Scaling IBGP in Large Networks Route Reflectors and Confederations

It is a common understanding that there exists a rule stating that IBGP neighbors must be fully meshed with each other. This section addresses why this is a requirement and how to avoid fully meshed IBGP.

It is important to understand two rules of prefix advertisement:

1. When a prefix is received from an EBGP neighbor, the router must advertise that prefix to all other EBGP and IBGP neighbors.

2. When a prefix is received from an IBGP neighbor, it can be advertised ONLY to EBGP neighbors, NOT to any other IBGP neighbors.

This second rule requires a fully meshed IBGP neighbor relationship; otherwise, prefixes are not advertised to all routers in a single AS.

IBGP full mesh can scale in networks where the number of IBGP running routers is small; however, in networks characteristic of a big ISP in which the number of routers running IBGP might reach several hundred, having an n(n? 1) (where n is the total number of routers in the AS) neighbor relationship and exchanging routes between all simply will not work. Figure 14-20 shows a fully meshed IBGP with only 12 routers running IBGP.

Figure 14-20. Twelve-Router, Fully Meshed IBGP Network

Figure 14-20. Twelve-Router, Fully Meshed IBGP Network

Imagine the nightmare caused by replacing the 12-router full mesh with a 500-router full mesh of IBGP. This limitation of full-mesh IBGP was the catalyst for the development of two mechanisms that address this problem:

• Route Reflection, as described in RFC 1966

• AS Confederations, as described in RFC 3065

The sections that follow briefly describe both mechanisms. For more detailed coverage of these mechanisms, you are encouraged to read the RFCs.

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