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RFC 2740 describes OSPF Version 3 for routing in IPv6 networks. Note that OSPFv3 is for IPv6 networks only and that it is not backward-compatible with OSPFv2 (used in IPv4). OSPF algorithms and mechanisms, such as flooding, router types, designated router election, areas, stub and NSSA, and shortest path first (SPF) calculations, remain the same. Changes are made for OSPF to support IPv6 addresses, address hierarchy, and IPv6 for transport. OSPFv3 uses multicast group FF02::5 for all OSPF routers and FF02::6 for all designated routers.

OSPFv3 Changes from OSPFv2

The following are the major changes for OSPFv3:

• Version number is 3— Obviously this is a newer version of OSPF, and it runs over IPv6 only.
• Support for IPv6 addressing— New LSAs created to carry IPv6 addresses and prefixes.
• Per-link processing— OSPFv2 uses per-subnet processing. With link processing, routers in the same link can belong to multiple subnets.
• Address semantics removed— Addresses are removed from the router and network LSAs. These LSAs now provide topology information.
• No authentication in the OSPFv3 protocol— OSPFv3 uses the authentication schemes inherited in IPv6.
• New Link LSA— For local-link flooding scope.
• New Intra-Area-Prefix LSA— Carries all the IPv6 prefix information. Similar to OSPFv2 router and network LSAs.
• Identifying neighbors by router ID— Neighbors are always identified by the router ID. This does not occur in OSPFv2 point-to-point and broadcast networks.

Note

In OSPFv3, the router IDs, area IDs, and LSA link state IDs remain at the size of 32 bits. Larger IPv6 addresses cannot be used.

OSPFv3 Areas and Router Types

OSPFv3 retains the same structure and concepts as OSPFv2. The area topology, interfaces, neighbors, link-state database, and routing table remain the same. RFC 2740 does not define new area types or router types.

The OSPF areas shown in Figure 11-2 and the router types shown in Figure 11-3 remain the same. The router types in relation to the OSPF areas are

• Internal router— Any router whose interfaces all belong to the same OSPF area. These routers keep only one link-state database.
• ABR— Routers that are connected to more than one area, in which one area is Area 0. These routers maintain a link-state database for each area they belong to. These routers generate summary LSAs.
• ASBR— Routers that inject external LSAs into the OSPF database (redistribution). These external routes are learned via either other routing protocols or static routes.
• Backbone router— Routers with at least one interface attached to Area 0.

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.

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.

Table 11-4. LSA Header S2 S1 Bits

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.

Table 11-5. OSPFv3 LSA Types

LSA Name	LS Type	Description
Router LSA	0x2001	State of router interface
Network LSA	0x2002	Generated by DR routers in broadcast or NBMA networks
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.

OSPFv3 Summary

OSPFv3 is used in large enterprise IPv6 networks. The network topology must be hierarchical. OSPF is used in the enterprise campus building access, distribution, and core layers. OSPF is also used in the enterprise data center, WAN/MAN, and branch offices.

The characteristics of OSPFv3 follow:

• Link-state routing protocol for IPv6.
• Uses IPv6 Next Header 89.
• Metric is cost (based on interface bandwidth by default).
• Sends partial route updates only when changes occur.
• Routes are labeled as intra-area, interarea, external Type 1, or external Type 2.
• Uses IPv6 for authentication.
• Uses the Dijkstra algorithm to calculate the SPF tree.
• Default administrative distance is 110.
• Uses multicast address FF02::5 (ALLSPFRouters).
• Uses multicast address FF02::6 (ALLDRouters).
• Recommended for large IPv6 networks.