs).
For each summary-LSA:
(1) If the cost specified by the LSA is LSInfinity, or if the
LSA's LS age is equal to MaxAge, then examine the the next
LSA.
(2) If the LSA was originated by the calculating router itself,
examine the next LSA.
(3) If it is a Type 3 summary-LSA, and the collection of
destinations described by the summary-LSA equals one of the
router's configured area address ranges (see Section 3.5),
and the particular area address range is active, then the
summary-LSA should be ignored. "Active" means that there
are one or more reachable (by intra-area paths) networks
contained in the area range.
(4) Else, call the destination described by the LSA N (for Type
3 summary-LSAs, N's address is obtained by masking the LSA's
Link State ID with the network/subnet mask contained in the
body of the LSA), and the area border originating the LSA
BR. Look up the routing table entry for BR having Area 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
LSA and consider the next in the list. Else, this LSA
describes an inter-area path to destination N, whose cost is
the distance to BR plus the cost specified in the LSA. Call
the cost of this inter-area path IAC.
(5) Next, look up the routing table entry for the destination N.
(If N is an AS boundary router, look up the "router" routing
table entry associated with Area A). If no entry exists for
N or if the entry's path type is "type 1 external" or "type
2 external", then install the inter-area path to N, with
associated area 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 LSA (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. Examining transit areas' summary-LSAs
This step is only performed by area border routers attached to
one or more non-backbone areas that are capable of carrying
transit traffic (i.e., "transit areas", or those areas whose
TransitCapability parameter has been set to TRUE in Step 2 of
the Dijkstra algorithm (see Section 16.1).
The purpose of the calculation below is to examine the transit
areas to see whether they provide any better (shorter) paths
than the paths previously calculated in Sections 16.1 and 16.2.
Any paths found that are better than or equal to previously
discovered paths are installed in the routing table.
The calculation also determines the actual next hop(s) for those
destinations whose next hop was calculated as a virtual link in
Sections 16.1 and 16.2. After completion of the calculation
below, any paths calculated in Sections 16.1 and 16.2 that still
have unresolved virtual next hops should be discarded.
The calculation proceeds as follows. All the transit areas'
summary-LSAs are examined in turn. Each such summary-LSA
describes a route through a transit area Area A to a Network N
(N's address is obtained by masking the LSA's Link State ID with
the network/subnet mask contained in the body of the LSA) or in
the case of a Type 4 summary-LSA, to an AS boundary router N.
Suppose also that the summary-LSA was originated by an area
border router BR.
(1) If the cost advertised by the summary-LSA is LSInfinity, or
if the LSA's LS age is equal to MaxAge, then examine the
next LSA.
(2) If the summary-LSA was originated by the calculating router
itself, examine the next LSA.
(3) Look up the routing table entry for N. (If N is an AS
boundary router, look up the "router" routing table entry
associated with the backbone area). If it does not exist, or
if the route type is other than intra-area or inter-area, or
if the area associated with the routing table entry is not
the backbone area, then examine the next LSA. In other
words, this calculation only updates backbone intra-area
routes found in Section 16.1 and inter-area routes found in
Section 16.2.
(4) Look up the routing table entry for the advertising router
BR associated with the Area A. If it is unreachable, examine
the next LSA. Otherwise, the cost to destination N is the
sum of the cost in BR's Area A routing table entry and the
cost advertised in the LSA. Call this cost IAC.
(5) If this cost is less than the cost occurring in N's routing
table entry, overwrite N's list of next hops with those used
for BR, and set N's routing table cost to IAC. Else, if IAC
is the same as N's current cost, add BR's list of next hops
to N's list of next hops. In any case, the area associated
with N's routing table entry must remain the backbone area,
and the path type (either intra-area or inter-area) must
also remain the same.
It is important to note that the above calculation never makes
unreachable destinations reachable, but instead just potentially
finds better paths to already reachable destinations. The
calculation installs any better cost found into the routing
table entry, from which it may be readvertised in summary-LSAs
to other areas.
As an example of the calculation, 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
........................ . Area 1 (transit) . + . . | . +---+1 1+---+100 | . |RT2|----------|RT4|=========| . 1/+---+********* +---+ | . /******* . | . 1/*Virtual . | 1+---+/* Link . Net|work =======|RT1|* . | N1 +---+\ . | . \ . | . \ . | . 1\+---+1 1+---+20 | . |RT3|----------|RT5|=========| . +---+ +---+ | . . | ........................ + Figure 17: Routing through transit areas 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-LSAs for Network N1 into Area 1. After the shortest-path tree has been calculated for the backbone in Section 16.1, Router RT1 (left end of the virtual link) will have calculated a path through Router RT4 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 examining Area 1's summary-LSAs by the above calculation, Router RT1 will also forward Network N1 traffic towards RT5. Note that in this example the virtual link enables transit data traffic to be forwarded through Area 1, but the actual path the transit data traffic takes does not follow the virtual link. 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.
16.4. Calculating AS external routes AS external routes are calculated by examining AS-external-LSAs. Each of the AS-external-LSAs is considered in turn. Most AS- external-LSAs describe routes to specific IP destinations. An AS-external-LSA can also describe a default route for the Autonomous System (Destination ID = DefaultDestination, network/subnet mask = 0x00000000). For each AS-external-LSA: (1) If the cost specified by the LSA is LSInfinity, or if the LSA's LS age is equal to MaxAge, then examine the next LSA. (2) If the LSA was originated by the calculating router itself, examine the next LSA. (3) Call the destination described by the LSA N. N's address is obtained by masking the LSA's Link State ID with the network/subnet mask contained in the body of the LSA. Look up the routing table entries (potentially one per attached area) for the AS boundary router (ASBR) that originated the LSA. If no entries exist for router ASBR (i.e., ASBR is unreachable), do nothing with this LSA and consider the next in the list. Else, this LSA describes an AS external path to destination N. Examine the forwarding address specified in the AS- external-LSA. This indicates the IP address to which packets for the destination should be forwarded. If the forwarding address is set to 0.0.0.0, packets should be sent to the ASBR itself. Among the multiple routing table entries for the ASBR, select the preferred entry as follows. If RFC1583Compatibility is set to "disabled", prune the set of routing table entries for the ASBR as described in Section 16.4.1. In any case, among the remaining routing table entries, select the routing table entry with the least cost; when there are multiple least cost routing table entries the entry whose associated area has the largest OSPF Area ID (when considered as an unsigned 32-bit integer) is chosen.
If the forwarding address is non-zero, look up the
forwarding address in the routing table.[24] The matching
routing table entry must specify an intra-area or inter-area
path; if no such path exists, do nothing with the LSA and
consider the next in the list.
(4) Let X be the cost specified by the preferred routing table
entry for the ASBR/forwarding address, and Y the cost
specified in the LSA. X is in terms of the link state
metric, and Y is a type 1 or 2 external metric.
(5) 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 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.
(6) Compare the AS external path described by the LSA with the
existing paths in N's routing table entry, as follows. If
the new path is preferred, it replaces the present paths in
N's routing table entry. If the new path is of equal
preference, it is added to N's routing table entry's list of
paths.
(a) Intra-area and inter-area paths are always preferred
over AS external paths.
(b) Type 1 external paths are always preferred over type 2
external paths. When all paths are type 2 external
paths, the paths with the smallest advertised type 2
metric are always preferred.
(c) If the new AS external path is still indistinguishable
from the current paths in the N's routing table entry,
and RFC1583Compatibility is set to "disabled", select
the preferred paths based on the intra-AS paths to the
ASBR/forwarding addresses, as specified in Section
16.4.1.
(d) If the new AS external path is still indistinguishable
from the current paths in the N's routing table entry,
select the preferred path based on a least cost
comparison. 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 advertising equal type 2 metrics are
compared by looking at the distance to the forwarding
addresses.
16.4.1. External path preferences
When multiple intra-AS paths are available to
ASBRs/forwarding addresses, the following rules indicate
which paths are preferred. These rules apply when the same
ASBR is reachable through multiple areas, or when trying to
decide which of several AS-external-LSAs should be
preferred. In the former case the paths all terminate at the
same ASBR, while in the latter the paths terminate at
separate ASBRs/forwarding addresses. In either case, each
path is represented by a separate routing table entry as
defined in Section 11.
This section only applies when RFC1583Compatibility is set
to "disabled".
The path preference rules, stated from highest to lowest
preference, are as follows. Note that as a result of these
rules, there may still be multiple paths of the highest
preference. In this case, the path to use must be determined
based on cost, as described in Section 16.4.
o Intra-area paths using non-backbone areas are always the
most preferred.
o The other paths, intra-area backbone paths and inter-
area paths, are of equal preference.
16.5. Incremental updates -- summary-LSAs
When a new summary-LSA is received, it is not necessary to
recalculate the entire routing table. Call the destination
described by the summary-LSA N (N's address is obtained by
masking the LSA's Link State ID with the network/subnet mask
contained in the body of the LSA), and let Area A be the area to
which the LSA belongs. There are then two separate cases:
Case 1: Area A is the backbone and/or the router is not an area
border router.
In this case, the following calculations must be performed.
First, if there is presently an inter-area route to the
destination N, N's routing table entry is invalidated,
saving the entry's values for later comparisons. Then the
calculation in Section 16.2 is run again for the single
destination N. In this calculation, all of Area A's
summary-LSAs that describe a route to N are examined. In
addition, if the router is an area border router attached to
one or more transit areas, the calculation in Section 16.3
must be run again for the single destination. If the
results of these calculations have changed the cost/path to
an AS boundary router (as would be the case for a Type 4
summary-LSA) or to any forwarding addresses, all AS-
external-LSAs will have to be reexamined by rerunning the
calculation in Section 16.4. Otherwise, if N is now newly
unreachable, the calculation in Section 16.4 must be rerun
for the single destination N, in case an alternate external
route to N exists.
Case 2: Area A is a transit area and the router is an area
border router.
In this case, the following calculations must be performed.
First, if N's routing table entry presently contains one or
more inter-area paths that utilize the transit area Area A,
these paths should be removed. If this removes all paths
from the routing table entry, the entry should be
invalidated. The entry's old values should be saved for
later comparisons. Next the calculation in Section 16.3 must
be run again for the single destination N. If the results of
this calculation have caused the cost to N to increase, the
complete routing table calculation must be rerun starting
with the Dijkstra algorithm specified in Section 16.1.
Otherwise, if the cost/path to an AS boundary router (as
would be the case for a Type 4 summary-LSA) or to any
forwarding addresses has changed, all AS-external-LSAs will
have to be reexamined by rerunning the calculation in
Section 16.4. Otherwise, if N is now newly unreachable, the
calculation in Section 16.4 must be rerun for the single
destination N, in case an alternate external route to N
exists.
16.6. Incremental updates -- AS-external-LSAs
When a new AS-external-LSA is received, it is not necessary to
recalculate the entire routing table. Call the destination
described by the AS-external-LSA N. N's address is obtained by
masking the LSA's Link State ID with the network/subnet mask
contained in the body of the LSA. If there is already an intra-
area or inter-area route to the destination, no recalculation is
necessary (internal routes take precedence).
Otherwise, the procedure in Section 16.4 will have to be
performed, but only for those AS-external-LSAs 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-LSAs 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 LSA must be
flushed from the routing domain by setting its LS 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, the corresponding virtual link is now
operational. An InterfaceUp 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
link's IP interface address and the virtual neighbor's
Neighbor IP address are also calculated.
If the entry indicates that the area border router is no
longer reachable, the virtual link and its associated
adjacency should be destroyed. This means an InterfaceDown
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-LSA 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 may specify 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.
Footnotes [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-LSA 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 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 SeqNumberMismatch 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]"Discard" entries are necessary to ensure that route
summarization at area boundaries will not cause packet looping.
[11]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.
[12]MaxAgeDiff is an architectural constant. It indicates the
maximum dispersion of ages, in seconds, that can occur for a single
LSA instance as it is flooded throughout the routing domain. If two
LSAs differ by more than this, they are assumed to be different
instances of the same LSA. This can occur when a router restarts
and loses track of the LSA's previous LS sequence number. See
Section 13.4 for more details.
[13]When two LSAs have different LS checksums, they are assumed to
be separate instances. This can occur when a router restarts, and
loses track of the LSA's previous LS sequence number. In the case
where the two LSAs have the same LS sequence number, it is not
possible to determine which LSA is actually newer. However, if the
wrong LSA is accepted as newer, the originating router will simply
originate another instance. See Section 13.4 for further details.
[14]There is one instance where a lookup must be done based on
partial information. This is during the routing table calculation,
when a network-LSA must be found based solely on its Link State ID.
The lookup in this case is still well defined, since no two
network-LSAs can have the same Link State ID.
[15]This is the way RFC 1583 specified point-to-point
representation. It has three advantages: a) it does not require
allocating a subnet to the point-to-point link, b) it tends to bias
the routing so that packets destined for the point-to-point
interface will actually be received over the interface (which is
useful for diagnostic purposes) and c) it allows network
bootstrapping of a neighbor, without requiring that the bootstrap
program contain an OSPF implementation.
[16]This is the more traditional point-to-point representation used
by protocols such as RIP.
[17]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.
[18]This clause is only invoked when a non-backbone Area A supports
transit data traffic (i.e., has TransitCapability set to TRUE). For
example, in the area configuration of Figure 6, Area 2 can support
transit traffic due to the configured virtual link between Routers
RT10 and RT11. As a result, Router RT11 need only originate a single
summary-LSA into Area 2 (having the collapsed destination N9-
N11,H1), since all of Router RT11's other eligible routes have next
hops belonging to Area 2 itself (and as such only need be advertised
by other area border routers; in this case, Routers RT10 and RT7).
[19]By keeping more information in the routing table, it is possible
for an implementation to recalculate the shortest path tree for only
a single area. In fact, there are incremental algorithms that allow
an implementation to recalculate only a portion of a single area's
shortest path tree [Ref1]. However, these algorithms are beyond the
scope of this specification.
[20]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.
[21]It should be a relatively rare occurrence for an LSA's LS age to
reach MaxAge in this fashion. Usually, the LSA will be replaced by
a more recent instance before it ages out.
[22]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.
[23]Note that the presence of any link back to V is sufficient; it
need not be the matching half of the link under consideration from V
to W. This is enough to ensure that, before data traffic flows
between a pair of neighboring routers, their link state databases
will be synchronized.
[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.
References [Ref1] McQuillan, J., I. Richer and E. Rosen, "ARPANET Routing Algorithm Improvements", BBN Technical Report 3803, April 1978. [Ref2] Digital Equipment Corporation, "Information processing systems -- Data communications -- Intermediate System to Intermediate System Intra-Domain Routing Protocol", October 1987. [Ref3] McQuillan, J., et.al., "The New Routing Algorithm for the ARPANET", IEEE Transactions on Communications, May 1980. [Ref4] Perlman, R., "Fault-Tolerant Broadcast of Routing Information", Computer Networks, December 1983. [Ref5] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981. [Ref6] McKenzie, A., "ISO Transport Protocol specification ISO DP 8073", RFC 905, April 1984. [Ref7] Deering, S., "Host extensions for IP multicasting", STD 5, RFC 1112, May 1988. [Ref8] McCloghrie, K., and M. Rose, "Management Information Base for network management of TCP/IP-based internets: MIB-II", STD 17, RFC 1213, March 1991. [Ref9] Moy, J., "OSPF Version 2", RFC 1583, March 1994. [Ref10] Fuller, V., T. Li, J. Yu, and K. Varadhan, "Classless Inter-Domain Routing (CIDR): an Address Assignment and Aggregation Strategy", RFC1519, September 1993. [Ref11] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC 1700, October 1994. [Ref12] Almquist, P., "Type of Service in the Internet Protocol Suite", RFC 1349, July 1992.
[Ref13] Leiner, B., et.al., "The DARPA Internet Protocol Suite", DDN
Protocol Handbook, April 1985.
[Ref14] Bradley, T., and C. Brown, "Inverse Address Resolution
Protocol", RFC 1293, January 1992.
[Ref15] deSouza, O., and M. Rodrigues, "Guidelines for Running OSPF
Over Frame Relay Networks", RFC 1586, March 1994.
[Ref16] Bellovin, S., "Security Problems in the TCP/IP Protocol
Suite", ACM Computer Communications Review, Volume 19,
Number 2, pp. 32-38, April 1989.
[Ref17] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.
[Ref18] Moy, J., "Multicast Extensions to OSPF", RFC 1584, March
1994.
[Ref19] Coltun, R., and V. Fuller, "The OSPF NSSA Option", RFC 1587,
March 1994.
[Ref20] Ferguson, D., "The OSPF External Attributes LSA", work in
progress.
[Ref21] Moy, J., "Extending OSPF to Support Demand Circuits", RFC
1793, April 1995.
[Ref22] Mogul, J., and S. Deering, "Path MTU Discovery", RFC 1191,
November 1990.
[Ref23] Rekhter, Y., and T. Li, "A Border Gateway Protocol 4 (BGP-
4)", RFC 1771, March 1995.
[Ref24] Hinden, R., "Internet Routing Protocol Standardization
Criteria", BBN, October 1991.
[Ref25] Moy, J., "OSPF Version 2", RFC 2178, July 1997.
[Ref26] Rosen, E., "Vulnerabilities of Network Control Protocols: An
Example", Computer Communication Review, July 1981.
A. OSPF data formats This appendix describes the format of OSPF protocol packets and OSPF LSAs. The OSPF protocol runs directly over the IP network layer. Before any data formats are described, the details of the OSPF encapsulation are explained. Next the OSPF Options field is described. This field describes various capabilities that may or may not be supported by pieces of the OSPF routing domain. The OSPF Options field is contained in OSPF Hello packets, Database Description packets and in OSPF LSAs. OSPF packet formats are detailed in Section A.3. A description of OSPF LSAs appears in Section A.4. A.1 Encapsulation of OSPF packets OSPF runs directly over the Internet Protocol's network layer. OSPF packets are therefore encapsulated solely by IP and local data-link headers. OSPF does not define a way to fragment its protocol packets, and depends on IP fragmentation when transmitting packets larger than the network MTU. If necessary, the length of OSPF packets can be up to 65,535 bytes (including the IP header). The OSPF packet types that are likely to be large (Database Description Packets, Link State Request, Link State Update, and Link State Acknowledgment packets) can usually be split into several separate protocol packets, without loss of functionality. This is recommended; IP fragmentation should be avoided whenever possible. Using this reasoning, an attempt should be made to limit the sizes of OSPF packets sent over virtual links to 576 bytes unless Path MTU Discovery is being performed (see [Ref22]). The other important features of OSPF's IP encapsulation are: o Use of IP multicast. Some OSPF messages are multicast, when sent over broadcast networks. Two distinct IP multicast addresses are used. Packets sent to these multicast addresses should never be forwarded; they are meant to travel a single hop only. To ensure that these packets will not travel multiple hops, their IP TTL must be set to 1.
AllSPFRouters
This multicast address has been assigned the value
224.0.0.5. All routers running OSPF should be prepared to
receive packets sent to this address. Hello packets are
always sent to this destination. Also, certain OSPF
protocol packets are sent to this address during the
flooding procedure.
AllDRouters
This multicast address has been assigned the value
224.0.0.6. Both the Designated Router and Backup Designated
Router must be prepared to receive packets destined to this
address. Certain OSPF protocol packets are sent to this
address during the flooding procedure.
o OSPF is IP protocol number 89. This number has been registered
with the Network Information Center. IP protocol number
assignments are documented in [Ref11].
o All OSPF routing protocol packets are sent using the normal
service TOS value of binary 0000 defined in [Ref12].
o Routing protocol packets are sent with IP precedence set to
Internetwork Control. OSPF protocol packets should be given
precedence over regular IP data traffic, in both sending and
receiving. Setting the IP precedence field in the IP header to
Internetwork Control [Ref5] may help implement this objective.
A.2 The Options field The OSPF Options field is present in OSPF Hello packets, Database Description packets and all LSAs. The Options field enables OSPF routers to support (or not support) optional capabilities, and to communicate their capability level to other OSPF routers. Through this mechanism routers of differing capabilities can be mixed within an OSPF routing domain. When used in Hello packets, the Options field allows a router to reject a neighbor because of a capability mismatch. Alternatively, when capabilities are exchanged in Database Description packets a router can choose not to forward certain LSAs to a neighbor because of its reduced functionality. Lastly, listing capabilities in LSAs allows routers to forward traffic around reduced functionality routers, by excluding them from parts of the routing table calculation. Five bits of the OSPF Options field have been assigned, although only one (the E-bit) is described completely by this memo. Each bit is described briefly below. Routers should reset (i.e. clear) unrecognized bits in the Options field when sending Hello packets or Database Description packets and when originating LSAs. Conversely, routers encountering unrecognized Option bits in received Hello Packets, Database Description packets or LSAs should ignore the capability and process the packet/LSA normally. +------------------------------------+ | * | * | DC | EA | N/P | MC | E | * | +------------------------------------+ The Options field E-bit This bit describes the way AS-external-LSAs are flooded, as described in Sections 3.6, 9.5, 10.8 and 12.1.2 of this memo. MC-bit This bit describes whether IP multicast datagrams are forwarded according to the specifications in [Ref18].
A.3 OSPF Packet Formats There are five distinct OSPF packet types. All OSPF packet types begin with a standard 24 byte header. This header is described first. Each packet type is then described in a succeeding section. In these sections each packet's division into fields is displayed, and then the field definitions are enumerated. All OSPF packet types (other than the OSPF Hello packets) deal with lists of LSAs. For example, Link State Update packets implement the flooding of LSAs throughout the OSPF routing domain. Because of this, OSPF protocol packets cannot be parsed unless the format of LSAs is also understood. The format of LSAs is described in Section A.4. The receive processing of OSPF packets is detailed in Section 8.2. The sending of OSPF packets is explained in Section 8.1.
A.3.1 The OSPF packet header Every OSPF packet starts with a standard 24 byte header. This header contains all the information necessary to determine whether the packet should be accepted for further processing. This determination is described in Section 8.2 of the specification. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Version # | Type | Packet length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Router ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Area ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Checksum | AuType | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Authentication | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Authentication | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Version # The OSPF version number. This specification documents version 2 of the protocol. Type The OSPF packet types are as follows. See Sections A.3.2 through A.3.6 for details.
Type Description
________________________________
1 Hello
2 Database Description
3 Link State Request
4 Link State Update
5 Link State Acknowledgment
Packet length
The length of the OSPF protocol packet in bytes. This length
includes the standard OSPF header.
Router ID
The Router ID of the packet's source.
Area ID
A 32 bit number identifying the area that this packet belongs
to. All OSPF packets are associated with a single area. Most
travel a single hop only. Packets travelling over a virtual
link are labelled with the backbone Area ID of 0.0.0.0.
Checksum
The standard IP checksum of the entire contents of the packet,
starting with the OSPF packet header but excluding the 64-bit
authentication field. This checksum is calculated as the 16-bit
one's complement of the one's complement sum of all the 16-bit
words in the packet, excepting the authentication field. If the
packet's length is not an integral number of 16-bit words, the
packet is padded with a byte of zero before checksumming. The
checksum is considered to be part of the packet authentication
procedure; for some authentication types the checksum
calculation is omitted.
AuType
Identifies the authentication procedure to be used for the
packet. Authentication is discussed in Appendix D of the
specification. Consult Appendix D for a list of the currently
defined authentication types.
Authentication
A 64-bit field for use by the authentication scheme. See
Appendix D for details.
A.3.2 The Hello packet Hello packets are OSPF packet type 1. These packets are sent periodically on all interfaces (including virtual links) in order to establish and maintain neighbor relationships. In addition, Hello Packets are multicast on those physical networks having a multicast or broadcast capability, enabling dynamic discovery of neighboring routers. All routers connected to a common network must agree on certain parameters (Network mask, HelloInterval and RouterDeadInterval). These parameters are included in Hello packets, so that differences can inhibit the forming of neighbor relationships. A detailed explanation of the receive processing for Hello packets is presented in Section 10.5. The sending of Hello packets is covered in Section 9.5. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Version # | 1 | Packet length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Router ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Area ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Checksum | AuType | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Authentication | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Authentication | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Network Mask | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | HelloInterval | Options | Rtr Pri | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | RouterDeadInterval | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Designated Router | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Backup Designated Router |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Neighbor |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
Network mask
The network mask associated with this interface. For example,
if the interface is to a class B network whose third byte is
used for subnetting, the network mask is 0xffffff00.
Options
The optional capabilities supported by the router, as documented
in Section A.2.
HelloInterval
The number of seconds between this router's Hello packets.
Rtr Pri
This router's Router Priority. Used in (Backup) Designated
Router election. If set to 0, the router will be ineligible to
become (Backup) Designated Router.
RouterDeadInterval
The number of seconds before declaring a silent router down.
Designated Router
The identity of the Designated Router for this network, in the
view of the sending router. The Designated Router is identified
here by its IP interface address on the network. Set to 0.0.0.0
if there is no Designated Router.
Backup Designated Router
The identity of the Backup Designated Router for this network,
in the view of the sending router. The Backup Designated Router
is identified here by its IP interface address on the network.
Set to 0.0.0.0 if there is no Backup Designated Router.
Neighbor
The Router IDs of each router from whom valid Hello packets have
been seen recently on the network. Recently means in the last
RouterDeadInterval seconds.
A.3.3 The Database Description packet Database Description packets are OSPF packet type 2. These packets are exchanged when an adjacency is being initialized. They describe the contents of the link-state database. Multiple packets may be used to describe the database. For this purpose a poll-response procedure is used. One of the routers is designated to be the master, the other the slave. The master sends Database Description packets (polls) which are acknowledged by Database Description packets sent by the slave (responses). The responses are linked to the polls via the packets' DD sequence numbers. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Version # | 2 | Packet length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Router ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Area ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Checksum | AuType | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Authentication | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Authentication | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Interface MTU | Options |0|0|0|0|0|I|M|MS +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | DD sequence number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | +- -+ | | +- An LSA Header -+ | | +- -+ | | +- -+ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... |
The format of the Database Description packet is very similar to
both the Link State Request and Link State Acknowledgment packets.
The main part of all three is a list of items, each item describing
a piece of the link-state database. The sending of Database
Description Packets is documented in Section 10.8. The reception of
Database Description packets is documented in Section 10.6.
Interface MTU
The size in bytes of the largest IP datagram that can be sent
out the associated interface, without fragmentation. The MTUs
of common Internet link types can be found in Table 7-1 of
[Ref22]. Interface MTU should be set to 0 in Database
Description packets sent over virtual links.
Options
The optional capabilities supported by the router, as documented
in Section A.2.
I-bit
The Init bit. When set to 1, this packet is the first in the
sequence of Database Description Packets.
M-bit
The More bit. When set to 1, it indicates that more Database
Description Packets are to follow.
MS-bit
The Master/Slave bit. When set to 1, it indicates that the
router is the master during the Database Exchange process.
Otherwise, the router is the slave.
DD sequence number
Used to sequence the collection of Database Description Packets.
The initial value (indicated by the Init bit being set) should
be unique. The DD sequence number then increments until the
complete database description has been sent.
The rest of the packet consists of a (possibly partial) list of the
link-state database's pieces. Each LSA in the database is described
by its LSA header. The LSA header is documented in Section A.4.1.
It contains all the information required to uniquely identify both
the LSA and the LSA's current instance.
A.3.4 The Link State Request packet Link State Request packets are OSPF packet type 3. After exchanging Database Description packets with a neighboring router, a router may find that parts of its link-state database are out-of-date. The Link State Request packet is used to request the pieces of the neighbor's database that are more up-to-date. Multiple Link State Request packets may need to be used. A router that sends a Link State Request packet has in mind the precise instance of the database pieces it is requesting. Each instance is defined by its LS sequence number, LS checksum, and LS age, although these fields are not specified in the Link State Request Packet itself. The router may receive even more recent instances in response. The sending of Link State Request packets is documented in Section 10.9. The reception of Link State Request packets is documented in Section 10.7. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Version # | 3 | Packet length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Router ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Area ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Checksum | AuType | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Authentication | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Authentication | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | LS type | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Link State ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Advertising Router | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... |
Each LSA requested is specified by its LS type, Link State ID, and
Advertising Router. This uniquely identifies the LSA, but not its
instance. Link State Request packets are understood to be requests
for the most recent instance (whatever that might be).
A.3.5 The Link State Update packet Link State Update packets are OSPF packet type 4. These packets implement the flooding of LSAs. Each Link State Update packet carries a collection of LSAs one hop further from their origin. Several LSAs may be included in a single packet. Link State Update packets are multicast on those physical networks that support multicast/broadcast. In order to make the flooding procedure reliable, flooded LSAs are acknowledged in Link State Acknowledgment packets. If retransmission of certain LSAs is necessary, the retransmitted LSAs are always sent directly to the neighbor. For more information on the reliable flooding of LSAs, consult Section 13. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Version # | 4 | Packet length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Router ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Area ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Checksum | AuType | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Authentication | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Authentication | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | # LSAs | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | +- +-+ | LSAs | +- +-+ | ... |
# LSAs
The number of LSAs included in this update.
The body of the Link State Update packet consists of a list of LSAs.
Each LSA begins with a common 20 byte header, described in Section
A.4.1. Detailed formats of the different types of LSAs are described
in Section A.4.
A.3.6 The Link State Acknowledgment packet Link State Acknowledgment Packets are OSPF packet type 5. To make the flooding of LSAs reliable, flooded LSAs are explicitly acknowledged. This acknowledgment is accomplished through the sending and receiving of Link State Acknowledgment packets. Multiple LSAs can be acknowledged in a single Link State Acknowledgment packet. Depending on the state of the sending interface and the sender of the corresponding Link State Update packet, a Link State Acknowledgment packet is sent either to the multicast address AllSPFRouters, to the multicast address AllDRouters, or as a unicast. The sending of Link State Acknowledgement packets is documented in Section 13.5. The reception of Link State Acknowledgement packets is documented in Section 13.7. The format of this packet is similar to that of the Data Description packet. The body of both packets is simply a list of LSA headers. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Version # | 5 | Packet length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Router ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Area ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Checksum | AuType | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Authentication | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Authentication | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | +- -+ | | +- An LSA Header -+ | | +- -+
| |
+- -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
Each acknowledged LSA is described by its LSA header. The LSA
header is documented in Section A.4.1. It contains all the
information required to uniquely identify both the LSA and the LSA's
current instance.
(next page on part 8)