BGP: Local Autonomous System

Technology: Routing
Area: IGP
Vendor: Cisco
Software: 12.X , 15.X, IP Base, IP Services
Platform: Catalyst 3560, 3750, 3850, 4500, 6500, ISR Routers

Functionality of Local Autonomous System allows to hide currently used AS number. Command instructs router to include local AS number in BGP OPEN message instead of AS number specified at routing process. This could be really helpful especially during the AS migration.

To configure example BGP process that use 200 AS number instead of AS 100  to connect with “CompanyB” neighbor, use the following command:

CompanyA#configure terminal
CompanyA(config)#router bgp 100
CompanyA(config-router)# neighbor 80.50.0.1 remote-as 101
CompanyA(config-router)# neighbor 80.50.0.1 local-as 200 [no-prepend replace-as dual-as]
CompanyA(config-router)# neighbor 80.50.0.1 description CompanyB
CompanyA(config-router)#exit

Verification commands:

CompanyA#show bgp ipv4 unicast summary
Neighbor        V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
80.50.0.1       4   101       5       5        3    0    0 00:01:39        1


CompanyA#show bgp ipv4 unicast neighbors 80.50.0.1
BGP neighbor is 80.50.0.1,  remote AS 101,  local AS 200, external link
Description: CompanyB
CompanyA#show bgp ipv4 unicast
Network          Next Hop            Metric LocPrf Weight Path
*> 80.80.1.0/24     80.50.0.1                0             0 200 101 i
*> 80.80.2.0/24     0.0.0.0                  0         32768 i


Extensions:

Command offers a couple of keyword extensions to influence a final result:

A. no-prepend – prefixes received from eBGP peer will not have a Local Autonomous System number prepended in our example AS 200.

Verification commands:

CompanyA#show bgp ipv4 unicast
Network          Next Hop            Metric LocPrf Weight Path
*> 80.80.1.0/24     80.50.0.1                0             0 101 i
*> 80.80.2.0/24     0.0.0.0                  0         32768 i

B. replace-as – prefixes advertised to eBGP peer will not have real process AS number prepended in our example AS 100.

Verification commands:

CompanyB#show bgp ipv4 unicast
Network          Next Hop            Metric LocPrf Weight Path
*> 80.80.1.0/24     0.0.0.0                  0         32768 i
*> 80.80.2.0/24     80.50.0.2                0             0 200 i

C. dual-as – used to establish BGP peering using local or real AS numbers.