Nokia’s implementation of IS-IS route leaking is performed in compliance with RFC 2966, Domain-wide Prefix Distribution with Two-Level IS-IS. As previously stated, IS-IS is a routing domain (an autonomous system running IS-IS) which can be divided into level 1 areas with a level 2-connected subset (backbone) of the topology that interconnects all of the level 1 areas. Within each level 1 area, the routers exchange link state information. Level 2 routers also exchange level 2 link state information to compute routes between areas.
Routers in a level 1 area typically only exchange information within the level 1 area. For IP destinations not found in the prefixes in the level 1 database, the level 1 router forwards PDUs to the nearest router that is in both level 1/level 2 with the attached bit set in its level 1 link-state PDU.
There are many reasons to implement domain-wide prefix distribution. The goal of domain-wide prefix distribution is to increase the granularity of the routing information within the domain. The routing mechanisms specified in RFC 1195 are appropriate in many situations and account for excellent scalability properties. However, in specific circumstances, the amount of scalability can be adjusted which can distribute more specific information than described by RFC 1195.
Distributing more prefix information can improve the quality of the resulting routes. A well-known property of default routing is that loss of information can occur. This loss of information affects the computation of a route based upon less information which can result in sub-optimal routes.