BGP AS Override

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
  • 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.

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

PE1(config)#router bgp 234
PE1(config-router)#address-family ipv4 vrf CUSTOMER
PE1(config-router-af)#neighbor 192.168.12.1 as-override
PE2(config)#router bgp 234
PE2(config-router)#address-family ipv4 vrf CUSTOMER
PE2(config-router-af)#neighbor 192.168.45.5 as-override

To speed things up, let’s clear the BGP neighbor adjacencies on the PE routers:

PE1 & PE2#clear ip bgp *

Let’s take another look at the CE routers:

CE1#show ip bgp 5.5.5.5        
BGP routing table entry for 5.5.5.5/32, version 7
Paths: (1 available, best #1, table default)
  Not advertised to any peer
  Refresh Epoch 1
  234 234
    192.168.12.2 from 192.168.12.2 (2.2.2.2)
      Origin IGP, localpref 100, valid, external, best
      rx pathid: 0, tx pathid: 0x0
CE2#show ip bgp 1.1.1.1
BGP routing table entry for 1.1.1.1/32, version 7
Paths: (1 available, best #1, table default)
  Not advertised to any peer
  Refresh Epoch 1
  234 234
    192.168.45.4 from 192.168.45.4 (4.4.4.4)
      Origin IGP, localpref 100, valid, external, best
      rx pathid: 0, tx pathid: 0x0

The CE routers have now learned each others prefixes. If you take a closer look, you can see that AS number 12 has been replaced with AS number 234.

One final check, 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 = 6/8/11 ms

Excellent this is working! Want to take a look at these configurations yourself?

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

Here you will find the startup configurations of each device.

Conclusion

AS override is a simple technique to change the AS number of updates that you advertise to your external BGP neighbors. Another solution is allow AS in but this is configured on the CE routers. Since we are “overruling” the external BGP loop prevention mechanism you have to make sure that you have a loop-free topology.

In this scenario there are no issues since the CE routers are stubs, they only have one exit path. When your customer sites are multihomed or have a backdoor link then you need to use the BGP SoO (Site of Origin) community to ensure you have a loop free topology. This is something we’ll 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