Observe the diagram. RTC is the hub router and RTA and RTB are the spokes. There are no
virtual circuits between the spoke locations. What is needed to successfully route traffic to
the 11.11.11.0/24 network from RTA?
A.
The neighbor 10.10.10.1 next-hop-self command on RTA.
B.
Nothing is required. This is the default behavior on this topology.
C.
The neighbor 10.10.10.1 next-hop-self command on RTB.
D.
The neighbor 10.10.10.1 next-hop-self command on RTC.
Explanation:
The following example illustrates the issue:
BGP Next Hop (NBMA)
If the common media as you see in the shaded area above is a frame relay or any NBMA
cloud then the exact behavior will occur as if we were connected via Ethernet. RTC will
advertise 180.20.0.0 to RTA with a next hop of 170.10.20.3.
The problem is that RTA does not have a direct PVC to RTD, and cannot reach the next
hop. In this case routing will fail.
In order to remedy this situation a command called next-hop-self is created.The next-hop-self Command
Because of certain situations with the next hop as we saw in the previous example, a
command called next-hop-self is created. The syntax is:
neighbor {ip-address|peer-group-name} next-hop-self
The next-hop-self command allows us to force BGP to use a specified IP address as the
next hop rather than letting the protocol choose the next hop.
In the previous example, the following configuration solves our problem:
RTC#
router bgp 300
neighbor 170.10.20.1 remote-as 100
neighbor 170.10.20.1 next-hop-self
RTC advertises 180.20.0.0 with a next hop = 170.10.20.2
http://www.cisco.com/warp/public/459/bgp-toc.html#bgpnexthop
