Configuring and Verifying External and Internal BGP

Basic BGP configuration is relatively simple on a Cisco router between two BGP peers. Figure 8.6 shows a basic BGP configuration.

Figure 8.6. A sample of a basic BGP configuration.

RouterA in AS 65410 and RouterB in AS 65420 are BGP peers (or neighbors) and will begin to advertise networks 172.18.0.0/16 and 172.19.0.0/16 respectively. Listing 8.2 shows the Cisco configuration commands.

Listing 8.2 A Sample Configuration for Basic BGP Neighbors

 RouterA(config)# router bgp 65410 RouterA(config-router)# neighbor 10.10.10.2 remote-as 65420 RouterA(config-router)# network 172.18.0.0 RouterB(config)# router bgp 65420 RouterA(config-router)# neighbor 10.10.10.1 remote-as 65410 RouterA(config-router)# network 172.19.0.0 

Sometimes, in a frame relay environment, you may need to modify the Next-hop attribute to force a router to advertise itself as the next hop. This can be accomplished with the neighbor next-hop-self router configuration command when a single path exists to the neighbor AS.

In other situations you may not need synchronization. If you are not passing traffic from another AS through your AS, or if all your AS routers are running BGP (full mesh), you can disable synchronization. Disabling this feature enables you to carry fewer routes in your IGP and enables BGP to converge more quickly. This process is not automatic. If all the routers in your AS are running BGP and you are not running any IGP, the router has no way of knowing it. Your router will be stuck waiting for an IGP update about a certain route before it can send it on to an external neighbor. You would need to manually disable synchronization in this case for routing to function properly.

You use the BGP configuration command no synchronization to disable the synchronization of routing information between the IGPs and BGP within a fully meshed, non-transit AS. Now your router can use and advertise a route learned by IBGP to an EBGP peer before they are learned within the IGP. The following snippet shows this command on a router running BGP:

 RouterA(config-router)#no synchronization RouterA(config-route-map)#  set community   {community-number [   additive   ]}   none  } 

The set community command is used to create a BGP Community. The optional keyword additive designates that this Community should be added to the already existing communities to which a destination belongs. The keyword none deletes the Community attribute from the specified destination network.

After you have established the Community, use the following router configuration command to send a Community attribute to a BGP neighbor:

 Router(config-router)#  neighbor  {  ip-address   peer-group-name  }  send-community  

The send-community attribute enables BGP routers to filter incoming or outgoing updates to manage the "flaggable" entries in a route map to keep the route from being advertised by a certain router. Commands should be used to create and manage communities within the unique route-map configuration mode after the route map is established.

To configure multiple, connected BGP routers to act as a peer group (a single administrative entity sharing routing update policies) named MyPeerGroup, you would use the following command:

 RouterA(config-router)#  Neighbor  MyPeerGroup  peer-group  

Use the maximum-paths router configuration command when your router has two parallel paths to two different routers in the same remote autonomous system. Without the maximum-paths command, you will not have more than one path to a destination in the routing table. For example, if you use the maximum-paths 2 command, two paths can appear in the routing table, although only one path is ultimately selected as the best one in the BGP table. This is indicated by the > symbol.

The clear ip bgp privileged EXEC command activates the configuration changes by resetting specific BGP connections. There are several possible connection parameters for this command, including ip address , which resets the connection with the specified neighbor and * , which resets all connections with all neighbors.

To reset the configuration for a peer group, use the following syntax:

 Router# clear ip bgp peer-group  peer-group-name  

You can use several important commands to verify BGP operations. Table 8.6 shows some of the more important commands. You should refer back to this table when reading Chapter 9, as well as know this information for your BSCI exam.

Table 8.6. BGP Routing Protocol Verification Commands

BGP Command	Description
show ip route	Will display the IP routing table. Other keywords can follow for more granular investigation.
show ip bgp	Shows the BGP routing table. You can also designate a network number for more granular information concerning a specific network.
show ip bgp summary	Displays the status of all BGP connections.
show ip bgp neighbors	Shows details concerning TCP and BGP neighbor connections.
debug ip bgp updates	Invokes a console display of BGP update messages. Because of its use of router resources, it should be run for only a short time and then shut off. Other options for debug ip bgp include dampening , events , and keepalives .