29.1.2 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 BGP4+ speaker. This BGP4+ speaker forms a peering relationship with another BGP4+ 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 information about member-to-member AS peering, see 29.4.10 Confederations.

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

Figure 29-1: Internal peers and external peers

<Structure of this section>

(1) External peer

External peer relations are formed between BGP4+ speakers that belong to different ASs. The link-local or global 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 Router 1 and Router 6, between Router 2 and Router 7, and between Router 3 and Router 8 in Figure 29-1.

(2) Internal peer

Internal peer relations are formed between BGP4+ speakers in the same AS. BGP4+ uses TCP (port 179) to establish connections between peers. Therefore, although there is no requirement for all BGP4+ speakers to be physically fully meshed, internal peers must be logically fully meshed with each BGP4+ 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.

The peers formed between routers 1 and 2, routers 1 and 3, and routers 2 and 3 in Figure 29-1 "Internal and external peers" are internal peers.

(3) Peering with a Device Address

In the Switch, an IPv6 address can be assigned to a device. This is called a device address. Using the device address as the IPv6 address of an external or internal peer can eliminate the effects of a particular physical interface (TCP connection) on peering.

For example, if IPv6 address of the interface is used for the internal peer between Router 1 and Router 2 in Figure 29-1: 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 IPv6 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 RIPng or OSPFv3). The Switch handles the device address as directly connected routing information.
Note on internal peering via a router that is not a BGP4+ speaker: When routing information is reported via an internal peer that is not a BGP4+ speaker (for example, via the route from Router 2 to Router 3), the non-BGP4+ speaker must have already learned the routing information via an IGP. This is necessary to prevent IPv6 packets sent from the destination BGP4+ speaker from being discarded by a non-BGP router that has not learned the route to the originating router. For example, Figure 29-1: Internal peers and external peers prevents IPv6 packets from entering Router 3 into Router 5 from being discarded by Router 5.