Identifying EBGP and IBGP Peers
CCNP BSCI
Chapter 6: BGP [ 61 ]
AS 65100
AS 65200
RtrC
RtrA
RtrB RtrC
RtrD
BGP Next-Hop Selection
The next hop for a route received from an EBGP neighbor is the IP address
of the neighbor that sent the update.
When a BGP router receives an update from an EBGP neighbor, it must pass
that update to its IBGP neighbors without changing the next-hop attribute.
The next-hop IP address is the IP address of an edge router belonging to the
next-hop autonomous system. Therefore, IBGP routers must have a route to
the network connecting their autonomous system to that edge router. For
example, in Figure 6-3, RtrA sends an update to RtrB, listing a next hop of
10.2.2.1, its serial interface. When RtrB forwards that update to RtrC, the
next-hop IP address will still be 10.2.2.1. RtrC needs to have a route to the
10.2.2.0 network in order to have a valid next hop.
To change this behavior, use the neighbor [ip address] next-hop-self
command in BGP configuration mode. In Figure 6-3, this configuration goes
on RtrB. After you give this command, RtrB will advertise its IP address to
RtrC as the next hop for networks from AS 65100, rather than the address of
RtrA. Thus, RtrC does not have to know about the external network between
RtrA and RtrB (network 10.2.2.0).