MPLS Layer 3 VPN BGP Allow-AS-In

External BGP uses a simple loop prevention mechanism: when you see your own AS number in the AS path, we don’t accept the prefix. There are some scenarios where this might be an issue. Take a look at the following topology:

Bgp As234 As12 Topology

Above we have a MPLS VPN network where the customer is using the same AS number (12) on both sites. CE1 and CE2 will be unable to learn each others prefixes since they are using the same AS number.

Let’s see if this is true, here are the configurations of all routers if you want to test this yourself:

  • Configurations
  • CE1
  • PE1
  • P
  • PE2
  • CE2

Here you will find the startup configurations of each device.

Each CE router has a loopback interface that was advertised in BGP (1.1.1.1/32 and 5.5.5.5/32). The first thing to check is to see if the PE routers have learned the prefixes from our 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

Above you can see that both PE routers have a VPN route for these prefixes. Did they advertise these prefixes to our 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

No issues there, our PE routers are advertising these prefixes to the CE routers. Let’s see what we find in the BGP tables of 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

The CE routers only have their own prefixes in their BGP tables. Why did they refuse the updates from the PE routers? Time for a debug:

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

Let’s reset the BGP neighbor adjacency:

CE1#clear ip bgp *

Here’s what you will see on the CE1 router:

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

As expected, the CE1 router denies the update since it sees its own AS number in the AS path. If we don’t want to change our AS numbers then there’s two ways to deal with this:

  • Use Allow-AS in to overrule the loop prevention mechanism of external BGP.
  • Use AS override to change the AS number on the PE routers.