MPLS Layer 3 VPN BGP AS Override – Network Professional Member Site

BGP has a simple loop prevention mechanism for external BGP. When you see your own AS number in the AS path, we do not accept the prefix. This mechanism is fine for Internet routing but there are some other scenarios where this might be an issue. Take a look at the following topology:

Above we have a small MPLS VPN network with two customer sites. The customer is using the same AS number (12) for both sites. When CE1 or CE2 receive an update from each other they will not accept it since their own AS number will be in the AS path.

Let’s find out if this is true. Here are the configurations of all routers:

Configurations
CE1
PE1
P
PE2
CE2

Here you will find the startup configurations of each device.

Let’s find out what is going on. First we’ll check if the PE routers have a VPN route for the prefixes from the CE routers:

PE1#show ip bgp vpnv4 all

     Network          Next Hop            Metric LocPrf Weight Path
Route Distinguisher: 1:1 (default for vrf CUSTOMER)
 *>  1.1.1.1/32       192.168.12.1             0             0 12 i
 *>i 5.5.5.5/32       4.4.4.4                  0    100      0 12 i

PE2#show ip bgp vpnv4 all 

     Network          Next Hop            Metric LocPrf Weight Path
Route Distinguisher: 1:1 (default for vrf CUSTOMER)
 *>i 1.1.1.1/32       2.2.2.2                  0    100      0 12 i
 *>  5.5.5.5/32       192.168.45.5             0             0 12 i

The PE routers have an entry for the loopback interfaces of the CE routers. Are they advertising these to the CE routers?

PE1#show ip bgp vpnv4 all neighbors 192.168.12.1 advertised-routes 
BGP table version is 16, 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, m multipath, b backup-path, f RT-Filter, 
              x best-external, a additional-path, c RIB-compressed, 
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
Route Distinguisher: 1:1 (default for vrf CUSTOMER)
 *>i 5.5.5.5/32       4.4.4.4                  0    100      0 12 i

Total number of prefixes 1

PE2#show ip bgp vpnv4 all neighbors 192.168.45.5 advertised-routes
BGP table version is 18, local router ID is 4.4.4.4
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, 
              r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter, 
              x best-external, a additional-path, c RIB-compressed, 
Origin codes: i - IGP, e - EGP, ? - incomplete
RPKI validation codes: V valid, I invalid, N Not found

     Network          Next Hop            Metric LocPrf Weight Path
Route Distinguisher: 1:1 (default for vrf CUSTOMER)
 *>i 1.1.1.1/32       2.2.2.2                  0    100      0 12 i

Total number of prefixes 1

The PE routers are advertising these to the CE routers. Let’s check the CE routers:

CE1#show ip bgp      

     Network          Next Hop            Metric LocPrf Weight Path
 *>  1.1.1.1/32       0.0.0.0                  0         32768 i

CE2#show ip bgp 

     Network          Next Hop            Metric LocPrf Weight Path
 *>  5.5.5.5/32       0.0.0.0                  0         32768 i

There’s nothing there…they only have the prefix on their own loopback interface in the BGP table. Let’s enable a debug on CE1 to figure out why it’s not accepting anything from PE1:

CE1#debug ip bgp all updates 
BGP updates debugging is on for all address families

Let’s reset the neighbor adjacency:

CE1#clear ip bgp *

Here’s what you will see:

CE1#
BGP(0): 192.168.12.2 rcv UPDATE about 5.5.5.5/32 -- DENIED due to: AS-PATH contains our own AS;

No surprises here…CE1 is denying the update since it sees its own AS number in the AS path. If we want to keep the same AS number on CE1 and CE2 then there are two possible solutions for this issue:

Allow-AS in: this can be configured on the CE routers which tells them to accept prefixes with their own AS number in the AS path.
AS override: this can be configured on the PE routers, the AS number will be replaced with the AS number from the service provider.