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Next Hops

If you use the neighbor remote-as command to configure the BGP neighbors, the next hop is passed according to the rules provided above when networks are advertised. Consider the network configuration shown in Figure 28. Router Jackson advertises 192.168.22.0/23 internally to router Memphis with a next hop of 10.2.2.1. Router Jackson advertises the same network externally to router Topeka with a next hop of 10.1.13.1.

Figure 28: Configuring Next-Hop Processing

Image g013185.gif

Router Memphis advertises 172.24.160/19 with a next hop of 10.2.2.2 to router Jackson. Router Jackson advertises this same network externally to router Topeka with a next hop of 10.1.13.1.

Router Topeka advertises network 192.168.32.0/19 with a next hop of 10.1.13.2 to router Jackson. Because this network originates outside AS 604, router Jackson then internally advertises this network (192.168.32.0/19) to router Memphis with the same next hop, 10.1.13.2 (the IP address of the external BGP speaker that advertised the route).

When router Memphis has traffic destined for 192.168.32.0/19, it must be able to reach the next hop by means of an IGP, because it has no direct connection to 10.1.13.2. Otherwise, router Memphis will drop packets destined for 192.168.32.0/19 because the next-hop address is not accessible. Router Memphis does a lookup in its IP routing table to determine how to reach 10.1.13.2:

Destination	Next Hop
10.1.13.0/24	10.2.2.1

The next hop is reachable through router Jackson, and the traffic can be forwarded.

The following commands configure the routers as shown in Figure 28:

To configure router Jackson:



host1(config)#router bgp 604 

host1(config-router)#neighbor 10.1.13.2 remote-as
25 

host1(config-router)#neighbor 10.2.2.2 remote-as
604 

host1(config-router)#network 192.168.22.0
mask 255.255.254.0

To configure router Memphis:



host2(config)#router bgp 604 

host2(config-router)#neighbor 10.2.2.1 remote-as
604 

host2(config-router)#network 172.24.160.0
mask 255.255.224.0

To configure router Topeka:



host3(config)#router bgp 25 

host3(config-router)#neighbor 10.1.13.1 remote-as
604 

host3(config-router)#network 172.31.64.0 mask
255.255.192.0

Additional configuration is required for routers Biloxi, Memphis, and Jackson; the details depend on the IGP running in AS 604.

neighbor remote-as

Use to add an entry to the BGP neighbor table.
Specifying a neighbor with an AS number that matches the AS number specified in the router bgp command identifies the neighbor as internal to the local AS. Otherwise, the neighbor is treated as an external neighbor.
If you specify a BGP peer group by using the peerGroupName argument, all the members of the peer group inherit the characteristic configured with this command unless it is overridden for a specific peer.
This command takes effect immediately.
Use the no version to remove an entry from the table.
See neighbor remote-as.

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