Chapter 13. Extensions for Multi-Topology Routing

Within the past few years, carriers and service providers have been reassessing their service offerings because "plain" IPv4 packet transport has become commoditized and unprofitable. Carriers and service providers worldwide recognize that to create new revenue streams, they must provide new services to their customers, and they must implement these new services economically by leveraging a single router-based infrastructure and eliminating traditional "overlay" ATM, Frame Relay, and telephony networks. The services themselves are seldom unique: voice and video transport, virtual private networks, and group-collaboration applications have been around for a long time. What is unique is offering them over a single IP infrastructure rather than over separateand hence expensivesingle-purpose infrastructures. Multiservice and next-generation are the current networking buzzwords.

Many of the intermediate or enabling technologies required for multiservice networks, such as MPLS, IP multicast, and IPv6, require extensions to OSPF and IS-IS; Chapters 11 and 12 explored those extensions. However, there is one more requirement for multiservice networks: Although running on the same IP infrastructure, some or all of these services might require different routing topologies. Your IPv6 topology might be a subset of your IPv4 topology (and in the not-so-distant future, your IPv4 topology might be a subset of your IPv6 topology). Similarly, your multicast topology might be a subset of your unicast topology. And your inband management network might be routed independently of your customer services.

The extensions to OSPF and IS-IS for support of multiple topologies (MT) are simple and surprisingly similar:

• Each logical topology is assigned a multi-topology identifier (MT ID).

• Each OSPF or IS-IS interface is assigned one or more MT IDs to designate what topologies run on that interface.

• Adjacencies are established with all neighbors as usual.

• LSAs and LSPs are tagged with the appropriate MT IDs.

• A separate SPF algorithm is run for each topology.

• The route entries resulting from the topology-specific SPF calculations are stored in separate routing information databases (RIBs).

You can create multiple OSPF and IS-IS topologies without the MT extensions by enabling a separate routing protocol instance on each router, if your protocol implementation supports multiple instances, for each desired topology. Authentication is then used to create adjacencies within each instance that follow the desired topology. The problem with this approach is inefficiency. Each protocol instance on a router maintains its own database, its own adjacencies, and sends its own Hellos. The MT extensions use a single database and maintain single adjacencies across a given link, no matter how many topologies actually use the link. Only the SPF calculations and the RIBs are maintained separately for each topology.