ISIS Multi Topology
Overview
Intermediate System to Intermediate System (ISIS) is a link-state routing protocol commonly used in large-scale service provider networks and enterprise networks. By default, ISIS is in a single topology with no separate Shortest Path First (SPF) process to differentiate between IPv4 and IPv6 topologies. If the topology in IPv6 is different from IPv4, the routing calculation encounters a problem as the routes are evaluated and chosen based on the common topology.
Multi Topology (MT) is a mechanism to run a set of independent IP topologies within a single ISIS domain. This means, both IPv4 and IPv6 have different topologies in the network and two SPF processes are run to find the route to each IPv4 and IPv6 destination independently.
Feature Characteristics
The main characteristics of ISIS Multi Topology are as follows:
| • | Enables ISIS to maintain separate topologies for IPv4 and IPv6 within the same ISIS area or domain. |
| • | Allows routers in the ISIS area (for Level 1 routing) or domain (for Level 2 routing) to support both IPv4 and IPv6 address families. |
| • | Performs multiple SPF calculations for each configured topology. |
| • | Defines new Type-Length-Value (TLV) encodings called Multi Topology TLV (MT TLV). It is used to advertise the multiple topologies supported by the routers and contains information about the topology, including the ID (MTID), flags, and MT metric. |
| • | MT TLV (229): Capability TLV advertised in Hello packets. |
| • | MT intermediate system TLV (222): Extended TLV that describes the adjacency between nodes once the adjacency is formed. |
| • | MT IPV6 reachability TLV (237): Reachability TLV that gives information on IPv6 routing. |
Benefits
The key benefits of ISIS Multi Topology are as follows:
| • | Enables the ability to make changes to the IPv6 topology without affecting the IPv4 topology, and vice-versa. |
| • | Leverages common adjacency and database tables. |
| • | Provides an independent SPF process for IPv4 and IPv6. |