3.6 Route redistribution

3.6 Route redistribution

We have seen that there are a number of protocol choices in the intradomain and interdomain space, including a number of legacy protocols, proprietary protocols, and several open standards.

At the AS boundary, interior and exterior routing protocols are typically incompatible. Within an AS there are likely to be multiple IGPs, either for historical reasons or because multiple vendor solutions are being used. The metric schemes associated with all of the routing protocols vary widely and may be incompatible. Even so, we often need to normalize these data to form a holistic view of the network topology. The problem of integrating and mapping routing information from multiple routing domains is dealt with by a process called route redistribution. Redistribution enables routing information derived from one routing protocol to be translated and used by another routing protocol. Redistribution might be used as a quick fix when merging two organizations or for migrating from an old IGP such as RIP to OSPF. Figure 3.24 illustrates some of the key interfaces at which redistribution is likely.

Figure 3.24: Examples of redistribution interfaces. (1) The AS border interface between exterior routing protocols such as BGP-4 or static routing. (2) The AS border interface between exterior routing protocols and interior routing protocols such as OSPF, EIGRP, and RIP. (3) The interior interface between two different routing domains (e.g., Domain-1 could run OSPF, Domain-3 could run EIGRP). (4) Same as (3), except that redistribution here is not mutual (e.g., Domain-3 runs RIP, and receives only a default route back from OSPF in Domain-1.).

It is important to understand that redistribution has direction, and we can choose to redistribute routing information symmetrically (mutual redistribution) or asymmetrically (hierarchical redistribution). It is also important to appreciate that standards do not really apply here, and the facilities available on a particular router platform are very much vendor dependent. In practice each router will have its own particular idiosyncrasies, and because of the differences inherent to various IGPs and EGPs, redistribution raises several potential problems that require detailed understanding and careful attention to detail. This is especially true at the AS boundary, where the consequences of getting things wrong can be quite extensive.