RFC 2178
OSPF Version 2
12.4. Originating LSAs Into any given OSPF area, a router will originate several LSAs. Each router originates a router-LSA. If the router is also the Designated Router for any of the area's networks, it will originate network-LSAs for those networks. Area border routers originate a single summary-LSA for each known inter-area destination. AS boundary routers originate a single AS- external-LSA 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 LSAs 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 LSAs into the backbone (one router-LSA, and one summary-LSA for each of the networks N1-N4). Router RT4 will also originate 8 distinct LSAs into Area 1 (one router-LSA and seven summary-LSAs as pictured in Figure 7). If RT4 has been selected as Designated Router for Network N3, it will also originate a network- LSA for N3 into Area 1. In this same figure, Router RT5 will be originating 3 distinct AS- external-LSAs (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 AS-external-LSAs for networks N12-N14
will not be flooded into area 3 (see Section 3.6). Instead, Router
RT11 would originate a default summary- LSA 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 an LSA is originated, its LS sequence
number is incremented, its LS age is set to 0, its LS checksum is
calculated, and the LSA is added to the link state database and
flooded out the appropriate interfaces. See Section 13.2 for details
concerning the installation of the LSA into the link state database.
See Section 13.3 for details concerning the flooding of newly
originated LSAs.
The ten events that can cause a new instance of an LSA to be
originated are:
(1) The LS age field of one of the router's self-originated LSAs
reaches the value LSRefreshTime. In this case, a new
instance of the LSA is originated, even though the contents
of the LSA (apart from the LSA header) will be the same.
This guarantees periodic originations of all LSAs. This
periodic updating of LSAs adds robustness to the link state
algorithm. LSAs 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 an LSA changes, a new LSA is
originated. However, two instances of the same LSA may not be
originated within the time period MinLSInterval. This may require
that the generation of the next instance be delayed by up to
MinLSInterval. The following events may cause the contents of an LSA
to change. These events should cause new originations if and only if
the contents of the new LSA 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-LSA.
(3) An attached network's Designated Router changes. A new
router-LSA should be originated. Also, if the router itself
is now the Designated Router, a new network-LSA should be
produced. If the router itself is no longer the Designated
Router, any network-LSA that it might have originated for
the network should be flushed from the routing domain (see
Section 14.1).
(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-LSA. Also, if the router is itself
the Designated Router for the attached network, a new
network-LSA should be produced.
The next four 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-
LSA (for this route) to be originated in each attached area
(possibly including 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-
LSA (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-LSAs into the newly attached
area for all pertinent intra-area and inter-area routes in
the router's routing table. See Section 12.4.3 for more
details.
(8) When the state of one of the router's configured virtual
links changes, it may be necessary to originate a new
router-LSA into the virtual link's Transit area (see the
discussion of the router-LSA's bit V in Section 12.4.1), as
well as originating a new router-LSA into the backbone.
The last two events concern AS boundary routers (and former AS
boundary routers) only:
(9) An external route gained through direct experience with an
external routing protocol (like BGP) changes. This will
cause an AS boundary router to originate a new instance of
an AS-external-LSA.
(10)
A router ceases to be an AS boundary router, perhaps after
restarting. In this situation the router should flush all
AS-external-LSAs that it had previously originated. These
LSAs can be flushed via the premature aging procedure
specified in Section 14.1.
The construction of each type of LSA is explained in detail below. In general, these sections describe the contents of the LSA body (i.e., the part coming after the 20-byte LSA header). For information concerning the building of the LSA header, see Section 12.1. 12.4.1. Router-LSAs A router originates a router-LSA for each area that it belongs to. Such an LSA describes the collected states of the router's links to the area. The LSA is flooded throughout the particular area, and no further. The format of a router-LSA is shown in Appendix A (Section A.4.2). The first 20 bytes of the LSA consist of the generic LSA header that was discussed in Section 12.1. router-LSAs have LS type = 1. A router also indicates whether it is an area border router, or an AS boundary router, by setting the appropriate bits .................................... . 192.1.2 Area 1 . . + . . | . . | 3+---+1 . . N1 |--|RT1|-----+ . . | +---+ \ . . | \ _______N3 . . + \/ \ . 1+---+ . * 192.1.1 *------|RT4| . + /\_______/ . +---+ . | / | . . | 3+---+1 / | . . N2 |--|RT2|-----+ 1| . . | +---+ +---+8 . 6+---+ . | |RT3|----------------|RT6| . + +---+ . +---+ . 192.1.3 |2 . 18.10.0.6|7 . | . | . +------------+ . . 192.1.4 (N4) . .................................... Figure 15: Area 1 with IP addresses shown (bit B and bit E, respectively) in its router-LSAs. This enables paths to those types of routers to be saved in the routing table, for later processing of summary-LSAs and AS-external-LSAs. Bit B should be set whenever the router is actively attached to two or more areas, even if the router is not currently attached to the OSPF backbone
area. Bit E should never be set in a router-LSA for a stub area
(stub areas cannot contain AS boundary routers).
In addition, the router sets bit V in its router-LSA for Area A if
and only if the router is the endpoint of one or more fully adjacent
virtual links having Area A as their Transit area. The setting of bit
V enables other routers in Area A to discover whether the area
supports transit traffic (see TransitCapability in Section 6).
The router-LSA then describes the router's working connections (i.e.,
interfaces or links) to the area. Each link is typed according to
the kind of attached network. Each link is also labelled with its
Link ID. This Link 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 Neighbor Router ID
link
2 Link to transit Interface address of
network Designated Router
3 Link to stub IP network number
network
4 Virtual link Neighbor Router ID
Table 18: Link descriptions in the
router-LSA.
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
transit networks, numbered point-to-point links and virtual links,
this field specifies the IP interface address of the associated
router interface (this is needed by the routing table calculation,
see Section 16.1.1). For links to stub networks, this field
specifies the stub network's IP address mask. For unnumbered point-
to-point links, the Link Data field should be set to the unnumbered
interface's MIB-II [Ref8] ifIndex value.
Finally, the cost of using the link for output is specified. The
output cost of a link is configurable. With the exception of links to
stub networks, the output cost must always be non-zero.
To further describe the process of building the list of link
descriptions, suppose a router wishes to build a router-LSA for 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 LSA, and the next interface should be
examined.
o If the state of the interface is Down, no links are added.
o 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 point-to-point network. 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 Otherwise, the link descriptions added to the router-LSA
depend on the OSPF interface type. Link descriptions used for
point-to-point interfaces are specified in Section 12.4.1.1, for
virtual links in Section 12.4.1.2, for broadcast and NBMA
interfaces in 12.4.1.3, and for Point-to-MultiPoint interfaces in
12.4.1.4.
After consideration of all the router interfaces, host links are
added to the router-LSA by examining the list of attached hosts
belonging to Area A. A host route is represented as a Type 3 link
(stub network) whose Link ID is the host's IP address, Link Data is
the mask of all ones (0xffffffff), and cost the host's configured
cost (see Section C.7).
12.4.1.1. Describing point-to-point interfaces
For point-to-point interfaces, one or more link descriptions are
added to the router-LSA as follows:
o If the neighboring router is fully adjacent, add a
Type 1 link (point-to-point). The Link ID should be set to the
Router ID of the neighboring router. For numbered point-to-point
networks, the Link Data should specify the IP interface address.
For unnumbered point-to-point networks, the Link Data field
should specify the interface's MIB-II [Ref8] ifIndex value. The
cost should be set to the output cost of the point-to-point
interface.
o In addition, as long as the state of the interface
is "Point-to-Point" (and regardless of the neighboring router
state), a Type 3 link (stub network) should be added. There are
two forms that this stub link can take:
Option 1
Assuming that the neighboring router's IP address is known, set
the Link ID of the Type 3 link to the neighbor's IP address, the
Link Data to the mask 0xffffffff (indicating a host route), and
the cost to the interface's configured output cost.[15]
Option 2
If a subnet has been assigned to the point-to-point link, set the
Link ID of the Type 3 link to the subnet's IP address, the Link
Data to the subnet's mask, and the cost to the interface's
configured output cost.[16]
12.4.1.2. Describing broadcast and NBMA interfaces
For operational broadcast and NBMA interfaces, a single link
description is added to the router-LSA as follows:
o If the state of the interface is Waiting, add a Type
3 link (stub network) with Link ID set to the IP network number
of the attached network, Link Data set to the attached network's
address mask, and cost equal to the interface's configured output
cost.
o Else, there has been a Designated Router elected 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) with Link ID set to the IP
interface address of the attached network's Designated Router
(which may be the router itself), Link Data set to the router's
own IP interface address, and cost equal to the interface's
configured output cost. Otherwise, add a link as if the
interface state were Waiting (see above).
12.4.1.3. Describing virtual links
For virtual links, a link description is added to the router-LSA only
when the virtual neighbor is fully adjacent. In this case, add a Type
4 link (virtual link) with Link ID set to the Router ID of the
virtual neighbor, Link Data set to the IP interface address
associated with the virtual link and cost set to the cost calculated
for the virtual link during the routing table calculation (see
Section 15).
12.4.1.4. Describing Point-to-MultiPoint interfaces For operational Point-to-MultiPoint interfaces, one or more link descriptions are added to the router-LSA as follows: o A single Type 3 link (stub network) is added with Link ID set to the router's own IP interface address, Link Data set to the mask 0xffffffff (indicating a host route), and cost set to 0. o For each fully adjacent neighbor associated with the interface, add an additional Type 1 link (point-to-point) with Link ID set to the Router ID of the neighboring router, Link Data set to the IP interface address and cost equal to the interface's configured output cost. 12.4.1.5. Examples of router-LSAs Consider the router-LSAs 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-LSAs, 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-LSA 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 an area border router.
; RT3's router-LSA for Area 1
LS age = 0 ;always true on origination
Options = (E-bit) ;
LS type = 1 ;indicates router-LSA
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 ;area border router
#links = 2
Link ID = 192.1.1.4 ;IP address of Desig. Rtr.
Link Data = 192.1.1.3 ;RT3's IP interface to net
Type = 2 ;connects to transit network
# TOS metrics = 0
metric = 1
Link ID = 192.1.4.0 ;IP Network number
Link Data = 0xffffff00 ;Network mask
Type = 3 ;connects to stub network
# TOS metrics = 0
metric = 2
Next RT3's router-LSA 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 an area border router.
; RT3's router-LSA for the backbone
LS age = 0 ;always true on origination
Options = (E-bit) ;
LS type = 1 ;indicates router-LSA
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 ;area border router
#links = 1
Link ID = 18.10.0.6 ;Neighbor's Router ID
Link Data = 0.0.0.3 ;MIB-II ifIndex of P-P link
Type = 1 ;connects to router
# TOS metrics = 0
metric = 8
12.4.2. Network-LSAs A network-LSA is generated for every transit broadcast or NBMA network. (A transit network is a network having two or more attached routers). The network-LSA describes all the routers that are attached to the network. The Designated Router for the network originates the LSA. The Designated Router originates the LSA only if it is fully adjacent to at least one other router on the network. The network-LSA is flooded throughout the area that contains the transit network, and no further. The network-LSA 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-LSA 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-LSA) 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-LSA that it had previously originated. This LSA is no longer used in the routing table calculation. It is flushed by prematurely incrementing the LSA's age to MaxAge and reflooding (see Section 14.1). In addition, in those rare cases where a router's Router ID has changed, any network-LSAs that were originated with the router's previous Router ID must be flushed. Since the router may have no idea what it's previous Router ID might have been, these network-LSAs are indicated by having their Link State ID equal to one of the router's IP interface addresses and their Advertising Router equal to some value other than the router's current Router ID (see Section 13.4 for more details). 12.4.2.1. Examples of network-LSAs Again consider the area configuration in Figure 6. Network-LSAs 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-LSA is generated by RT4 on behalf of Network N3 (see Figure 15 for the address assignments):
; Network-LSA for Network N3
LS age = 0 ;always true on origination
Options = (E-bit) ;
LS type = 2 ;indicates network-LSA
Link State ID = 192.1.1.4 ;IP address of Desig. Rtr.
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-LSAs
The destination described by a summary-LSA is either an IP network,
an AS boundary router or a range of IP addresses. Summary-LSAs are
flooded throughout a single area only. The destination described is
one that is external to the area, yet still belongs to the Autonomous
System.
Summary-LSAs 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) in accordance with the
algorithm described below. Note that only intra-area routes are
advertised into the backbone, while 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. Remember that each
routing table entry describes a set of equal-cost best paths to a
particular destination:
o Only Destination Types of network and AS boundary router
are advertised in summary-LSAs. 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-LSAs.
If the routing table entry has Path-type of 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-LSA for the
route.[17]
o Else, if the next hops associated with this set of paths
belong to Area A itself, do not generate a summary-LSA for the
route.[18] This is the logical equivalent of a Distance Vector
protocol's split horizon logic.
o Else, if the routing table cost equals or exceeds the
value LSInfinity, a summary-LSA cannot be generated for this
route.
o Else, if the destination of this route is an AS boundary
router, a summary-LSA should be originated if and only if the
routing table entry describes the preferred path to the AS
boundary router (see Step 3 of Section 16.4). If so, a Type 4
summary-LSA is originated for the destination, with Link State ID
equal to the AS boundary router's Router ID and metric equal to
the routing table entry's cost. Note: these LSAs 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 summary-LSA for the
destination, with Link State ID equal to the network's address (if
necessary, the Link State ID can also have one or more of the
network's host bits set; see Appendix E for details) 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-LSAs. Remember that an area
has a configured list of address ranges, each range consisting of
an [address,mask] pair and a status indication of either Advertise
or DoNotAdvertise. At most a single Type 3 summary-LSA is
originated for each range. When the range's status indicates
Advertise, a Type 3 summary-LSA is generated with Link State ID
equal to the range's address (if necessary, the Link State ID can
also have one or more of the range's "host" bits set; see Appendix
E for details) and cost equal to the largest cost of any of the
component networks. When the range's status indicates
DoNotAdvertise, the Type 3 summary-LSA is suppressed and the
component networks remain hidden from other areas.
By default, if a network is not contained in any explicitly
configured address range, a Type 3 summary-LSA is generated with Link
State ID equal to the network's address (if necessary, the Link State
ID can also have one or more of the network's "host" bits set; see
Appendix E for details) and metric equal to the network's routing
table cost.
If an area is capable of carrying transit traffic (i.e., its TransitCapability is set to TRUE), routing information concerning backbone networks should not be condensed before being summarized into the area. Nor should the advertisement of backbone networks into transit areas be suppressed. In other words, the backbone's configured ranges should be ignored when originating summary-LSAs into transit areas. If a router advertises a summary-LSA for a destination which then becomes unreachable, the router must then flush the LSA 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 LSA should also be flushed from the routing domain. 12.4.3.1. Originating summary-LSAs into stub areas The algorithm in Section 12.4.3 is optional when Area A is an OSPF stub area. Area border routers connecting to a stub area can originate summary-LSAs into the area according to the Section 12.4.3's algorithm, or can choose to originate only a subset of the summary-LSAs, possibly under configuration control. The fewer LSAs originated, the smaller the stub area's link state database, further reducing the demands on its routers' resources. However, omitting LSAs may also lead to sub-optimal inter-area routing, although routing will continue to function. As specified in Section 12.4.3, Type 4 summary-LSAs (ASBR-summary- LSAs) are never originated into stub areas. In a stub area, instead of importing external routes each area border router originates a "default summary-LSA" into the area. The Link State ID for the default summary-LSA is set to DefaultDestination, and the metric set to the (per-area) configurable parameter StubDefaultCost. Note that StubDefaultCost need not be configured identically in all of the stub area's area border routers. 12.4.3.2. Examples of summary-LSAs 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-LSAs. Consider in particular Router RT4. Its routing table was calculated as the example in Section 11.3. RT4 originates summary-LSAs into both the backbone and Area 1. Into the backbone, Router RT4 originates separate LSAs for each of the
networks N1-N4. Into Area 1, Router RT4 originates separate LSAs for
networks N6-N8 and the AS boundary routers RT5,RT7. It also
condenses host routes Ia and Ib into a single summary-LSA. Finally,
the routes to networks N9,N10,N11 and Host H1 are advertised by a
single summary-LSA. This condensation was originally performed by
the router RT11.
These LSAs are illustrated graphically in Figures 7 and 8. Two of
the summary-LSAs originated by Router RT4 follow. The actual IP
addresses for the networks and routers in question have been assigned
in Figure 15.
; Summary-LSA for Network N1,
; originated by Router RT4 into the backbone
LS age = 0 ;always true on origination
Options = (E-bit) ;
LS type = 3 ;Type 3 summary-LSA
Link State ID = 192.1.2.0 ;N1's IP network number
Advertising Router = 192.1.1.4 ;RT4's ID
metric = 4
; Summary-LSA for AS boundary router RT7
; originated by Router RT4 into Area 1
LS age = 0 ;always true on origination
Options = (E-bit) ;
LS type = 4 ;Type 4 summary-LSA
Link State ID = Router RT7's ID
Advertising Router = 192.1.1.4 ;RT4's ID
metric = 14
12.4.4. AS-external-LSAs
AS-external-LSAs describe routes to destinations external to the
Autonomous System. Most AS-external-LSAs describe routes to specific
external destinations; in these cases the LSA's Link State ID is set
to the destination network's IP address (if necessary, the Link State
ID can also have one or more of the network's "host" bits set; see
Appendix E for details). However, a default route for the Autonomous
System can be described in an AS-external-LSA by setting the LSA's
Link State ID to DefaultDestination (0.0.0.0). AS-external-LSAs are
originated by AS boundary routers. An AS boundary router originates
a single AS-external-LSA for each external route that it has learned,
either through another routing protocol (such as BGP), or through
configuration information.
AS-external-LSAs are the only type of LSAs that are flooded throughout the entire Autonomous System; all other types of LSAs are specific to a single area. However, AS-external-LSAs 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. If a router advertises an AS-external-LSA for a destination which then becomes unreachable, the router must then flush the LSA from the routing domain by setting its age to MaxAge and reflooding (see Section 14.1). 12.4.4.1. Examples of AS-external-LSAs Consider once again the AS pictured in Figure 6. There are two AS boundary routers: RT5 and RT7. Router RT5 originates three AS- external-LSAs, for networks N12-N14. Router RT7 originates two AS- external-LSAs, for networks N12 and N15. Assume that RT7 has learned its route to N12 via BGP, and that it wishes to advertise a Type 2 metric to the AS. RT7 would then originate the following LSA for N12: ; AS-external-LSA for Network N12, ; originated by Router RT7 LS age = 0 ;always true on origination Options = (E-bit) ; LS type = 5 ;AS-external-LSA Link State ID = N12's IP network number Advertising Router = Router RT7's ID bit E = 1 ;Type 2 metric 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 BGP information with the non-OSPF router RTX. RTA must then originate AS- external-LSAs for those destinations it has learned from RTX. By using the AS- external-LSA'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 BGP peer RTX.
The resulting AS-external-LSA 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 = (E-bit) ;
LS type = 5 ;AS-external-LSA
Link State ID = DefaultDestination ; default route
Advertising Router = Router RTA's ID
bit E = 1 ;Type 2 metric
metric = 1
Forwarding address = RTX's IP address
In figure 16, suppose instead that both RTA and RTB exchange BGP
information with RTX. In this case, RTA and RTB would originate the
same set of AS-external-LSAs. These LSAs, 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 AS-external-LSAs, 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 LSAs
(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-LSAs (i.e., same destination, cost and non-
zero forwarding address), then the LSA originated by the router
having the highest OSPF Router ID is used. The router having the
lower OSPF Router ID can then flush its LSA. Flushing an LSA is
discussed in Section 14.1.
13. The Flooding Procedure
Link State Update packets provide the mechanism for flooding LSAs. A
Link State Update packet may contain several distinct LSAs, and
floods each LSA one hop further from its point of origination. To
make the flooding procedure reliable, each LSA must be acknowledged
separately. Acknowledgments are transmitted in Link State
Acknowledgment packets. Many separate acknowledgments can also be
grouped together into a single packet.
The flooding procedure starts when a Link State Update packet has
been received. Many consistency checks have been made on the
received packet before being handed to the flooding procedure (see
Section 8.2). In particular, the Link State Update packet has been
associated with a particular neighbor, and a particular area. If the
neighbor is in a lesser state than Exchange, the packet should be
dropped without further processing.
+
|
+---+.....|.BGP
|RTA|-----|.....+---+
+---+ |-----|RTX|
| +---+
+---+ |
|RTB|-----|
+---+ |
|
+---+ |
|RTC|-----|
+---+ |
|
+
Figure 16: Forwarding address example
All types of LSAs, other than AS-external-LSAs, are associated with a
specific area. However, LSAs do not contain an area field. An LSA's
area must be deduced from the Link State Update packet header.
For each LSA contained in a Link State Update packet, the following
steps are taken:
(1) Validate the LSA's LS checksum. If the checksum turns out to be
invalid, discard the LSA and get the next one from the Link
State Update packet.
(2) Examine the LSA's LS type. If the LS type is unknown, discard
the LSA and get the next one from the Link State Update Packet.
This specification defines LS types 1-5 (see Section 4.3).
(3) Else if this is an AS-external-LSA (LS type = 5), and the area
has been configured as a stub area, discard the LSA and get the
next one from the Link State Update Packet. AS-external-LSAs
are not flooded into/throughout stub areas (see Section 3.6).
(4) Else if the LSA's LS age is equal to MaxAge, and there is
currently no instance of the LSA in the router's link state
database, then take the following actions:
(a) Acknowledge the receipt of the LSA by sending a Link State
Acknowledgment packet back to the sending neighbor (see
Section 13.5).
(b) Purge all outstanding requests for equal or previous
instances of the LSA from the sending neighbor's Link State
Request list (see Section 10).
(c) If the sending neighbor is in state Exchange or in state
Loading, then install the MaxAge LSA in the link state
database. Otherwise, simply discard the LSA. In either
case, examine the next LSA (if any) listed in the Link State
Update packet.
(5) Otherwise, find the instance of this LSA that is currently
contained in the router's link state database. If there is no
database copy, or the received LSA is more recent than the
database copy (see Section 13.1 below for the determination of
which LSA is more recent) the following steps must be performed:
(a) If there is already a database copy, and if the database
copy was installed less than MinLSArrival seconds ago,
discard the new LSA (without acknowledging it) and examine
the next LSA (if any) listed in the Link State Update
packet.
(b) Otherwise immediately flood the new LSA out some subset of
the router's interfaces (see Section 13.3). In some cases
(e.g., the state of the receiving interface is DR and the
LSA was received from a router other than the Backup DR) the
LSA will be flooded back out the receiving interface. This
occurrence should be noted for later use by the
acknowledgment process (Section 13.5).
(c) Remove the current database copy from all neighbors' Link
state retransmission lists.
(d) Install the new LSA in the link state database (replacing
the current database copy). This may cause the routing
table calculation to be scheduled. In addition, timestamp
the new LSA with the current time (i.e., the time it was
received). The flooding procedure cannot overwrite the
newly installed LSA until MinLSArrival seconds have elapsed.
The LSA installation process is discussed further in Section
13.2.
(e) Possibly acknowledge the receipt of the LSA by sending a
Link State Acknowledgment packet back out the receiving
interface. This is explained below in Section 13.5.
(f) If this new LSA indicates that it was originated by the
receiving router itself (i.e., is considered a self-
originated LSA), the router must take special action, either
updating the LSA or in some cases flushing it from the
routing domain. For a description of how self-originated
LSAs are detected and subsequently handled, see Section
13.4.
(6) Else, if there is an instance of the LSA on the sending
neighbor's Link state request list, an error has occurred in the
Database Exchange process. In this case, restart the Database
Exchange process by generating the neighbor event BadLSReq for
the sending neighbor and stop processing the Link State Update
packet.
(7) Else, if the received LSA is the same instance as the database
copy (i.e., neither one is more recent) the following two steps
should be performed:
(a) If the LSA is listed in the Link state retransmission list
for the receiving adjacency, the router itself is expecting
an acknowledgment for this LSA. The router should treat the
received LSA as an acknowledgment by removing the LSA from
the Link state retransmission list. This is termed an
"implied acknowledgment". Its occurrence should be noted
for later use by the acknowledgment process (Section 13.5).
(b) Possibly acknowledge the receipt of the LSA by sending a
Link State Acknowledgment packet back out the receiving
interface. This is explained below in Section 13.5.
(8) Else, the database copy is more recent. If the database copy
has LS age equal to MaxAge and LS sequence number equal to
MaxSequenceNumber, simply discard the received LSA without
acknowledging it. (In this case, the LSA's LS sequence number is
wrapping, and the MaxSequenceNumber LSA must be completely
flushed before any new LSA instance can be introduced).
Otherwise, send the database copy back to the sending neighbor,
encapsulated within a Link State Update Packet. The Link State
Update Packet should be unicast to the neighbor. In so doing, do
not put the database copy of the LSA on the neighbor's link
state retransmission list, and do not acknowledge the received
(less recent) LSA instance.
13.1. Determining which LSA is newer
When a router encounters two instances of an LSA, it must determine
which is more recent. This occurred above when comparing a received
LSA to its database copy. This comparison must also be done during
the Database Exchange procedure which occurs during adjacency bring-
up.
An LSA is identified by its LS type, Link State ID and Advertising
Router. For two instances of the same LSA, the LS sequence number,
LS age, and LS checksum fields are used to determine which instance
is more recent:
o The LSA having the newer LS sequence number is more recent.
See Section 12.1.6 for an explanation of the LS sequence number
space. If both instances have the same LS sequence number, then:
o If the two instances have different LS checksums, then the
instance having the larger LS checksum (when considered as a 16-
bit unsigned integer) is considered more recent.
o Else, if only one of the instances has its LS age field set
to MaxAge, the instance of age MaxAge is considered to be more
recent.
o Else, if the LS age fields of the two instances differ by
more than MaxAgeDiff, the instance having the smaller (younger)
LS age is considered to be more recent.
o Else, the two instances are considered to be identical.
13.2. Installing LSAs in the database Installing a new LSA in the database, either as the result of flooding or a newly self-originated LSA, may cause the OSPF routing table structure to be recalculated. The contents of the new LSA should be compared to the old instance, if present. If there is no difference, there is no need to recalculate the routing table. When comparing an LSA to its previous instance, the following are all considered to be differences in contents: o The LSA's Options field has changed. o One of the LSA instances has LS age set to MaxAge, and the other does not. o The length field in the LSA header has changed. o The body of the LSA (i.e., anything outside the 20-byte LSA header) has changed. Note that this excludes changes in LS Sequence Number and LS Checksum. If the contents are different, the following pieces of the routing table must be recalculated, depending on the new LSA's LS type field: Router-LSAs and network-LSAs The entire routing table must be recalculated, starting with the shortest path calculations for each area (not just the area whose link-state database has changed). The reason that the shortest path calculation cannot be restricted to the single changed area has to do with the fact that AS boundary routers may belong to multiple areas. A change in the area currently providing the best route may force the router to use an intra-area route provided by a different area.[19] Summary-LSAs The best route to the destination described by the summary-LSA must be recalculated (see Section 16.5). If this destination is an AS boundary router, it may also be necessary to re-examine all the AS-external-LSAs. AS-external-LSAs The best route to the destination described by the AS-external-LSA must be recalculated (see Section 16.6). Also, any old instance of the LSA must be removed from the database when the new LSA is installed. This old instance must also be removed from all neighbors' Link state retransmission lists (see Section 10).
13.3. Next step in the flooding procedure When a new (and more recent) LSA has been received, it must be flooded out some set of the router's interfaces. This section describes the second part of flooding procedure (the first part being the processing that occurred in Section 13), namely, selecting the outgoing interfaces and adding the LSA to the appropriate neighbors' Link state retransmission lists. Also included in this part of the flooding procedure is the maintenance of the neighbors' Link state request lists. This section is equally applicable to the flooding of an LSA that the router itself has just originated (see Section 12.4). For these LSAs, this section provides the entirety of the flooding procedure (i.e., the processing of Section 13 is not performed, since, for example, the LSA has not been received from a neighbor and therefore does not need to be acknowledged). Depending upon the LSA's LS type, the LSA can be flooded out only certain interfaces. These interfaces, defined by the following, are called the eligible interfaces: AS-external-LSAs (LS Type = 5) AS-external-LSAs are flooded throughout the entire AS, with the exception of stub areas (see Section 3.6). The eligible interfaces are all the router's interfaces, excluding virtual links and those interfaces attaching to stub areas. All other LS types All other types are specific to a single area (Area A). The eligible interfaces are all those interfaces attaching to the Area A. If Area A is the backbone, this includes all the virtual links. Link state databases must remain synchronized over all adjacencies associated with the above eligible interfaces. This is accomplished by executing the following steps on each eligible interface. It should be noted that this procedure may decide not to flood an LSA out a particular interface, if there is a high probability that the attached neighbors have already received the LSA. However, in these cases the flooding procedure must be absolutely sure that the neighbors eventually do receive the LSA, so the LSA is still added to each adjacency's Link state retransmission list. For each eligible interface:
(1) Each of the neighbors attached to this interface are
examined, to determine whether they must receive the new
LSA. The following steps are executed for each neighbor:
(a) If the neighbor is in a lesser state than Exchange, it
does not participate in flooding, and the next neighbor
should be examined.
(b) Else, if the adjacency is not yet full (neighbor state
is Exchange or Loading), examine the Link state request
list associated with this adjacency. If there is an
instance of the new LSA on the list, it indicates that
the neighboring router has an instance of the LSA
already. Compare the new LSA to the neighbor's copy:
o If the new LSA is less recent, then examine the next
neighbor.
o If the two copies are the same instance, then delete
the LSA from the Link state request list, and
examine the next neighbor.[20]
o Else, the new LSA is more recent. Delete the LSA
from the Link state request list.
(c) If the new LSA was received from this neighbor, examine
the next neighbor.
(d) At this point we are not positive that the neighbor has
an up-to-date instance of this new LSA. Add the new LSA
to the Link state retransmission list for the adjacency.
This ensures that the flooding procedure is reliable;
the LSA will be retransmitted at intervals until an
acknowledgment is seen from the neighbor.
(2) The router must now decide whether to flood the new LSA out
this interface. If in the previous step, the LSA was NOT
added to any of the Link state retransmission lists, there
is no need to flood the LSA out the interface and the next
interface should be examined.
(3) If the new LSA was received on this interface, and it was
received from either the Designated Router or the Backup
Designated Router, chances are that all the neighbors have
received the LSA already. Therefore, examine the next
interface.
(4) If the new LSA was received on this interface, and the
interface state is Backup (i.e., the router itself is the
Backup Designated Router), examine the next interface. The
Designated Router will do the flooding on this interface.
However, if the Designated Router fails the router (i.e.,
the Backup Designated Router) will end up retransmitting the
updates.
(5) If this step is reached, the LSA must be flooded out the
interface. Send a Link State Update packet (including the
new LSA as contents) out the interface. The LSA's LS age
must be incremented by InfTransDelay (which must be > 0)
when it is copied into the outgoing Link State Update packet
(until the LS age field reaches the maximum value of
MaxAge).
On broadcast networks, the Link State Update packets are
multicast. The destination IP address specified for the
Link State Update Packet depends on the state of the
interface. If the interface state is DR or Backup, the
address AllSPFRouters should be used. Otherwise, the
address AllDRouters should be used.
On non-broadcast networks, separate Link State Update
packets must be sent, as unicasts, to each adjacent neighbor
(i.e., those in state Exchange or greater). The destination
IP addresses for these packets are the neighbors' IP
addresses.
13.4. Receiving self-originated LSAs
It is a common occurrence for a router to receive self-originated
LSAs via the flooding procedure. A self-originated LSA is detected
when either 1) the LSA's Advertising Router is equal to the router's
own Router ID or 2) the LSA is a network-LSA and its Link State ID is
equal to one of the router's own IP interface addresses.
However, if the received self-originated LSA is newer than the last
instance that the router actually originated, the router must take
special action. The reception of such an LSA indicates that there
are LSAs in the routing domain that were originated by the router
before the last time it was restarted. In most cases, the router
must then advance the LSA's LS sequence number one past the received
LS sequence number, and originate a new instance of the LSA.
It may be the case the router no longer wishes to originate the
received LSA. Possible examples include: 1) the LSA is a summary-LSA
or AS-external-LSA and the router no longer has an (advertisable)
route to the destination, 2) the LSA is a network-LSA but the router is no longer Designated Router for the network or 3) the LSA is a network-LSA whose Link State ID is one of the router's own IP interface addresses but whose Advertising Router is not equal to the router's own Router ID (this latter case should be rare, and it indicates that the router's Router ID has changed since originating the LSA). In all these cases, instead of updating the LSA, the LSA should be flushed from the routing domain by incrementing the received LSA's LS age to MaxAge and reflooding (see Section 14.1). 13.5. Sending Link State Acknowledgment packets Each newly received LSA must be acknowledged. This is usually done by sending Link State Acknowledgment packets. However, acknowledgments can also be accomplished implicitly by sending Link State Update packets (see step 7a of Section 13). Many acknowledgments may be grouped together into a single Link State Acknowledgment packet. Such a packet is sent back out the interface which received the LSAs. The packet can be sent in one of two ways: delayed and sent on an interval timer, or sent directly (as a unicast) to a particular neighbor. The particular acknowledgment strategy used depends on the circumstances surrounding the receipt of the LSA. Sending delayed acknowledgments accomplishes several things: 1) it facilitates the packaging of multiple acknowledgments in a single Link State Acknowledgment packet, 2) it enables a single Link State Acknowledgment packet to indicate acknowledgments to several neighbors at once (through multicasting) and 3) it randomizes the Link State Acknowledgment packets sent by the various routers attached to a common network. The fixed interval between a router's delayed transmissions must be short (less than RxmtInterval) or needless retransmissions will ensue. Direct acknowledgments are sent to a particular neighbor in response to the receipt of duplicate LSAs. These acknowledgments are sent as unicasts, and are sent immediately when the duplicate is received. The precise procedure for sending Link State Acknowledgment packets is described in Table 19. The circumstances surrounding the receipt of the LSA are listed in the left column. The acknowledgment action then taken is listed in one of the two right columns. This action depends on the state of the concerned interface; interfaces in state Backup behave differently from interfaces in all other states. Delayed acknowledgments must be delivered to all adjacent routers associated with the interface. On broadcast networks, this is accomplished by sending the delayed Link State Acknowledgment packets
as multicasts. The Destination IP address used depends on the state
of the interface. If the interface state is DR or Backup, the
destination AllSPFRouters is used. In all other states, the
destination AllDRouters is used. On non-broadcast networks, delayed
Link State Acknowledgment packets must be unicast separately over
each adjacency (i.e., neighbor whose state is >= Exchange).
Action taken in state
Circumstances Backup All other states
_______________________________________________________________
LSA has No acknowledgment No acknowledgment
been flooded back sent. sent.
out receiving in-
terface (see Sec-
tion 13, step 5b).
_______________________________________________________________
LSA is Delayed acknowledg- Delayed ack-
more recent than ment sent if adver- nowledgment sent.
database copy, but tisement received
was not flooded from Designated
back out receiving Router, otherwise
interface do nothing
_______________________________________________________________
LSA is a Delayed acknowledg- No acknowledgment
duplicate, and was ment sent if adver- sent.
treated as an im- tisement received
plied acknowledg- from Designated
ment (see Section Router, otherwise
13, step 7a). do nothing
_______________________________________________________________
LSA is a Direct acknowledg- Direct acknowledg-
duplicate, and was ment sent. ment sent.
not treated as an
implied ack-
nowledgment.
_______________________________________________________________
LSA's LS Direct acknowledg- Direct acknowledg-
age is equal to ment sent. ment sent.
MaxAge, and there is
no current instance
of the LSA
in the link state
database (see
Section 13, step 4).
Table 19: Sending link state acknowledgments.
The reasoning behind sending the above packets as multicasts is best explained by an example. Consider the network configuration depicted in Figure 15. Suppose RT4 has been elected as Designated Router, and RT3 as Backup Designated Router for the network N3. When Router RT4 floods a new LSA to Network N3, it is received by routers RT1, RT2, and RT3. These routers will not flood the LSA back onto net N3, but they still must ensure that their link-state databases remain synchronized with their adjacent neighbors. So RT1, RT2, and RT4 are waiting to see an acknowledgment from RT3. Likewise, RT4 and RT3 are both waiting to see acknowledgments from RT1 and RT2. This is best achieved by sending the acknowledgments as multicasts. The reason that the acknowledgment logic for Backup DRs is slightly different is because they perform differently during the flooding of LSAs (see Section 13.3, step 4). 13.6. Retransmitting LSAs LSAs flooded out an adjacency are placed on the adjacency's Link state retransmission list. In order to ensure that flooding is reliable, these LSAs are retransmitted until they are acknowledged. The length of time between retransmissions is a configurable per- interface value, RxmtInterval. If this is set too low for an interface, needless retransmissions will ensue. If the value is set too high, the speed of the flooding, in the face of lost packets, may be affected. Several retransmitted LSAs may fit into a single Link State Update packet. When LSAs are to be retransmitted, only the number fitting in a single Link State Update packet should be sent. Another packet of retransmissions can be sent whenever some of the LSAs are acknowledged, or on the next firing of the retransmission timer. Link State Update Packets carrying retransmissions are always sent as unicasts (directly to the physical address of the neighbor). They are never sent as multicasts. Each LSA's LS age must be incremented by InfTransDelay (which must be > 0) when it is copied into the outgoing Link State Update packet (until the LS age field reaches the maximum value of MaxAge). If an adjacent router goes down, retransmissions may occur until the adjacency is destroyed by OSPF's Hello Protocol. When the adjacency is destroyed, the Link state retransmission list is cleared.
13.7. Receiving link state acknowledgments Many consistency checks have been made on a received Link State Acknowledgment packet before it is handed to the flooding procedure. In particular, it has been associated with a particular neighbor. If this neighbor is in a lesser state than Exchange, the Link State Acknowledgment packet is discarded. Otherwise, for each acknowledgment in the Link State Acknowledgment packet, the following steps are performed: o Does the LSA acknowledged have an instance on the Link state retransmission list for the neighbor? If not, examine the next acknowledgment. Otherwise: o If the acknowledgment is for the same instance that is contained on the list, remove the item from the list and examine the next acknowledgment. Otherwise: o Log the questionable acknowledgment, and examine the next one. 14. Aging The Link State Database Each LSA has an LS age field. The LS age is expressed in seconds. An LSA's LS age field is incremented while it is contained in a router's database. Also, when copied into a Link State Update Packet for flooding out a particular interface, the LSA's LS age is incremented by InfTransDelay. An LSA's LS age is never incremented past the value MaxAge. LSAs having age MaxAge are not used in the routing table calculation. As a router ages its link state database, an LSA's LS age may reach MaxAge.[21] At this time, the router must attempt to flush the LSA from the routing domain. This is done simply by reflooding the MaxAge LSA just as if it was a newly originated LSA (see Section 13.3). When creating a Database summary list for a newly forming adjacency, any MaxAge LSAs present in the link state database are added to the neighbor's Link state retransmission list instead of the neighbor's Database summary list. See Section 10.3 for more details. A MaxAge LSA must be removed immediately from the router's link state database as soon as both a) it is no longer contained on any neighbor Link state retransmission lists and b) none of the router's neighbors are in states Exchange or Loading.
When, in the process of aging the link state database, an LSA's LS age hits a multiple of CheckAge, its LS checksum should be verified. If the LS checksum is incorrect, a program or memory error has been detected, and at the very least the router itself should be restarted. 14.1. Premature aging of LSAs An LSA can be flushed from the routing domain by setting its LS age to MaxAge and reflooding the LSA. This procedure follows the same course as flushing an LSA whose LS age has naturally reached the value MaxAge (see Section 14). In particular, the MaxAge LSA is removed from the router's link state database as soon as a) it is no longer contained on any neighbor Link state retransmission lists and b) none of the router's neighbors are in states Exchange or Loading. We call the setting of an LSA's LS age to MaxAge "premature aging". Premature aging is used when it is time for a self-originated LSA's sequence number field to wrap. At this point, the current LSA instance (having LS sequence number MaxSequenceNumber) must be prematurely aged and flushed from the routing domain before a new instance with sequence number equal to InitialSequenceNumber can be originated. See Section 12.1.6 for more information. Premature aging can also be used when, for example, one of the router's previously advertised external routes is no longer reachable. In this circumstance, the router can flush its AS- external-LSA from the routing domain via premature aging. This procedure is preferable to the alternative, which is to originate a new LSA for the destination specifying a metric of LSInfinity. Premature aging is also be used when unexpectedly receiving self- originated LSAs during the flooding procedure (see Section 13.4). A router may only prematurely age its own self-originated LSAs. The router may not prematurely age LSAs that have been originated by other routers. An LSA is considered self- originated when either 1) the LSA's Advertising Router is equal to the router's own Router ID or 2) the LSA is a network-LSA and its Link State ID is equal to one of the router's own IP interface addresses. 15. Virtual Links The single backbone area (Area ID = 0.0.0.0) cannot be disconnected, or some areas of the Autonomous System will become unreachable. To establish/maintain connectivity of the backbone, virtual links can be configured through non-backbone areas. Virtual links serve to connect physically separate components of the backbone. The two endpoints of a virtual link are area border routers. The virtual
link must be configured in both routers. The configuration information in each router consists of the other virtual endpoint (the other area border router), and the non-backbone area the two routers have in common (called the Transit area). Virtual links cannot be configured through stub areas (see Section 3.6). The virtual link is treated as if it were an unnumbered point-to- point network belonging to the backbone and joining the two area border routers. An attempt is made to establish an adjacency over the virtual link. When this adjacency is established, the virtual link will be included in backbone router-LSAs, and OSPF packets pertaining to the backbone area will flow over the adjacency. Such an adjacency has been referred to in this document as a "virtual adjacency". In each endpoint router, the cost and viability of the virtual link is discovered by examining the routing table entry for the other endpoint router. (The entry's associated area must be the configured Transit area). This is called the virtual link's corresponding routing table entry. The InterfaceUp event occurs for a virtual link when its corresponding routing table entry becomes reachable. Conversely, the InterfaceDown event occurs when its routing table entry becomes unreachable. In other words, the virtual link's viability is determined by the existence of an intra-area path, through the Transit area, between the two endpoints. Note that a virtual link whose underlying path has cost greater than hexadecimal 0xffff (the maximum size of an interface cost in a router-LSA) should be considered inoperational (i.e., treated the same as if the path did not exist). The other details concerning virtual links are as follows: o AS-external-LSAs are NEVER flooded over virtual adjacencies. This would be duplication of effort, since the same AS-external-LSAs are already flooded throughout the virtual link's Transit area. For this same reason, AS-external-LSAs are not summarized over virtual adjacencies during the Database Exchange process. o The cost of a virtual link is NOT configured. It is defined to be the cost of the intra-area path between the two defining area border routers. This cost appears in the virtual link's corresponding routing table entry. When the cost of a virtual link changes, a new router-LSA should be originated for the backbone area. o Just as the virtual link's cost and viability are determined by the routing table build process (through construction of the routing table entry for the other endpoint), so are the IP interface address for the virtual interface and the virtual neighbor's IP address.
These are used when sending OSPF protocol packets over the virtual link. Note that when one (or both) of the virtual link endpoints connect to the Transit area via an unnumbered point-to-point link, it may be impossible to calculate either the virtual interface's IP address and/or the virtual neighbor's IP address, thereby causing the virtual link to fail. o In each endpoint's router-LSA for the backbone, the virtual link is represented as a Type 4 link whose Link ID is set to the virtual neighbor's OSPF Router ID and whose Link Data is set to the virtual interface's IP address. See Section 12.4.1 for more information. o A non-backbone area can carry transit data traffic (i.e., is considered a "transit area") if and only if it serves as the Transit area for one or more fully adjacent virtual links (see TransitCapability in Sections 6 and 16.1). Such an area requires special treatment when summarizing backbone networks into it (see Section 12.4.3), and during the routing calculation (see Section 16.3). o The time between link state retransmissions, RxmtInterval, is configured for a virtual link. This should be well over the expected round-trip delay between the two routers. This may be hard to estimate for a virtual link; it is better to err on the side of making it too large.
(page 137 continued on part 6)
