RFC 1247
OSPF Version 2
12. Link State Advertisements Each router in the Autonomous System originates one or more link state advertisements. There are five distinct types of link state advertisements, which are described in Section 4.3. The collection of link state advertisements forms the link state or topological database. Each separate type of advertisement has a separate function. Router links and network links advertisements describe how an area's routers and networks are interconnected. Summary link advertisements provide a way of condensing an area's routing information. AS external advertisements provide a way of transparently advertising externally- derived routing information throughout the Autonomous System. Each link state advertisement begins with a standard 20-byte header. This link state header is discussed below.
Type Dest Area Path Type Cost Next Hop(s) Adv. Router(s)
_______________________________________________________________________________
N N1 1 intra-area 4 RT1 *
N N2 1 intra-area 4 RT2 *
N N3 1 intra-area 1 * *
N N4 1 intra-area 3 RT3 *
BR RT3 1 intra-area 1 * *
_______________________________________________________________________________
N Ib 0 intra-area 22 RT5 *
N Ia 0 intra-area 27 RT5 *
BR RT3 0 intra-area 21 RT5 *
BR RT7 0 intra-area 14 RT5 *
BR RT10 0 intra-area 22 RT5 *
BR RT11 0 intra-area 25 RT5 *
ASBR RT5 0 intra-area 8 * *
ASBR RT7 0 intra-area 14 RT5 *
_______________________________________________________________________________
N N6 0 inter-area 15 RT5 RT7
N N7 0 inter-area 19 RT5 RT7
N N8 0 inter-area 18 RT5 RT7
N N9-N11,H1 0 inter-area 26 RT5 RT11
_______________________________________________________________________________
N N12 * type 1 external 16 RT5 RT5,RT7
N N13 * type 1 external 16 RT5 RT5
N N14 * type 1 external 16 RT5 RT5
N N15 * type 1 external 23 RT5 RT7
Table 13: Router RT4's routing table in the presence of areas.
Type Dest Area Path Type Cost Next Hop(s) Adv. Router(s)
__________________________________________________________________________
N Ib 0 intra-area 16 RT3 *
N Ia 0 intra-area 21 RT3 *
BR RT3 0 intra-area 1 * *
BR RT10 0 intra-area 16 RT3 *
BR RT11 0 intra-area 19 RT3 *
__________________________________________________________________________
N N9-N11,H1 0 inter-area 20 RT3 RT11
Table 14: Changes resulting from an additional virtual link.
12.1 The Link State Header
The link state header contains the LS type, Link State ID and Advertising Router fields. The combination of these three fields uniquely identifies the link state advertisement. There may be several instances of an advertisement present in the Autonomous System, all at the same time. It must then be determined which instance is more recent. This determination is made be examining the LS sequence, LS checksum and LS age fields. These fields are also contained in the 20-byte link state header. Several of the OSPF packet types list link state advertisements. When the instance is not important, an advertisement is referred to by its LS type, Link State ID and Advertising Router (see Link State Request Packets). Otherwise, the LS sequence number, LS age and LS checksum fields must also be referenced. A detailed explanation of the fields contained in the link state header follows. 12.1.1 LS age This field is the age of the link state advertisement in seconds. It should be processed as an unsigned 16-bit integer. It is set to 0 when the link state advertisement is originated. It must be incremented by InfTransDelay on every hop of the flooding procedure. Link state advertisements are also aged as they are held in each router's database. The age of a link state advertisement is never incremented past MaxAge. Advertisements having age MaxAge are not used in the routing table calculation. When an advertisement's age first reaches MaxAge, it is reflooded. A link state advertisement of age MaxAge is finally flushed from the database when it is no longer contained on any neighbor Link state retransmission lists. This indicates that it has been acknowledged by all adjacent neighbors. For more information on the aging of link state advertisements, consult Section 14. Ages are examined when a router receives two instances of a link state advertisement, both having identical sequence numbers and checksums. An instance of age MaxAge is then always accepted as most recent; this allows old advertisements to be flushed quickly from the routing domain. Otherwise, if the ages differ by more than MaxAgeDiff, the instance having the smaller age is accepted as most recent.[11] See Section 13.1 for more details.
12.1.2 Options The options field in the link state header indicates which optional capabilities are associated with the advertisement. OSPF's optional capabilities are described in Section 4.5. There are currently two optional capabilities defined; they are represented by the T-bit and E- bit found in the options field. The rest of the options field should be set to zero. The E-bit represents OSPF's external routing capability. This bit should be set in all advertisements associated with the backbone, and all advertisements associated with non-stub areas (see Section 3.6). It should also be set in all AS external advertisements. It should be reset in all router links, network links and summary link advertisements associated with a stub area. For all link state advertisements, the setting of the E-bit is for informational purposes only; it does not affect the routing table calculation. The T-bit represents OSPF's TOS routing capability. This bit should be set in a router links advertisement if and only if the router is capable of calculating separate routes for each IP TOS (see Section 2.4). The T-bit should always be set in network links advertisements. It should be set in summary link and AS external link advertisements if and only if the advertisement describes paths for all TOS values, instead of just the TOS 0 path. Note that, with the T-bit set, there may still be only a single metric in the advertisement (the TOS 0 metric). This would mean that paths for non-zero TOS exist, but are equivalent to the TOS 0 path. A link state advertisement's T-bit is examined when calculating the routing table's non-zero TOS paths (see Section 16.9). 12.1.3 LS type The LS type field dictates the format and function of the link state advertisement. Advertisements of different types have different names (e.g., router links or network links). All advertisement types, except the AS external link advertisements (LS type = 5), are flooded throughout a single area only. AS external link advertisements are flooded throughout the entire Autonomous System, excluding stub areas (see Section 3.6). Each separate advertisement type is briefly described below in Table 15. LS Type Advertisement description __________________________________________________ 1 These are the router links advertisements. They describe the
LS Type Advertisement description
__________________________________________________
collected states of the router's
interfaces. For more information,
consult Section 12.4.1.
__________________________________________________
2 These are the network links
advertisements. They describe the set
of routers attached to the network. For
more information, consult
Section 12.4.2.
__________________________________________________
3 or 4 These are the summary link
advertisements. They describe
inter-area routes, and enable the
condensation of routing information at
area borders. Originated by area border
routers, the Type 3 advertisements
describe routes to networks while the
Type 4 advertisements describe routes to
AS boundary routers.
__________________________________________________
5 These are the AS external link
advertisements. Originated by AS
boundary routers, they describe routes
to destinations external to the
Autonomous System. A default route for
the Autonomous System can also be
described by an AS external link
advertisement.
Table 15: OSPF link state advertisements.
12.1.4 Link State ID
This field identifies the piece of the routing domain that is being
described by the advertisement. Depending on the advertisement's LS
type, the Link State ID takes on the values listed in Table 16.
LS Type Link State ID
______________________________________________________________________
1 The originating router's Router ID.
2 The IP interface address of the network's Designated Router.
3 The destination network's IP address.
4 The Router ID of the described AS boundary router.
5 The destination network's IP address.
Table 16: The advertisement's Link State ID.
When the link state advertisement is describing a network, the Link
State ID is either the network's IP address (as in type 3 summary link
advertisements and in AS external link advertisements) or the network's
IP address is easily derivable from the Link State ID (note that masking
a network links advertisement's Link State ID with the network's subnet
mask yields the network's IP address). When the link state
advertisement is describing a router, the Link State ID is always the
described router's OSPF Router ID.
When an AS external advertisement (LS Type = 5) is describing a default
route, its Link State ID is set to DefaultDestination (0.0.0.0).
12.1.5 Advertising Router
This field specifies the OSPF Router ID of the advertisement's
originator. For router links advertisements, this field is identical to
the Link State ID field. Network link advertisements are originated by
the network's Designated Router. Summary link advertisements are
originated by area border routers. Finally, AS external link
advertisements are originated by AS boundary routers.
12.1.6 LS sequence number
The sequence number field is a signed 32-bit integer. It is used to
detect old and duplicate link state advertisements. The space of
sequence numbers is linearly ordered. The larger the sequence number
(when compared as signed 32-bit integers) the more recent the
advertisement. To describe to sequence number space more precisely, let
N refer in the discussion below to the constant 2**31.
The sequence number -N (0x80000000) is reserved (and unused). This
leaves -N + 1 (0x80000001) as the smallest (and therefore oldest)
sequence number. A router uses this sequence number the first time it
originates any link state advertisement. Afterwards, the advertisement's sequence number is incremented each time the router originates a new instance of the advertisement. When an attempt is made to increment the sequence number past the maximum value of of N - 1 (0x7fffffff), the current instance of the advertisement must first be flushed from the routing domain. This is done by prematurely aging the advertisement (see Section 14.1) and reflooding it. As soon as this flood has been acknowledged by all adjacent neighbors, a new instance can be originated with sequence number of -N + 1 (0x80000001). The router may be forced to promote the sequence number of one of its advertisements when a more recent instance of the advertisement is unexpectedly received during the flooding process. This should be a rare event. This may indicate that an out-of-date advertisement, originated by the router itself before its last restart/reload, still exists in the Autonomous System. For more information see Section 13.4. ,uh "12.1.7 LS checksum" This field is the checksum of the complete contents of the advertisement, excepting the age field. The age field is excepted so that an advertisement's age can be incremented without updating the checksum. The checksum used is the same that is used for ISO connectionless datagrams; it is commonly referred to as the Fletcher checksum. It is documented in Annex C of [RFC 994]. The link state header also contains the length of the advertisement in bytes; subtracting the size of the age field (two bytes) yields the amount of data to checksum. The checksum is used to detect data corruption of an advertisement. This corruption can occur while an advertisement is being flooded, or while it is being held in a router's memory. The LS checksum field cannot take on the value of zero; the occurrence of such a value should be considered a checksum failure. In other words, calculation of the checksum is not optional. The checksum of a link state advertisement is verified in two cases: a) when it is received in a Link State Update Packet and b) at times during the aging of the link state database. The detection of a checksum failure leads to separate actions in each case. See Sections 13 and 14 for more details. Whenever the LS sequence number field indicates that two instances of an advertisement are the same, the LS checksum field is examined. If there is a difference, the instance with the larger checksum is considered to be most recent.[12] See Section 13.1 for more details.
12.2 The link state database A router has a separate link state database for every area to which it belongs. The link state database has been referred to elsewhere in the text as the topological database. All routers belonging to the same area have identical topological databases for the area. The databases for each individual area are always dealt with separately. The shortest path calculation is performed separately for each area (see Section 16). Components of the area topological database are flooded throughout the area only. Finally, when an adjacency (belonging to Area A) is being brought up, only the database for Area A is synchronized between the two routers. The area database is composed of router links advertisements, network links advertisements, and summary link advertisements (all listed in the area data structure). In addition, external routes (AS external advertisements) are included in all non-stub area databases (see Section 3.6). An implementation of OSPF must be able to access individual pieces of an area database. This lookup function is based on an advertisement's LS type, Link State ID and Advertising Router.[13] There will be a single instance (the most up-to-date) of each link state advertisement in the database. The database lookup function is invoked during the link state flooding procedure (Section 13) and the routing table calculation (Section 16). In addition, using this lookup function the router can determine whether it has itself ever originated a particular link state advertisement, and if so, with what LS sequence number. A link state advertisement is added to a router's database when either a) it is received during the flooding process (Section 13) or b) it is originated by the router itself (Section 12.4). A link state advertisement is deleted from a router's database when either a) it has been overwritten by a newer instance during the flooding process (Section 13) or b) the router originates a newer instance of one of its self-originated advertisements (Section 12.4) or c) the advertisement ages out and is flushed from the routing domain (Section 14). Whenever a link state advertisement is deleted from the database it must also be removed from all neighbors' Link state retransmission lists (see Section 10). 12.3 Representation of TOS All OSPF link state advertisements (with the exception of network links advertisements) specify metrics. In router links advertisements, the metrics indicate the costs of the described interfaces. In summary link
and AS external link advertisements, the metric indicates the cost of the described path. In all of these advertisements, a separate metric can be specified for each IP TOS. TOS is encoded in an OSPF link state advertisement as the following mapping of the Delay (D), Throughput (T) and Reliability (R) flags found in the IP packet header's TOS field (see [RFC 791]). OSPF encoding D T R _________________________ 0 0 0 0 4 0 0 1 8 0 1 0 12 0 1 1 16 1 0 0 20 1 0 1 24 1 1 0 28 1 1 1 Table 17: Representing TOS in OSPF. Each OSPF link state advertisement must specify the TOS 0 metric. Other TOS metrics, if they appear, must appear in order of increasing TOS encoding. For example, the TOS 8 (high throughput) metric must always appear before the TOS 16 (low delay) metric when both are specified. If a metric for some non-zero TOS is not specified, its cost defaults to the cost for TOS 0, unless the T-bit is reset in the advertisement's options field (see Section 12.1.2 for more details). Note that if more TOS types are defined in a future IP architecture, OSPF's TOS encoding can be extended in a straightforward manner. 12.4 Originating link state advertisements A router may originate many types of link state advertisements. A router originates a router links advertisement for each area to which it belongs. If the router is also the Designated Router for any of its attached networks, it will originate a network links advertisement for that network. Area border routers originate a single summary links advertisement for each known inter-area destination. AS boundary routers originate a single AS external links advertisement for each known AS external destination. Destinations are advertised one at a time so that the
change in any single route can be flooded without reflooding the entire
collection of routes. During the flooding procedure, many link state
advertisements can be carried by a single Link State Update packet.
As an example, consider router RT4 in Figure 6. It is an area border
router, having a connection to Area 1 and the backbone. Router RT4
originates 5 distinct link state advertisements into the backbone (one
router links, and one summary link for each of the networks N1-N4).
Router RT4 will also originate 8 distinct link state advertisements into
Area 1 (one router links and seven summary link advertisements as
pictured in Figure 7). If RT4 has been selected as Designated Router
for network N3, it will also originate a network links advertisement for
N3 into Area 1.
In this same figure, router RT5 will be originating 3 distinct AS
external link advertisements (one for each of the networks N12-N14).
These will be flooded throughout the entire AS, assuming that none of
the areas have been configured as stubs. However, if area 3 has been
configured as a stub area, the external advertisements for networks
N12-N14 will not be flooded into area 3 (see Section 3.6). Instead,
router RT11 would originate a default summary link advertisement that
would be flooded throughout area 3 (see Section 12.4.3). This instructs
all of area 3's internal routers to send their AS external traffic to
RT11.
Whenever a new instance of a link state advertisement is originated, its
LS sequence number is incremented, its LS age is set to 0, its LS
checksum is calculated, and the advertisement is added to the link state
database and flooded out the appropriate interfaces. See Section 13.2
for details concerning the installation of the advertisement into the
link state database. See Section 13.3 for details concerning the
flooding of newly originated advertisements.
The eight events that cause a new instance of a link state advertisement
to be originated are:
(1) The LS refresh timer firing. There is a LS refresh timer for each
link state advertisement that the router has originated. The LS
refresh timer is an interval timer, with length LSRefreshTimer. The
LS refresh timer guarantees periodic originations regardless of any
other events that cause new instances. This periodic updating of
link state advertisements adds robustness to the link state
algorithm. Link state advertisements that solely describe
unreachable destinations should not be refreshed, but should instead
be flushed from the routing domain (see Section 14.1).
When whatever is being described by a link state advertisement changes,
a new advertisement is originated. Two instances of the same link state
advertisement may not be originated within the time period
MinLSInterval. This may require that the generation of the next
instance to be delayed by up to MinLSInterval. The following changes
may cause a router to originate a new instance of an advertisement.
These changes should cause new originations only if the contents of the
new advertisement would be different.
(2) An interface's state changes (see Section 9.1). This may mean that
it is necessary to produce a new instance of the router links
advertisement.
(3) An attached network's Designated Router changes. A new router links
advertisement should be originated. Also, if the router itself is
now the Designated Router, a new network links advertisement should
be produced.
(4) One of the neighboring routers changes to/from the FULL state. This
may mean that it is necessary to produce a new instance of the
router links advertisement. Also, if the router is itself the
Designated Router for the attached network, a new network links
advertisement should be produced.
The next three events concern area border routers only.
(5) An intra-area route has been added/deleted/modified in the routing
table. This may cause a new instance of a summary links
advertisement (for this route) to be originated in each attached
area (this includes the backbone).
(6) An inter-area route has been added/deleted/modified in the routing
table. This may cause a new instance of a summary links
advertisement (for this route) to be originated in each attached
area (but NEVER for the backbone).
(7) The router becomes newly attached to an area. The router must then
originate summary link advertisements into the newly attached area
for all pertinent intra-area and inter-area routes in its routing
table. See Section 12.4.3 for more details.
The last event concerns AS boundary routers only.
(8) An external route gained through direct experience with an external
routing protocol (like EGP) changes. This will cause the AS
boundary router to originate a new instance of an external links
advertisement.
The construction of each type of the link state advertisement is
explained below. In general, these sections describe the contents of
the advertisement body (i.e., the part coming after the 20-byte
advertisement header). For information concerning the building of the
link state advertisement header, see Section 12.1.
12.4.1 Router links
A router originates a router links advertisement for each area that it
belongs to. Such an advertisement describes the collected states of the
router's links to the area. The advertisement is flooded throughout the
particular area, and no further.
The format of a router links advertisement is shown in Appendix A
(Section A.4.2). The first 20 bytes of the advertisement consist of the
generic link state header that was discussed in Section 12.1. Router
links advertisements have LS type = 1. The router indicates whether it
is willing to calculate separate routes for each IP TOS by setting (or
resetting) the T-bit of the link state advertisement's Options field.
A router also indicates whether it is an area border router, or an AS
boundary router, by setting the appropriate bits in its router links
advertisements. This enables paths to those types of routers to be
saved in the routing table, for later processing of summary link
advertisements and AS external link advertisements.
The router links advertisement then describes the router's working
connections (links) to the area. Each link is typed according to the
_________________________________________
(Figure not included in text version.)
Figure 15: Area 1 with IP addresses shown
Figure 16: Forwarding address example
_________________________________________
kind of attached network. Each link is also labelled with its Link ID.
This ID gives a name to the entity that is on the other end of the link.
Table 18 summarizes the values used for the type and Link ID fields.
Link type Description Link ID
____________________________________________________________________________
1 Point-to-point link Neighbor Router ID
2 Link to transit network Interface address of Designated Router
3 Link to stub network IP network number
4 Virtual link Neighbor Router ID
Table 18: Link descriptions in the router links advertisement.
In addition, the Link Data field is specified for each link. This field
gives 32 bits of extra information for the link. For links to routers
and transit networks, this field specifies the IP interface address of
the associated router interface (this is needed by the routing table
calculation, see Section 16.3). For links to stub networks, this field
specifies the network's IP address mask.
Finally, the cost of using the link for output (possibly specifying a
different cost for each type of service) is specified. The output cost
of a link is configurable. It must always be non-zero.
To further describe the process of building the list of link records,
suppose a router wishes to build router links advertisement for an Area
A. The router examines its collection of interface data structures.
For each interface, the following steps are taken:
o If the attached network does not belong to Area A, no links are
added to the advertisement, and the next interface should be
examined.
o Else, if the state of the interface is Down, no links are added.
o Else, if the state of the interface is Point-to-Point, then add
links according to the following:
- If the neighboring router is fully adjacent, add a Type 1 link
(point-to-point) if this is an interface to a point-to-point
network, or add a type 4 link (virtual link) if this is a
virtual link. The Link ID should be set to the Router ID of the
neighboring router, and the Link Data should specify the
interface IP address.
- If this is a numbered point-to-point network (i.e, not a virtual
link and not an unnumbered point-to-point network) and the
neighboring router's IP address is known, add a Type 3 link
(stub network) whose Link ID is the neighbor's IP address, whose
Link Data is the mask 0xffffffff indicating a host route, and
whose cost is the interface's configured output cost.
o Else if the state of the interface is Loopback, add a Type 3 link
(stub network) as long as this is not an interface to an unnumbered
serial line. The Link ID should be set to the IP interface address,
the Link Data set to the mask 0xffffffff (indicating a host route),
and the cost set to 0.
o Else if the state of the interface is Waiting, add a Type 3 link
(stub network) whose Link ID is the IP network number of the
attached network and whose Link Data is the attached network's
address mask.
o Else, there has been a Designated Router selected for the attached
network. If the router is fully adjacent to the Designated Router,
or if the router itself is Designated Router and is fully adjacent
to at least one other router, add a single Type 2 link (transit
network) whose whose link ID is the IP interface address of the
attached network's Designated Router (which may be the router
itself) and whose Link Data is the interface IP address. Otherwise,
add a link as if the interface state were Waiting (see above).
Unless otherwise specified, the cost of each link generated by the above
procedure is equal to the output cost of the associated interface. Note
that in the case of serial lines, multiple links may be generated by a
single interface.
After consideration of all the router interfaces, host links are added
to the advertisement by examining the list of attached hosts. A host
route is represented as a Type 3 link (stub network) whose link ID is
the host's IP address and whose Link Data is the mask of all ones
(0xffffffff).
As an example, consider the router links advertisements generated by
router RT3, as pictured in Figure 6. The area containing router RT3
(Area 1) has been redrawn, with actual network addresses, in Figure 15.
Assume that the last byte of all of RT3's interface addresses is 3,
giving it the interface addresses 192.1.1.3 and 192.1.4.3, and that the
other routers have similar addressing schemes. In addition, assume that
all links are functional, and that Router IDs are assigned as the
smallest IP interface address.
RT3 originates two router links advertisements, one for Area 1 and one
for the backbone. Assume that router RT4 has been selected as the
Designated router for network 192.1.1.0. RT3's router links
advertisement for Area 1 is then shown below. It indicates that RT3 has
two connections to Area 1, the first a link to the transit network
192.1.1.0 and the second a link to the stub network 192.1.4.0. Note
that the transit network is identified by the IP interface of its
Designated Router (i.e., the Link ID = 192.1.1.4 which is the Designated
Router RT4's IP interface to 192.1.1.0). Note also that RT3 has
indicated that it is capable of calculating separate routes based on IP
TOS, through setting the T-bit in the Options field. It has also
indicated that it is an area border router.
; RT3's router links advertisement for Area 1
LS age = 0 ;always true on origination
Options = (T-bit|E-bit) ;TOS-capable
LS type = 1 ;indicates router links
Link State ID = 192.1.1.3 ;RT3's Router ID
Advertising Router = 192.1.1.3 ;RT3's Router ID
bit E = 0 ;not an AS boundary router
bit B = 1 ;RT3 is an area border router
#links = 2
Link ID = 192.1.1.4 ;IP address of Designated Router
Link Data = 192.1.1.3 ;RT3's IP interface to net
Type = 2 ;connects to transit network
# other metrics = 0
TOS 0 metric = 1
Link ID = 192.1.4.0 ;IP Network number
Link Data = 0xffffff00 ;Network mask
Type = 3 ;connects to stub network
# other metrics = 0
TOS 0 metric = 2
Next RT3's router links advertisement for the backbone is shown. It
indicates that RT3 has a single attachment to the backbone. This
attachment is via an unnumbered point-to-point link to router RT6. RT3
has again indicated that it is TOS-capable, and that it is an area
border router.
; RT3's router links advertisement for the backbone
LS age = 0 ;always true on origination
Options = (T-bit|E-bit) ;TOS-capable
LS type = 1 ;indicates router links
Link State ID = 192.1.1.3 ;RT3's router ID
Advertising Router = 192.1.1.3 ;RT3's router ID
bit E = 0 ;not an AS boundary router
bit B = 1 ;RT3 is an area border router
#links = 1
Link ID = 18.10.0.6 ;Neighbor's Router ID
Link Data = 0.0.0.0 ;Interface to unnumbered SL
Type = 1 ;connects to router
# other metrics = 0
TOS 0 metric = 8
Even though router RT3 has indicated that it is TOS-capable in the above
examples, only a single metric (the TOS 0 metric) has been specified for
each interface. Different metrics can be specified for each TOS. The
encoding of TOS in OSPF link state advertisements is described in
Section 12.3.
As an example, suppose the point-to-point link between routers RT3 and
RT6 in Figure 15 is a satellite link. The AS administrator may want to
encourage the use of the line for high bandwidth traffic. This would be
done by setting the metric artificially low for that TOS. Router RT3
would then originate the following router links advertisement for the
backbone (IP TOS 8 = high bandwidth):
; RT3's router links advertisement for the backbone
LS age = 0 ;always true on origination
Options = (T-bit|E-bit) ;TOS-capable
LS type = 1 ;indicates router links
Link State ID = 192.1.1.3 ;RT3's Router ID
Advertising Router = 192.1.1.3
bit E = 0 ;not an AS boundary router
bit B = 1 ;RT3 is an area border router
#links = 1
Link ID = 18.10.0.6 ; Neighbor's Router ID
Link Data = 0.0.0.0 ;Interface to unnumbered SL
Type = 1 ;connects to router
# other metrics = 1
TOS 0 metric = 8
TOS = 8 ;High bandwidth
metric = 1 ;traffic preferred
12.4.2 Network links
A network links advertisement is generated for every transit multi-
access network. (A transit network is a network having two or more
attached routers). The network links advertisement describes all the
routers that are attached to the network.
The Designated Router for the network originates the advertisement. The
Designated Router originates the advertisement only if it is fully
adjacent to at least one other router on the network. The network links
advertisement is flooded throughout the area that contains the transit
network, and no further. The networks links advertisement lists those
routers that are fully adjacent to the Designated Router; each fully
adjacent router is identified by its OSPF Router ID. The Designated
Router includes itself in this list.
The Link State ID for a network links advertisement is the IP interface
address of the Designated Router. This value, masked by the network's
address mask (which is also contained in the network links
advertisement) yields the network's IP address.
A router that has formerly been the Designated Router for a network, but
is no longer, should flush the network links advertisement that it had
previously originated. This advertisement is no longer used in the
routing table calculation. It is flushed by prematurely incrementing
the advertisement's age to MaxAge and reflooding (see Section 14.1).
As an example of a network links advertisement, again consider the area
configuration in Figure 6. Network links advertisements are originated
for network N3 in Area 1, networks N6 and N8 in Area 2, and network N9
in Area 3. Assuming that router RT4 has been selected as the Designated
Router for network N3, the following network links advertisement is
generated by RT4 on behalf of network N3 (see Figure 15 for the address
assignments):
; network links advertisement for network N3
LS age = 0 ;always true on origination
Options = (T-bit|E-bit) ;TOS-capable
LS type = 2 ;indicates network links
Link State ID = 192.1.1.4 ;IP address of Designated Router
Advertising Router = 192.1.1.4 ;RT4's Router ID
Network Mask = 0xffffff00
Attached Router = 192.1.1.4 ;Router ID
Attached Router = 192.1.1.1 ;Router ID
Attached Router = 192.1.1.2 ;Router ID
Attached Router = 192.1.1.3 ;Router ID
12.4.3 Summary links
Each summary link advertisement describes a route to a single
destination. Summary link advertisements are flooded throughout a
single area only. The destination described is one that is external to
the area, yet still belonging to the Autonomous System.
The DefaultDestination can also be specified in summary link
advertisements. This is used when implementing OSPF's stub area
functionality (see Section 3.6). In a stub area, instead of importing
external routes each area border router originates a "default summary
link" (Link State ID = DefaultDestination) into the area.
Summary link advertisements are originated by area border routers. The
precise summary routes to advertise into an area are determined by
examining the routing table structure (see Section 11). Only intra-area
routes are advertised into the backbone. Both intra-area and inter-area
routes are advertised into the other areas.
To determine which routes to advertise into an attached Area A, each
routing table entry is processed as follows:
o Only Destination types of network and AS boundary router are
advertised in summary link advertisements. If the routing table
entry's Destination type is area border router, examine the next
routing table entry.
o AS external routes are never advertised in summary link
advertisements. If the routing table entry has Path-type type 1
external or type 2 external, examine the next routing table entry.
o Else, if the area associated with this set of paths is the Area A
itself, do not generate a summary link advertisement for the
route.[14]
o Else, if the destination of this route is an AS boundary router,
generate a Type 4 link state advertisement for the destination, with
Link State ID equal to the AS boundary router's ID and metric equal
to the routing table entry's cost. These advertisements should not
be generated if area A has been configured as a stub area.
o Else, the Destination type is network. If this is an inter-area
route, generate a Type 3 advertisement for the destination, with
Link State ID equal to the network's address and metric equal to the
routing table cost.
o The one remaining case is an intra-area route to a network. This
means that the network is contained in one of the router's directly
attached areas. In general, this information must be condensed
before appearing in summary link advertisements. Remember that an
area has been defined as a list of address ranges, each range
consisting of an [address,mask] pair. A single Type 3 advertisement
must be made for each range, with Link State ID equal to the range's
address and cost equal to the smallest cost of any of the component
networks.
If virtual links are being used to provide/increase connectivity of
the backbone, routing information concerning the backbone networks
should not be condensed before being summarized into the virtual
links' transit areas. In other words, the backbone ranges should be
ignored when originating summary links into these areas. The
existence of virtual links can be determined during the shortest
path calculation for the backbone (see Section 16.1).
In addition, if area A has been configured as a stub area and the router
is an area border router, it should advertise a default summary link
into Area A. The Link State ID for the advertisement should be set to
DefaultDestination, and the metric set to the (per-area) configurable
parameter StubDefaultCost.
If a router advertises a summary advertisement for a destination which
then becomes unreachable, the router must then flush the advertisement
from the routing domain by setting its age to MaxAge and reflooding (see
Section 14.1). Also, if the destination is still reachable, yet can no
longer be advertised according to the above procedure (e.g., it is now
an inter-area route, when it used to be an intra-area route associated
with some non-backbone area; it would thus no longer be advertisable to
the backbone), the advertisement should also be flushed from the routing
domain.
For an example of summary link advertisements, consider again the area
configuration in Figure 6. Routers RT3, RT4, RT7, RT10 and RT11 are all
area border routers, and therefore are originating summary links
advertisements. Consider in particular router RT4. Its routing table
was calculated as the example in Section 11.3. RT4 originates summary
link advertisements into both the backbone and Area 1. Into the
backbone, router RT4 originates separate advertisements for each of the
networks N1-N4. Into Area 1, router RT4 originates separate
advertisements for networks N6-N8 and the AS boundary routers RT5,RT7.
It also condenses host routes Ia and Ib into a single summary
advertisement. Finally, the routes to networks N9,N10,N11 and host H9
are advertised by a single summary link. This condensation was
originally performed by the router RT11.
These advertisements are illustrated graphically in Figures 7 and 8.
Two of the summary link advertisements originated by router RT4 follow.
The actual IP addresses for the networks and routers in question have
been assigned in Figure 15.
; summary link advertisement for network N1,
; originated by router RT4 into the backbone
LS age = 0 ;always true on origination
Options = (T-bit|E-bit) ;TOS-capable
LS type = 3 ;indicates summary link to IP net
Link State ID = 192.1.2.0 ;N1's IP network number
Advertising Router = 192.1.1.4 ;RT4's ID
TOS = 0
metric = 4
; summary link advertisement for AS boundary router RT7
; originated by router RT4 into Area 1
LS age = 0 ;always true on origination
Options = (T-bit|E-bit) ;TOS-capable
LS type = 4 ;indicates summary link to ASBR
Link State ID = router RT7's ID
Advertising Router = 192.1.1.4 ;RT4's ID
TOS = 0
metric = 14
Summary link advertisements pertain to a single destination (IP network
or AS boundary router). However, for a single destination there may be
separate sets of paths, and therefore separate routing table entries,
for each Type of Service. All these entries must be considered when
building the summary link advertisement for the destination; a single
advertisement must specify the separate costs (if they exist) for each
TOS. The encoding of TOS in OSPF link state advertisements is described
in Section 12.3.
Clearing the T-bit in the Options field of a summary link advertisement
indicates that there is a TOS 0 path to the destination, but no paths
for non-zero TOS. This can happen when non-TOS capable routers exist in
the routing domain (see Section 2.4).
12.4.4 AS external links
AS external link advertisements describe routes to destinations external
to the Autonomous System. Most AS external link advertisements describe
routes to specific external destinations. However, a default route for
the Autonomous System can be described in an AS external advertisement
by setting the advertisement's Link State ID to DefaultDestination
(0.0.0.0). AS external link advertisements are originated by AS
boundary routers. An AS boundary router originates a single AS external
link advertisement for each external route that it has learned, either
through another routing protocol (such as EGP), or through configuration
information.
In general, AS external link advertisements are the only type of link
state advertisements that are flooded throughout the entire Autonomous
System; all other types of link state advertisements are specific to a
single area. However, AS external advertisements are not flooded
into/throughout stub areas (see Section 3.6). This enables a reduction
in link state database size for routers internal to stub areas.
The metric that is advertised for an external route can be one of two
types. Type 1 metrics are comparable to the link state metric. Type 2
metrics are assumed to be larger than the cost of any intra-AS path. As
with summary link advertisements, if separate paths exist based on TOS,
separate TOS costs can be included in the AS external link
advertisement. The encoding of TOS in OSPF link state advertisements is
described in Section 12.3. If the T-bit of the advertisement's Options
field is clear, no non-zero TOS paths to the destination exist.
If a router advertises an AS external link advertisement for a
destination which then becomes unreachable, the router must then flush
the advertisement from the routing domain by setting its age to MaxAge
and reflooding (see Section 14.1).
For an example of AS external link advertisements, consider once again
the AS pictured in Figure 6. There are two AS boundary routers: RT5 and
RT7. Router RT5 originates three external link advertisements, for
networks N12-N14. Router RT7 originates two external link
advertisements, for networks N12 and N15. Assume that RT7 has learned
its route to N12 via EGP, and that it wishes to advertise a Type 2
metric to the AS. RT7 would then originate the following advertisement
for N12:
; AS external link advertisement for network N12,
; originated by router RT7
LS age = 0 ;always true on origination
Options = (T-bit|E-bit) ;TOS-capable
LS type = 5 ;indicates AS external link
Link State ID = N12's IP network number
Advertising Router = Router RT7's ID
bit E = 1 ;Type 2 metric
TOS = 0
metric = 2
Forwarding address = 0.0.0.0
In the above example, the forwarding address field has been set to
0.0.0.0, indicating that packets for the external destination should be
forwarded to the advertising OSPF router (RT7). This is not always
desirable. Consider the example pictured in Figure 16. There are three
OSPF routers (RTA, RTB and RTC) connected to a common network. Only one
of these routers, RTA, is exchanging EGP information with the non-OSPF
router RTX. RTA must then originate AS external link state
advertisements for those destinations it has learned from RTX. By using
the AS external advertisement's forwarding address field, RTA can
specify that packets for these destinations be forwarded directly to
RTX. Without this feature, routers RTB and RTC would take an extra hop
to get to these destinations.
Note that when the forwarding address field is non-zero, it should point
to a router belonging to another Autonomous System.
A forwarding address can also be specified for the default route. For
example, in figure 16 RTA may want to specify that all externally-
destined packets should by default be forwarded to its EGP peer RTX.
The resulting AS external link advertisement is pictured below. Note
that the Link State ID is set to DefaultDestination.
; Default route, originated by router RTA
; Packets forwarded through RTX
LS age = 0 ;always true on origination
Options = (T-bit|E-bit) ;TOS-capable
LS type = 5 ;indicates AS external link
Link State ID = DefaultDestination ; default route
Advertising Router = Router RTA's ID
bit E = 1 ;Type 2 metric
TOS = 0
metric = 1
Forwarding address = RTX's IP address
In figure 16, suppose instead that both RTA and RTB exchange EGP
information with RTX. In this case, RTA and RTB would originate the
same set of external advertisements. These advertisements, if they
specify the same metric, would be functionally equivalent since they
would specify the same destination and forwarding address (RTX). This
leads to a clear duplication of effort. If only one of RTA or RTB
originated the set of external advertisements, the routing would remain
the same, and the size of the link state database would decrease.
However, it must be unambiguously defined as to which router originates
the advertisements (otherwise neither may, or the identity of the
originator may oscillate). The following rule is thereby established:
if two routers, both reachable from one another, originate functionally
equivalent AS external advertisements (i.e., same destination, cost and
non-zero forwarding address), then the advertisement originated by the
router having the highest OSPF Router ID is used. The router having the
lower OSPF Router ID can then flush its advertisement. Flushing a link
state advertisement is discussed in Section 14.1.
(page 103 continued on part 5)
