External and Internal BGP

BGP uses TCP to communicate. Any two routers that have formed a TCP connection to exchange BGP routing information—in other words, a BGP connection—are called peers or neighbors. BGP peers can be either internal or external to the AS.

When BGP is running between routers within one AS, it is called internal BGP (IBGP). IBGP is run within an AS to exchange BGP information so that all internal BGP speakers have the same BGP routing information about outside autonomous systems, and so that this information can be passed to other autonomous systems. As long as they can reach each other, routers that run IBGP do not have to be directly connected to each other; static routes or routes learned from an IGP running within the AS provide reachability.

When BGP runs between routers in different autonomous systems, it is called external BGP (EBGP). Routers that run EBGP are usually connected directly to each other. Figure 7-13 illustrates IBGP and EBGP neighbors.

Figure 7-13 Routers That Have Formed a BGP Connection Are BGP Peers or Neighbors, Either External or Internal

Figure 7-13 Routers That Have Formed a BGP Connection Are BGP Peers or Neighbors, Either External or Internal

The primary use for IBGP is to carry EBGP (inter-AS) routes through an AS. IBGP can be run on all routers or on specific routers inside the AS.

KEY All routers in the path between IBGP neighbors within an AS, known as the transit path, POINT must also be running BGP. These IBGP sessions must be fully meshed.

IBGP is usually not the only protocol running in the AS; there is usually an IGP running also. Instead of redistributing the entire Internet routing table (learned via EBGP) into the IGP, IBGP carries the EBGP routes across the AS. This is necessary because in most cases the EBGP tables are too large for an IGP to handle. Even if EBGP has a small table, the loss of external routes triggering extensive computations in the IGP should be prevented. Other IBGP uses include the following:

■ Applying policy-based routing within an AS using BGP path attributes.

■ QoS Policy Propagation on BGP, which uses IBGP to send common QoS parameters (such as Type of Service [ToS]) between routers in a network and results in a synchronized QoS policy.

■ Multiprotocol Label Switching (MPLS) virtual private networks (VPN) where the multiprotocol version of BGP is used to carry MPLS VPN information.

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