Overview

The neighbor as-override command rewrites the AS_PATH attribute of a BGP route to advertise to a specific neighbor. This section describes the neighbor as-override command.

What is neighbor as-override command ?

The neighbor as-override command rewrites the AS_PATH attribute of BGP route. When advertising a BGP route to a specific neighbor, the AS number at the beginning of AS_PATH is rewritten to its own AS number.

The primary case for using the neighbor as-override command is when connecting between sites via MPLS-VPN. When using MPLS-VPN to connect between sites and using BGP between PE and CE, the AS numbers of the CE routers may be the same. Then the BGP route will be invalid due to loop prevention by AS_PATH.

Figure Connecting between sites with MPLS-VPN Part 1
Figure Connecting between sites with MPLS-VPN Part 1

So, use the neighbor-as-override command on the PE router. In the following figure, PE2 configures the neighbor-as-override command for CE2. The AS_PATH attribute attached to the BGP route 192.168.1.0/24 is “65100”; the PE2 router rewrites this to its own AS number “65001”. And because the CE2 router is an EBGP neighbor, PE2 router’s AS number is added (prepended) to the AS_PATH of the BGP route 192.168.1.0/24 and becomes “65001 65001”.

Since the AS_PATH does not contain its own AS number, the CE2 router can successfully receive the BGP route 192.168.1.0/24.

Figure Connecting between sites with MPLS-VPN Part 2
Figure Connecting between sites with MPLS-VPN Part 2

Related article

Instead of neighbor as-override, neighbor allowas-in can also be configured. neighbor allowas-in command is explained in the following article.

N-Study
BGP neighbor allowas-in command | N-Study
https://www.n-study.com/en/bgp-detail/bgp-neighbor-allowas-in/
The neighbor allowas-in command disables AS_PATH loop prevention for BGP routes received from a specific neighbor.

neighbor as-override Command and Verification commands

Command Format

The neighbor as-override command is configured in BGP IPv4 VRF configuration mode; it cannot be configured directly under BGP configuration mode. VRF must also be configured beforehand.

neighbor as-override

(config)#router bgp <AS>
(config-router)#address-family ipv4 vrf <vrf-name>
(config-router-af)#neighbor <ip-address> as-override

<AS> : AS number
<vrf-name> : VRF name
<ip-address> : Neighbor IP address

When advertising a BGP route to a specified neighbor, the first AS number in AS_PATH is rewritten to its own AS. Processed before the AS_PATH prepend when advertising a BGP route to an EBGP neighbor.

Related article

Please also see the following article about VRF.

N-Study
Overview of VRF/VRF-Lite – Virtually separating the router – | N-Study
https://www.n-study.com/en/advanced-iprouting/vrf-overview/
VRF (Virtual Routing & Forwarding) is a technology for virtually separating the router. This page provides an overview of VRF.Carrier Networks
N-Study
Cisco VRF Configuration and Verification Commands | N-Study
https://www.n-study.com/en/advanced-iprouting/cisco-vrf-configuration/
This article describes the configuration and verification commands for virtually partitioning one router by VRF on a Cisco router.

Verifycation Command

If the neighbor details are viewed with the show bgp vpnv4 unicast vrf neighbor command, it can be verified that the neighbor as-override command has been configured.

show bgp vpnv4 unicast vrf neighbor

R1#show bgp vpnv4 unicast vrf AAA neighbors
BGP neighbor is 10.1.2.2,  vrf AAA,  remote AS 65100, external link
  BGP version 4, remote router ID 2.2.2.2
  BGP state = Established, up for 00:42:45
  Last read 00:00:44, last write 00:00:44, hold time is 180, keepalive interval is 60 seconds
  Neighbor capabilities:
    Route refresh: advertised and received(old & new)
    Address family IPv4 Unicast: advertised and received
  Message statistics:
    InQ depth is 0
    OutQ depth is 0
                         Sent       Rcvd
    Opens:                  2          2
    Notifications:          0          0
    Updates:                4          2
    Keepalives:            48         48
    Route Refresh:          0          0
    Total:                 54         52
  Default minimum time between advertisement runs is 0 seconds

 For address family: VPNv4 Unicast
  Translates address family IPv4 Unicast for VRF AAA
  BGP table version 9, neighbor version 9/0
  Output queue size: 0
  Index 1, Offset 0, Mask 0x2
  1 update-group member
  Overrides the neighbor AS with my AS before sending updates
～省略～

neighbor as-override Configuration Example

Network Diagram

Consider the following network diagram for the neighbor as-override command.

Figure neighbor as-override command configuration example
Figure neighbor as-override command configuration example

初期設定

Here is an excerpt of the initial configuration for each router: R1 defines VRF “AAA”; R1-R2 and R1-R3 establish BGP neighbors; R2 advertises 192.168.1.0/24 via BGP; R3 advertises 192.168.3.0/24 via BGP.

R1 Initial Configuration(Click)

hostname R1
!
ip vrf AAA
 rd 65001:1
 route-target export 65001:1
 route-target import 65001:1
!
interface FastEthernet0/0
 ip vrf forwarding AAA
 ip address 10.1.2.1 255.255.255.0
!
interface FastEthernet0/1
 ip vrf forwarding AAA
 ip address 10.1.3.1 255.255.255.0
!
router bgp 65001
 no synchronization
 bgp router-id 1.1.1.1
 bgp log-neighbor-changes
 no auto-summary
 !
 address-family ipv4 vrf AAA
  neighbor 10.1.2.2 remote-as 65100
  neighbor 10.1.2.2 activate
  neighbor 10.1.2.2 as-override
  neighbor 10.1.3.3 remote-as 65100
  neighbor 10.1.3.3 activate
  neighbor 10.1.3.3 as-override
  no synchronization
 exit-address-family

R2 Initial Configuration(Click)

hostname R2
!
interface Loopback0
 ip address 192.168.2.2 255.255.255.0
!
interface FastEthernet0/0
 ip address 10.1.2.2 255.255.255.0
!
router bgp 65100
 no synchronization
 bgp router-id 2.2.2.2
 bgp log-neighbor-changes
 network 192.168.2.0
 neighbor 10.1.2.1 remote-as 65001
 no auto-summary

R3 Initial Configuration(Click)

hostname R3
!
interface Loopback0
 ip address 192.168.3.3 255.255.255.0
!
interface FastEthernet0/0
 ip address 10.1.3.3 255.255.255.0
!
router bgp 65100
 no synchronization
 bgp router-id 3.3.3.3
 bgp log-neighbor-changes
 network 192.168.3.0
 neighbor 10.1.3.1 remote-as 65001
 no auto-summary

Configuration and Verifycation

Step1:Verify loop prevention by AS_PATH

Verify that if the AS_PATH attribute contains its own AS number, BGP route is considered a loop.Enable debugging of BGP UPDATE messages on R3 and receive BGP routes again.

R3 Verify loop prevention by AS_PATH

R3#debug ip bgp updates
BGP updates debugging is on for address family: IPv4 Unicast
R3#clear ip bgp * in
R3#
*Mar  1 00:25:46.339: BGP(0): 10.1.3.1 rcv UPDATE w/ attr: nexthop 10.1.3.1, origin i, originator 0.0.0.0, path 65001 65100, community , extended community
*Mar  1 00:25:46.343: BGP(0): 10.1.3.1 rcv UPDATE about 192.168.3.0/24 -- DENIED due to: AS-PATH contains our own AS;
*Mar  1 00:25:46.347: BGP(0): 10.1.3.1 rcv UPDATE w/ attr: nexthop 10.1.3.1, origin i, originator 0.0.0.0, path 65001 65100, community , extended community
*Mar  1 00:25:46.351: BGP(0): 10.1.3.1 rcv UPDATE about 192.168.2.0/24 -- DENIED due to: AS-PATH contains our own AS;
R3#undebug all
All possible debugging has been turned off

The BGP route 192.168.2.0/24, which is considered a loop, is not received and is not in the BGP table.

R3 show ip bgp

R3#show ip bgp
BGP table version is 4, local router ID is 3.3.3.3
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*> 192.168.3.0      0.0.0.0                  0         32768 i

Figure Loop Prevention by AS_PATH
Figure Loop Prevention by AS_PATH

Step2:R1 configure neighbor as-override

On R1, configure the neighbor as-override command for neighbor R3: when advertising a BGP route from R1 to R3, the leading AS number “65100” is rewritten to its own AS number “65001”.

R1 neighbor as-override

router bgp 65001
 address-family ipv4 vrf AAA
  neighbor 10.1.3.3 as-override

Step3:R3 Verify BGP route

R3 receives the BGP route 192.168.2.0/24 and makes it the best path. Then, R3 registers BGP route 192.168.2.0/24 in the routing table.

R3 Verify BGP route

R3#show ip bgp
BGP table version is 5, local router ID is 3.3.3.3
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*> 192.168.2.0      10.1.3.1                               0 65001 65001 i
*> 192.168.3.0      0.0.0.0                  0         32768 i
R3#show ip route
～省略～

Gateway of last resort is not set

     10.0.0.0/24 is subnetted, 1 subnets
C       10.1.3.0 is directly connected, FastEthernet0/0
B    192.168.2.0/24 [20/0] via 10.1.3.1, 00:01:09
C    192.168.3.0/24 is directly connected, Loopback0

Figure neighbor as-override
Figure neighbor as-override

Step4:R1 Configure neighbor as-override

Configure the neighbor as-override command for R2 in the same way.

R2 neighbor as-override

router bgp 65001
 address-family ipv4 vrf AAA
  neighbor 10.1.2.2 as-override

Step5:Verify Communication

Now communication is possible in the network between distant ASes with the same AS number. verify the BGP table and routing table of R2/R3.

R2/R3 BGP Table and Routing Table

R2#show ip bgp
BGP table version is 11, local router ID is 2.2.2.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*> 192.168.2.0      0.0.0.0                  0         32768 i
*> 192.168.3.0      10.1.2.1                               0 65001 65001 i
R2#show ip route bgp
B    192.168.3.0/24 [20/0] via 10.1.2.1, 00:00:22

R3#show ip bgp
BGP table version is 7, local router ID is 3.3.3.3
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
              r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path
*> 192.168.2.0      10.1.3.1                               0 65001 65001 i
*> 192.168.3.0      0.0.0.0                  0         32768 i
R3#show ip route bgp
B    192.168.2.0/24 [20/0] via 10.1.3.1, 00:00:48

Then verify that communication is possible between 192.168.2.0/24 and 192.168.3.0/24. Ping from R2 to R3.

Ping from R2 to R3

R2#ping 192.168.3.3 source 192.168.2.2

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.3.3, timeout is 2 seconds:
Packet sent with a source address of 192.168.2.2
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 12/18/20 ms

Summary

Points

  • The neighbor as-override command rewrites the AS number at the beginning of AS_PATH to its own AS number when advertising a BGP route to a specific neighbor.
  • Enter the following command in configuration mode for the VRF address family of BGP
    • (config-router-af)#neighbor <ip-address> as-override

How the BGP works