Concept Guide

You can constrain the number of passive sessions accepted by the neighbor. The limit keyword allows you to set the total number of
sessions the neighbor will accept, between 2 and 265. The default is 256 sessions.
1 Congure a peer group that does not initiate TCP connections with other peers.
CONFIG-ROUTER-BGP mode
neighbor {ip-address | ipv6–address | peer-group-name} peer-group passive limit
Enter the limit keyword to restrict the number of sessions accepted.
2 Assign a subnet to the peer group.
CONFIG-ROUTER-BGP mode
neighbor {ip-address | ipv6–address | peer-group-name} subnet subnet-number mask
The peer group responds to OPEN messages sent on this subnet.
3 Enable the peer group.
CONFIG-ROUTER-BGP mode
neighbor {ip-address | ipv6–address | peer-group-name} no shutdown
4 Create and specify a remote peer for BGP neighbor.
CONFIG-ROUTER-BGP mode
neighbor {ip-address | ipv6–address | peer-group-name} remote-as as-number
Only after the peer group responds to an OPEN message sent on the subnet does its BGP state change to ESTABLISHED. After the peer
group is ESTABLISHED, the peer group is the same as any other peer group.
For more information about peer groups, refer to Congure Peer Groups.
Maintaining Existing AS Numbers During an AS Migration
The local-as feature smooths out the BGP network migration operation and allows you to maintain existing ASNs during a BGP network
migration.
When you complete your migration, be sure to recongure your routers with the new information and disable this feature.
• Allow external routes from this neighbor.
CONFIG-ROUTERBGP mode
neighbor {ip-address | ipv6-address | peer-group-name} local-as as number [no prepend]
– peer-group-name: 16 characters.
– AS-number: 0 to 65535 (2-Byte) or 1 to 4294967295 (4-Byte) or 0.1 to 65535.65535 (Dotted format).
– No Prepend: species that local AS values are not prepended to announcements from the neighbor.
Format: IPv4 Address: A.B.C.D and IPv6 address: X:X:X:X::X.
You must Congure Peer Groups before assigning it to an AS. This feature is not supported on passive peer groups.
Example of the Verifying that Local AS Numbering is Disabled
The rst line in bold shows the actual AS number. The second two lines in bold show the local AS number (6500) maintained during
migration.
To disable this feature, use the no neighbor local-as command in CONFIGURATION ROUTER BGP mode.
R2(conf-router_bgp)#show conf
!
router bgp 65123
210
Border Gateway Protocol (BGP)