OSPFv3 Fundamentals
OSPFv3 is different from OSPFv2 in the following ways:
Support for multiple address families: OSPFv3 supports IPv4 and IPv6 address families.
New LSA types: New LSA types have been created to carry IPv6 prefixes.
Removal of addressing semantics: The IP prefix information is no longer present in the OSPF packet headers. Instead, it is carried as LSA payload information, making the protocol essentially address family independent, much like IS-IS. OSPFv3 uses the term link instead of network because the SPT calculations are per link instead of per subnet.
LSA flooding: OSPFv3 includes a new link-state type field that is used to determine the flooding scope of LSA, as well as the handling of unknown LSA types.
Packet format: OSPFv3 runs directly over IPv6, and the number of fields in the packet header has been reduced.
Router ID: The router ID is used to identify neighbors, regardless of the network type in OSPFv3. When you’re configuring OSPFv3 on IOS routers, the ID must always be manually assigned in the routing process.
Authentication: Neighbor authentication has been removed from the OSPF protocol and is now performed through IPsec extension headers in the IPv6 packet.
Neighbor adjacencies: OSPFv3 inter-router communication is handled by IPv6 link-local addressing. Neighbors are not automatically detected over non-broadcast multiple access (NBMA) interfaces. A neighbor must be manually specified using the link-local address. IPv6 allows for multiple subnets to be assigned to a single interface, and OSPFv3 allows for neighbor adjacency to form even if the two routers do not share a common subnet.
Multiple instances: OSPFv3 packets include an instance ID field that may be used to manipulate which routers on a network segment are allowed to form adjacencies.
OSPFv3 Link-State Advertisement
The OSPF link-state database information is organized and advertised differently in Version 3 than in Version 2. OSPFv3 modifies the structure of the router LSA (type 1), renames the network summary LSA to inter-area prefix LSA, and renames the ASBR summary LSA to inter-area router LSA. The principal difference is that the router LSA is only responsible for announcing interface parameters such as the interface type (point-to-point, broadcast, NBMA, point-to-multipoint, and virtual links) and metric (cost).
IP address information is advertised independently by two new LSA types:
Intra-area prefix LSA
Link LSA
The OSPF Dijkstra calculation used to determine the shortest path tree (SPT) only examines the router and network LSAs. Advertising the IP prefix information using new LSA types eliminates the need for OSPF to perform full shortest path first (SPF) tree calculations every time a new IP address (prefix) is added or changed on an interface. The OSPFv3 link-state database (LSDB) creates a shortest path topology tree based on links instead of networks.
OSPFv3 Communication
OSPFv3 packets use protocol number 89 in the IPv6 header, and routers communicate with each other using the local interface’s IPv6 link-local address as the source. It also uses the same five packet types and logic as OSPFv2. Depending on the packet type, the destination address is either a unicast link-local address or a multicast link-local scoped address:
Answers to the “Do I Know This Already?” quiz:
1 C 2 F 3 C 4 B 5 B
FF02::05: OSPFv3 AllSPFRouters
FF02::06: OSPFv3 AllDRouters
Every router uses the AllSPFRouters multicast address FF02::5 to send OSPF hello messages to routers on the same link. The hello messages are used for neighbor discovery and detecting whether a neighbor relationship is down. The DR and BDR routers also use this address to send link-state update and flooding acknowledgment messages to all routers.
Non-DR/BDR routers send an update or link-state acknowledgment message to the DR and BDR by using the AllDRouters address FF02::6.