In Cisco IOS Release 12.2(33)SXH, support for BGP dynamic neighbors was introduced. Perform this task to implement the dynamic creation of BGP neighbors using subnet ranges.

In this task, a BGP peer group is created on Router B in the figure below, a global limit is set on the number of dynamic BGP neighbors, and a subnet range is associated with a peer group. Configuring the subnet range enables the dynamic BGP neighbor process. The peer group is added to the BGP neighbor table of the local router, and an alternate autonomous system number is also configured. The peer group is activated under the IPv4 address family.

The next step is to move to another router--Router E in the figure below--where a BGP session is started and the neighbor router, Router B, is configured as a remote BGP peer. The peering configuration opens a TCP session and triggers Router B to create a dynamic BGP neighbor because the IP address that starts the TCP session (192.168.3.2) is within the configured subnet range for dynamic BGP peers. The task moves back to the first router, Router B, to run three show commands that have been modified to display dynamic BGP peer information.

Figure 1

BGP Dynamic Neighbor Topology

In the following example, two BGP peer groups are created on Router B in the figure below, a global limit is set on the number of dynamic BGP neighbors, and a subnet range is associated with a peer group. Configuring the subnet range enables the dynamic BGP neighbor process. The peer groups are added to the BGP neighbor table of the local router, and an alternate autonomous system number is also configured for one of the peer groups, group192. The subnet range peer groups and a standard BGP peer are then activated under the IPv4 address family.

The configuration moves to another router--Router A in the figure below--where a BGP session is started and the neighbor router, Router B, is configured as a remote BGP peer. The peering configuration opens a TCP session and triggers Router B to create a dynamic BGP neighbor because the IP address that starts the TCP session (192.168.1.2) is within the configured subnet range for dynamic BGP peers.

A third router--Router E in the figure below--also starts a BGP peering session with Router B. Router E is in the autonomous system 50000, which is the configured alternate autonomous system. Router B responds to the resulting TCP session by creating another dynamic BGP peer.

This example concludes with the output of the show ip bgp summary command entered on Router B.

Figure 2

BGP Dynamic Neighbor Topology

Router B

enable
configure terminal
router bgp 45000
 bgp log-neighbor-changes
 bgp listen limit 200
 bgp listen range 172.21.0.0/16 peer-group group172  
 bgp listen range 192.168.0.0/16 peer-group group192   
 neighbor group172 peer-group
 neighbor group172 remote-as 45000
 neighbor group192 peer-group     
 neighbor group192 remote-as 40000 alternate-as 50000
 neighbor 172.16.1.2 remote-as 45000
 address-family ipv4 unicast 
 neighbor group172 activate
 neighbor group192 activate
 neighbor 172.16.1.2 activate
 end

Router A

enable
configure terminal
router bgp 40000
 neighbor 192.168.1.1 remote-as 45000
 exit

Router E

enable
configure terminal
router bgp 50000
 neighbor 192.168.3.1 remote-as 45000
 exit

After both Router A and Router E are configured, the show ip bgp summary command is run on Router B. The output displays the regular BGP neighbor, 172.16.1.2, and the two BGP neighbors that were created dynamically when Router A and Router E initiated TCP sessions for BGP peering to Router B. The output also shows information about the configured listen range subnet groups.

BGP router identifier 192.168.3.1, local AS number 45000
BGP table version is 1, main routing table version 1
Neighbor        V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
172.16.1.2      4 45000      15      15        1    0    0 00:12:20        0
*192.168.1.2    4 40000       3       3        1    0    0 00:00:37        0
*192.168.3.2    4 50000       6       6        1    0    0 00:04:36        0
* Dynamically created based on a listen range command
Dynamically created neighbors: 2/(200 max), Subnet ranges: 2
BGP peergroup group172 listen range group members: 
  172.21.0.0/16  
BGP peergroup group192 listen range group members: 
  192.168.0.0/16