13.1.2 Type of peer and connection type

Because BGP4 operates between ASs, the routing information it handles consists of AS path information to a destination network (the series of ASs that a packet traverses to reach the destination network). A router running BGP4 is known as a BGP speaker. This BGP speaker forms a peering relationship with another BGP speaker in order to exchange routing information.

The Switch uses two types of peers: external peers and internal peers. In addition to these two types, a third category called a member AS peer is used when configuring a confederation. For details about member-to-member AS peering, see 13.4.10 Confederations.

Use the appropriate peer type for your network configuration. The following figure shows internal and external peers.

Figure 13-1: Internal peers and external peers

<Structure of this section>

(1) External peer

External peer relations are formed between BGP speakers that belong to different ASs. The interface address of the directly connected interface is used as the IP address for peering. The peers can be connected by a different address (for example, the device address) by using the neighbor ebgp-multihop configuration command.

The external peers are formed between routers 1 and 6, routers 2 and 7, and routers 3 and 8 in Figure 13-1 "Internal and external peers."

(2) Internal peer

Internal peer relations are formed between BGP speakers in the same AS. BGP4 uses TCP (port 179) to establish connections between peers. Therefore, although there is no requirement for all BGP speakers to be physically fully meshed, internal peers must be logically fully meshed with each BGP speaker within the AS. This is because an internal peer does not announce received routing information to other internal peers. When route reflection or confederation is used, this condition is relaxed.

In Figure 13-1: Internal peers and external peers, the internal peers are formed between Router 1 and Router 2, between Router 1 and Router 3, and between Router 2 and Router 3.

(3) Peering with a Device Address

In the Switch, the state of a particular physical interface (TCP connection) can affect peering. Such effects can be eliminated by using the IP address of the loopback interface (the device address) as the IP address of an external or internal peer.

For example, if IP address of the interface is used for the internal peer between Router 1 and Router 2 in Figure 13-1: Internal peer and external peer, if a failure occurs between Router 1 and Router 2 and the interface cannot be used, the internal peer between Router 1 and Router 2 cannot be established. However, if the device address is used as IP address of the internal peer, the internal peer can be established through Router 4 and Router 5 even if the interface between Router 1 and Router 2 is not available.

Notes on using the device address: Before peers can use a device address for interconnectivity, they must learn information about routes to that address via static routing or by an IGP (such as RIP or OSPF). The Switch handles the device address as directly connected routing information.
Notes on internal peering via a router that is not a BGP speaker: When routing information is reported via an internal peer that is not a BGP speaker (for example, via the route from Router 2 to Router 3), the non-BGP speaker must have already learned the routing information via an IGP. This is necessary to prevent IP packets sent from the notified BGP speaker from being discarded by a non-BGP router that has not learned the route to the originating router. For example, Fig. 13-1: Internal peers and external peers prevents IP packets from entering Router 3 into Router 5 from being discarded by Router 5.