11. Configuration and Deployment Considerations

The BGP protocol provides no way for a Client to identify itself dynamically as a Client of an RR. The simplest way to achieve this is by manual configuration.

One of the key component of the route reflection approach in addressing the scaling issue is that the RR summarizes routing information and only reflects its best path.

Both Multi-Exit Discriminators (MEDs) and Interior Gateway Protocol (IGP) metrics may impact the BGP route selection. Because MEDs are not always comparable and the IGP metric may differ for each router, with certain route reflection topologies the route reflection approach may not yield the same route selection result as that of the full IBGP mesh approach. A way to make route selection the same as it would be with the full IBGP mesh approach is to make sure that route reflectors are never forced to perform the BGP route selection based on IGP metrics that are significantly different from the IGP metrics of their clients, or based on incomparable MEDs. The former can be achieved by configuring the intra-cluster IGP metrics to be better than the inter-cluster IGP metrics, and maintaining full mesh within the cluster. The latter can be achieved by

• setting the local preference of a route at the border router to reflect the MED values, or

• making sure the AS-path lengths from different ASes are different when the AS-path length is used as a route selection criteria, or

• configuring community-based policies to influence the route selection.

One could argue though that the latter requirement is overly restrictive, and perhaps impractical in some cases. One could further argue that as long as there are no routing loops, there are no compelling reasons to force route selection with route reflectors to be the same as it would be with the full IBGP mesh approach.

To prevent routing loops and maintain consistent routing view, it is essential that the network topology be carefully considered in designing a route reflection topology. In general, the route reflection topology should be congruent with the network topology when there exist multiple paths for a prefix. One commonly used approach is the reflection based on Point of Presence (POP), in which each POP maintains its own route reflectors serving clients in the POP, and all route reflectors are fully meshed. In addition, clients of the reflectors in each POP are often fully meshed for the purpose of optimal intra-POP routing, and the intra-POP IGP metrics are configured to be better than the inter-POP IGP metrics.