Overview of local-as

local-as allows a BGP neighbor to establish a BGP neighbor and exchange routes with an AS other than the original AS number for a specific EBGP neighbor. Local-as allows for a smooth transition between old and new AS numbers, for example, when you want to change the AS numbers due to ISP consolidation, but the neighbor has not yet been configured.

The following figure shows an overview of local-as.

Figure Overview of local-as
Figure Overview of local-as

R1 in this figure assumes that we have changed the AS number from AS100 to AS1000. However, ISP1 has not yet been able to handle the change in R1’s AS number and has set R1’s AS number as 100. R1 can behave as if it were AS100 to ISP1 with local-as.

AS_PATH attribute in the local-as configuration.

When emulating other AS numbers by local-as, the route’s AS_PATH attribute looks like this

  • Receiving a route : [old AS number] to be prepended to AS_PATH.
  • Sending a route : AS_PATH is prepended with [old AS number] [new AS number].
Figure AS_PATH attribute when local-as is configured.
Figure AS_PATH attribute when local-as is configured.

Configuration of local-as

To configure lobal-as, use the following command in BGP’s configuration mode.

Configuration of local-as

Router(config)#router bgp <AS-num1>
Router(config-router)#neighbor <ip-address> local-as <AS-num2>

<AS-num1> : AS number
<ip-address> : neighbor IP address
<AS-num2> : AS number to pretend to be a neighbor

Example of local-as configuration

Let’s look at an example configuration for local-as.

The ISP1 in this figure recognizes the AS number of R1 as 100 and configures BGP as shown below.

ISP1 BGP configuration

router bgp 1
 bgp log-neighbor-changes
 network 100.1.1.0 mask 255.255.255.0
 network 100.1.2.0 mask 255.255.255.0
 network 100.1.3.0 mask 255.255.255.0
 neighbor 172.16.1.1 remote-as 100
 no auto-summary

In R1, the AS number is actually changed to AS1000, but it is configured in local-as to be AS100 for ISP1.

R1 local-as configuration

router bgp 1000
 neighbor 172.16.1.11 remote-as 1
 neighbor 172.16.1.11 local-as 100
 network 100.100.1.0 mask 255.255.255.0
 network 100.100.2.0 mask 255.255.255.0
 network 100.100.3.0 mask 255.255.255.0

When you verify the BGP neighbor on R1, it has successfully established a neighbor with ISP1.

R1 neighbor verification

R1#show ip bgp summary
-- omitted --
Neighbor        V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
172.16.1.11     4     1       8       8        7    0    0 00:03:49        3

And, if you look at the BGP table in R1, it looks like this

R1 BGP table

R1#show ip bgp
-- omitted --
Network          Next Hop            Metric LocPrf Weight Path
*> 100.1.1.0/24     172.16.1.11              0             0 100 1 i
*> 100.1.2.0/24     172.16.1.11              0             0 100 1 i
*> 100.1.3.0/24     172.16.1.11              0             0 100 1 i
*> 100.100.1.0/24   192.168.13.3             2         32768 i
*> 100.100.2.0/24   192.168.13.3             2         32768 i
*> 100.100.3.0/24   192.168.13.3             2         32768 i

You will see that the route received from ISP1 has an additional AS number prepended to the route that you specified in local-as.

And if you look at the BGP table for ISP1, it looks like this

ISP1 BGP table

ISP1#show ip bgp
-- omitted --
Network          Next Hop            Metric LocPrf Weight Path
*> 100.1.1.0/24     0.0.0.0                  0         32768 i
*> 100.1.2.0/24     0.0.0.0                  0         32768 i
*> 100.1.3.0/24     0.0.0.0                  0         32768 i
*> 100.100.1.0/24   172.16.1.1               2             0 100 1000 i
*> 100.100.2.0/24   172.16.1.1               2             0 100 1000 i
*> 100.100.3.0/24   172.16.1.1               2             0 100 1000 i

The AS_PATH attribute of the route to be received from R1 at ISP1 is now “100 1000” and the AS number specified in local-as is additionally prepended.

How the BGP works