BGP Allow AS In

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:

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.

This lesson is about allow-AS in so that’s what we will do this time:

CE1(config)#router bgp 12
CE1(config-router)#neighbor 192.168.12.2 allowas-in

CE1 is now configured to allow prefixes with its own AS number from the PE1 router. If you left the debug enabled then you will see this:

CE1#
BGP(0): Revise route installing 1 of 1 routes for 5.5.5.5/32 -> 192.168.12.2(global) to main IP table

That should take care of our problem. Let’s see if the prefix has been installed:

CE1#show ip route 5.5.5.5
Routing entry for 5.5.5.5/32
  Known via "bgp 12", distance 20, metric 0
  Tag 234, type external
  Last update from 192.168.12.2 00:01:13 ago
  Routing Descriptor Blocks:
  * 192.168.12.2, from 192.168.12.2, 00:01:13 ago
      Route metric is 0, traffic share count is 1
      AS Hops 2
      Route tag 234
      MPLS label: none

There we go, it’s in the routing table. Don’t forget to configure the same change on CE2:

CE2(config)#router bgp 12
CE2(config-router)#neighbor 192.168.45.4 allowas-in

CE2 should now accept 1.1.1.1/32:

CE2#show ip route 1.1.1.1
Routing entry for 1.1.1.1/32
  Known via "bgp 12", distance 20, metric 0
  Tag 234, type external
  Last update from 192.168.45.4 00:00:45 ago
  Routing Descriptor Blocks:
  * 192.168.45.4, from 192.168.45.4, 00:00:45 ago
      Route metric is 0, traffic share count is 1
      AS Hops 2
      Route tag 234
      MPLS label: none

That’s looking good. One final check left, let’s see if there is connectivity between 1.1.1.1 and 5.5.5.5:

CE1#ping 5.5.5.5 source loopback 0
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 5.5.5.5, timeout is 2 seconds:
Packet sent with a source address of 1.1.1.1 
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 7/9/11 ms

Excellent it’s working.

Want to take a look at the complete configurations yourself?

Here you will find the startup configurations of each device.

Conclusion

The allow-AS command is a simple trick to overrule the loop prevention mechanism of external BGP. In this example it’s safe to disable it since CE1 and CE2 are stub routers, they only have one exit path through the PE routers. This solution allowed us to solve the problem on the CE routers. We can also fix it by making a change on the PE routers, I’ll show you how to do this in the AS override lesson.

When your customer sites are multihomed or have a backdoor link between them then you have to be careful as this solution can introduce loops. The BGP SoO (Site of Origin) communitry attribute is then used as a loop prevention mechanism. This is something we will cover in another lesson.

Unit 1: Introduction to BGP

Unit 2: BGP Neighbor Adjacency

Unit 3: BGP Attributes

Unit 4: BGP Communities

Unit 5: BGP Filtering

Unit 6: Advanced BGP Features

Unit 7: BGP Convergence