Routing

OSPFv3 is different from regular OSPF in an number of ways: Supports multiple address families: IPv4 is only supported in OSPFv2, OSPFv3 introduces support for IPv6 too! Additional LSA Types: For carrying IPv6 addresses. Removal of addressing semantics: Prefix information is no longer present in the OSPF packet headers. It is carried as a LSA…

Route filtering allows traffic flows to be manipulated at the area border router by filtering advertised routes to other routers. Filter by Summarisation The most simplistic method to filter a route for advertisement is to use the not-advertise option when configuring prefix summarisation. Using the not-advertise keyword prevents any type 3 LSAs being generated for…

Scalability of the routing protocol is an important factor for large networks such as service providers. By splitting up an OSPF network into multiple OSPF areas, it can reduce the size of the LSDB for that area. The number of routers and networks remain the same but Type 1 and Type 2 LSAs are exchanged…

If an OSPF topology is created, where traffic will need to cross a non-backbone area to reach a destination, it is known as a discontiguous network. The fix for a discontigious network is to ensure that Area 0 is contiguous. There are workarounds, such as GRE tunnels or a OSPF virtual link.

A Type 3 LSA represents an advertisement of a network from another area. Area Border Routers (ABRs) do not forward Type 1 or 2 LSAs between areas. If a ABR receives a Type 1 LSA from a area, it creates a Type 3 LSA referencing the network from the Type 1 LSA. The Type 2…

The Type 2 LSA represents a multi-access network segment (such as Ethernet) that utilises a DR/BDR. The DR always advertises the Type 2 LSA, which identifies all routers attached to the multi-access network segment. If a DR is not yet elected in the segment, there are no Type 2 LSAs in the LSDB. If a…

Every router in an OSPF network advertises a Type 1 LSA. The Type 1 LSA is a essential part within the link state database. An entry exists as a Type 1 LSA for every OSPF enabled link including its interface, and attached networks. Type 1 LSAs are not advertised outside of an area. The packet…

When OSPF routers become neighbours, they synchronise their Link State Databases between them. If a OSPF router adds or removes a directly connected route from its database, it floods a Link State Advertisment (LSA) out of the OSPF enabled interfaces. There are six types of LSAs that are used in an OSPF network: Type LSA…

Intra-Area Routes A network route learned from another router within the same area is known as a intra-area route. In show ip route ospf , any routes learned this way are displayed with the key symbol O These routes are advertised using a Type 1 LSA, and are preferred over Type 3 LSAs. If multiple…

An OSPF area is a logical grouping of OSPF enabled interfaces across multiple routers. Area IDs The area is identified by a number or dotted decimal that is assigned at an interface level. The interface can only belong to one area. The area number is included in OSPF Hello packets that are sent by the…