Reference Guide
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 reconfigure your routers with the new information and disable this
feature.
• Allow external routes from this neighbor.
CONFIG-ROUTERBGP mode
neighbor {IP 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: specifies that local AS values are not prepended to announcements from the neighbor.
Format: IP Address: A.B.C.D.
You must Configure Peer Groups
before
assigning it to an AS. This feature is not supported on passive peer groups.
The first 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.
Example of the Verifying that Local AS Numbering is Disabled
R2(conf-router_bgp)#show conf
!
router bgp 65123
bgp router-id 192.168.10.2
network 10.10.21.0/24
network 10.10.32.0/24
network 100.10.92.0/24
network 192.168.10.0/24
bgp four-octet-as-support
neighbor 10.10.21.1 remote-as 65123
neighbor 10.10.21.1 filter-list Laura in
neighbor 10.10.21.1 no shutdown
neighbor 10.10.32.3 remote-as 65123
neighbor 10.10.32.3 no shutdown
neighbor 100.10.92.9 remote-as 65192
neighbor 100.10.92.9 local-as 6500
neighbor 100.10.92.9 no shutdown
neighbor 192.168.10.1 remote-as 65123
neighbor 192.168.10.1 update-source Loopback 0
neighbor 192.168.10.1 no shutdown
neighbor 192.168.12.2 remote-as 65123
neighbor 192.168.12.2 update-source Loopback 0
neighbor 192.168.12.2 no shutdown
R2(conf-router_bgp)#
Allowing an AS Number to Appear in its Own AS Path
This command allows you to set the number of times a particular AS number can occur in the AS path.
The allow-as feature permits a BGP speaker to allow the ASN to be present for a specified number of times in the
update received from the peer, even if that ASN matches its own. The AS-PATH loop is detected if the local ASN is
present more than the specified number of times in the command.
181