A question regarding BGP configuration.

mysql1988

What is the difference between the following commands?

R1
router bgp 200
neighbor 192.168.1.1 remote-as 400

and

R1
router bgp 200
neighbor 192.168.1.1 next-hop-self

EdTheLad

"neighbor 192.168.1.1 remote-as 400" is used to specify an external neighbor in as 400, when an update arrives it will have a next-hop of the exit interface of the external neighbor.This next-hop attribute is advertised throughout the local AS via ibgp so all routers within the AS must have a route to the network joining the external peers.This is usually done by injecting the network between the ebgp peers into the igp.

"neighbor 192.168.1.1 next-hop-self" is used between ebgp peers to modify the next-hop address, this means updates sent throughout the AS via ibgp will have the next-hop set to the exit interface of the Ebgp peer local to the AS.Since this interface is internal to the AS it should already be advertised by the IGP.This can create suboptimal routing,but it makes life easier.

Cucumber

Sidenote, regarding the command
neighbor x.x.x.x next-hop-self

On iBGP, if you are also using the command
neighbor x.x.x.x source-update interfacetype interfacenumber

The address of said interface will be used as the next hop for the updates sent to the neighbor x.x.x.x.