MPLS Layer 3 VPN BGP AS Override

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:

Bgp As234 As12 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

Here you will find the startup configurations of each device.

CE1

PE1

P

PE2

CE2

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.

This lesson is about AS override so that’s what we will do. Let’s configure the PE routers:

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Forum Replies

  1. Hi Rene,

    So I’ve learned so far that BGP Allow_AS_IN and BPG Override achieve a similar outcome when confronted with using same ASes for MPLS VPN customer sites that use eBGP for the last mile, but from different end points: CE vs PE. So my question is, is there an advantage/disadvantage using one over another? Or in what context would one method be preferred over another?

    Rgds,

    Shannon

  2. Shannon,
    This is a good question, and there is a lot going on behind the scene to answer it. As you know, AllowAS-In and AS-Override are both essentially tricks that can be used to deal with what would normally be considered a BGP looped route (since the ASN number will appear in the AS Path).

    The important consideration is whether the BGP “loop” route will actually be filtered on the PE side or the CE side. AllowAS-In assumes the filtering is being done on the CE side, but in reality, most PEs would have already filtered the route before it got to the CE.

    Fo

    ... Continue reading in our forum

  3. Hi Andrew,

    Awesome response & much appreciated! It’s clear now, thank you.

    Rgds,

    Shannon

  4. Hi Rene,
    Thanks for your nice article !
    Is AS Override only support on MPLS environment ??

    br//
    zaman

  5. Hi Zaman,

    It is a BGP feature so it’s not only for MPLS. MPLS VPN however is one of those situations where this can be useful.

    Rene

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