To remove private AS numbers from the AS path on the outbound side of an eBGP neighbor, perform the following task. To also replace private AS numbers with the local router’s AS number, include the all replace-as keywords in Step 17.

The examples in this task reflect the configuration for Router 2 in the scenario in the figure below.

In the example below, Router A has the neighbor remove-private-as command configured, which removes private AS numbers in updates sent to the neighbor at 172.30.0.7. The subsequent show command asks for information about the route to host 1.1.1.1. The output includes private AS numbers 65200, 65201, 65201 in the AS path of 1001 65200 65201 65201 1002 1003 1003.

To prove that the private AS numbers were removed from the AS path, the show command on Router B also asks for information about the route to host 1.1.1.1. The output indicates a shorter AS path of 100 1001 1002 1003 1003, which excludes private AS numbers 65200, 65201, and 65201. The 100 prepended in the path is Router B’s own AS number.

Router A

router bgp 100
 bgp log-neighbor-changes
 neighbor 19.0.101.1 remote-as 1001
 neighbor 172.30.0.7 remote-as 200
 neighbor 172.30.0.7 remove-private-as all
 no auto-summary
 
RouterA# show ip bgp 1.1.1.1
BGP routing table entry for 1.1.1.1/32, version 2
Paths: (1 available, best #1, table default)
  Advertised to update-groups:
     1          2
  1001 65200 65201 65201 1002 1003 1003
    19.0.101.1 from 19.0.101.1 (19.0.101.1)
      Origin IGP, localpref 100, valid, external, best RouterA#

Router B (All Private ASNs Have Been Removed)

RouterB# show ip bgp 1.1.1.1

BGP routing table entry for 1.1.1.1/32, version 3
Paths: (1 available, best #1, table default)
  Not advertised to any peer
  100 1001 1002 1003 1003
    172.30.0.6 from 172.30.0.6 (19.1.0.1)
      Origin IGP, localpref 100, valid, external, best RouterB#

In the following example, when Router A sends prefixes to the peer 172.30.0.7, all private ASNs in the AS path are replaced with the router’s own ASN, which is 100.

Router A

router bgp 100
 bgp log-neighbor-changes
 neighbor 172.16.101.1 remote-as 1001
 neighbor 172.16.101.1 update-source Loopback0
 neighbor 172.30.0.7 remote-as 200
 neighbor 172.30.0.7 remove-private-as all replace-as
 no auto-summary

Router A receives 1.1.1.1 from peer 172.16.101.1 which has some private ASNs (65200, 65201, and 65201) in the AS path list, as shown in the following output:

RouterA# show ip bgp 1.1.1.1

BGP routing table entry for 1.1.1.1/32, version 2
Paths: (1 available, best #1, table default)
   Advertised to update-groups:
      1          2
   1001 65200 65201 65201 1002 1003 1003
     172.16.101.1 from 172.16.101.1 (172.16.101.1)
       Origin IGP, localpref 100, valid, external, best RouterA#

Because Router A is configured with neighbor 172.30.0.7 remove-private-as all replace-as, Router A sends prefix 1.1.1.1 with all private ASNs replaced with 100:

Router B

RouterB# show ip bgp 1.1.1.1

BGP routing table entry for 1.1.1.1/32, version 3
Paths: (1 available, best #1, table default)
   Not advertised to any peer
   100 1001 100 100 100 1002 1003 1003
     172.30.0.6 from 172.30.0.6 (192.168.1.2)
       Origin IGP, localpref 100, valid, external, best RouterB#

Router B

router bgp 200
 bgp log-neighbor-changes
 neighbor 172.30.0.6 remote-as 100
 no auto-summary

In this example, Router 3 uses private ASN 65000. Router 1 and Router 2 use public ASNs AS 1 and AS 5 respectively.

The figure below illustrates Router 2 belonging to a service provider, with Router 1 and Router 3 as its clients.

Figure 1. Removing Private AS Numbers

In this example, Router 2, belonging to the Service Provider, removes private AS numbers as follows.

1. Router 3 advertises the network 10.0.0.0/24 with the AS path attribute 65000 to Router 2.

2. Router 2 receives the update from Router 3 and makes an entry for the network 10.0.0.0/24 in its routing table with the next hop as 172.16.0.1 (serial interface S0 on Router 3).

3. Router 2 (service provider device), when configured with the neighbor 192.168.0.2 remove-private-as command, strips off the private AS number and constructs a new update packet with its own AS number as the AS path attribute for the 10.0.0.0/24 network and sends the packet to Router 1.

4. Router 1 receives the eBGP update for network 10.0.0.0/24 and makes an entry in its routing table with the next hop as 192.168.0.1 (serial interface S1 on Router 2). The AS path attribute for this network as seen on Router 1 is AS 5 (Router 2). Thus, the private AS numbers are prevented from entering the BGP tables of the Internet.

The configurations of Router 3, Router 2, and Router 1 follow.

Router 3

interface gigabitethernet 0/0
 ip address 10.0.0.1 255.255.255.0
!
interface Serial 0
 ip address 172.16.0.1 255.255.255.0
!
router bgp 65000
 network 10.0.0.0 mask 255.255.255.0
 neighbor 172.16.0.2 remote-as 5
!---Configures Router 2 as an eBGP neighbor in public AS 5.
!
end

Router 2

interface gigabitethernet 0/0
 ip address 172.30.1.1 255.255.0.0
!
interface Serial 0
 ip address 172.16.0.2 255.255.255.0
!
interface Serial 1
 ip address 192.168.0.1 255.255.255.0
!
router bgp 5
 network 172.30.0.0
 network 192.168.0.0
 neighbor 172.16.0.1 remote-as 65000
!---Configures Router 3 as an eBGP neighbor in private AS 65000.
 neighbor 192.168.0.2 remote-as 1
!---Configures Router 1 as an eBGP neighbor in public AS 1.
 neighbor 192.168.0.2 remove-private-as
!---Removes the private AS numbers from outgoing eBGP updates.
!
end

Router 1

version 12.2
!
!
interface Serial 0
 ip address 192.168.0.2 255.255.255.0
!
router bgp 1
 neighbor 192.168.0.1 remote-as 5
!---Configures Router 2 as an eBGP neighbor in public AS 5.
!
end

Related Documents

MIBs

Technical Assistance

The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.