OSPFv3 Link State Advertisements
OSPFv3 retains the LSA types used by OSPFv2 with some modifications and introduces two new LSAs: Link LSA and Intra-Area-Prefix.
All LSAs use a common 20-byte header that indicates the LS type, the advertising router, and the sequence number. Figure 11-7 shows the format of the LSA header.
Figure 11-7 LSA Header
0 1 2 3
01234567890123456789012345678901
LS Age
LS Type
Link State ID
Advertising Router
LS Sequence Number
LS Checksum
Length
The LS age indicates the time in seconds since the LSA was generated.
The LS type indicates the function performed by this LSA. This field includes a U bit and S2 and S1 bits. When the U bit is set to 0, the LSA is only flooded locally. When the U bit is set to 1, the LSA is stored and flooded. The S1 and S2 bits have the functions indicated in Table 11-4.
|
S2 S1 |
Flooding Scope |
|
00 |
Link-local scope |
|
01 |
Flood to all routers within the area |
|
10 |
Flood to all routers within the AS |
|
11 |
Reserved |
The Link State ID is used with the LS type and advertising router to identify the link-state database. The Advertising Router field contains the 32-bit router ID of the router that generated the LSA. The LS Sequence Number is used to detect old or duplicate LSAs. The LS Checksum is for error checking. The Length field indicates the length of the LSA, including the header.
Table 11-5 summarizes the nine LSAs that can be used in OSPF. Most LSAs retain the same function used in OSPFv2 for IPv4. Each OSPFv3 LSA is described in more detail following the table.
|
LSA Name |
LS Type |
Description |
|
0x2001 |
State of router interfaces |
|
|
0x2002 |
Generated by DR routers in broadcast or NBMA networks |
continues continues
|
LSA Name |
LS Type |
Description |
|
Inter-Area-Prefix LSA |
0x2003 |
Routes to prefixes in other areas |
|
Inter-Area-Router LSA |
0x2004 |
Routes to routers in other areas |
|
AS-External LSA |
0x4005 |
Routes to networks external to the AS |
|
Group-Membership LSA |
0x2006 |
Networks that contain multicast groups |
|
NSSA Type 7 LSA |
0x2007 |
Routers to networks external to the AS, injected into the NSSA |
|
Link LSA |
0x0008 |
Link-local addresses and list IPv6 prefixes associated with the link |
|
Intra-Area-Prefix LSA |
0x2009 |
IPv6 prefixes associated with a router, a stub network, or an associated transit network segment |
Router LSAs describe the cost and state of all the originating router's interfaces. These LSAs are flooded within the area only. Router LSAs are LS type 0x2001. No IPv6 prefixes are contained in this LSA.
Network LSAs are originated by DRs in broadcast or NBMA networks. They describe all routers attached to the link that are adjacent to the DR. These LSAs are flooded within the area only. The LS type is 0x2002. No IPv6 prefixes are contained in this LSA.
Inter-Area-Prefix LSAs describe routes to IPv6 prefixes that belong to other areas. They are similar to OSPFv2 type 3 summary LSAs. The Inter-Area-Prefix LSA is originated by the ABR and has an LS type of 0x2003. It is also used to send the default route in stub areas. These LSAs are flooded within the area only.
Each Inter-Area-Router LSA describes a route to a router in another area. It is similar to OSPF type 4 summary LSAs. It is originated by the ABR and has an LS type of 0x2004. These LSAs are flooded within the area only.
AS-External LSAs describe networks that are external to the autonomous system (AS). These LSAs are originated by ASBRs, have an LS type of 0x4005, and thus are flooded to all routers in the AS.
The group-membership LSA describes the directly attached networks that contain members of a multicast group. This LSA is limited to the area and has an LS type of 0x2006. This LSA is described further in RFC 1584.
Type-7 LSAs describe networks that are external to the AS, but they are flooded to the NSSA area only. NSSAs are covered in RFC 1587. This LSA is generated by the NSSA ASBR and has a type of 0x2007.
Link LSAs describe the router's link-local address and a list of IPv6 prefixes associated with the link. This LSA is flooded to the local link only and has a type of 0x0008.
The Intra-Area-Prefix LSA is a new LSA type that is used to advertise IPv6 prefixes associated with a router, a stub network, or an associated transit network segment. This LSA contains information that used to be part of the router-LSAs and network-LSAs.
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