MPLS Layer 3 VPN BGP AS Override

(Rene Molenaar) #1

This topic is to discuss the following lesson:

(Shannon S) #2

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?



(Andrew P) #3

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.

For example, in IOS-XR, the default behavior in this situation is different than the regular IOS. With IOS-XR, the PE would do the filtering because it is more efficient to do this. The logic is that the PE router knows what the CE’s ASN is, so why bother sending it a BGP router that it “knows” the CE will filter anyway?

For this reason, it is probably best to have the PE control this situation, and therefore, use AS-Override instead of AllowAS-In.

(Shannon S) #4

Hi Andrew,

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



(Mohammad Hasanuz Zaman) #5

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


(Rene Molenaar) #6

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.


(Mohammad Hasanuz Zaman) #7

Hi Rene,
Got your point . Many Thanks


(kapil k) #9

Hi Rene,

Thanks for the Great explanation. I was wondering how does MPLS work & configured between different autonomous systems. Like when there are more than 2 Service providers involved. I checked in google but couldn’t find any article in simple terms… Could you please help ?

Thank you

(Lazaros Agapides) #10

Hello Kapil

Rene doesn’t yet have a lesson on MPLS connectivity via dual ISPs, however, take a look at Cisco’s latest design guide concerning such a scenario.

(see pg 21)

If you would like Rene to take a more detailed look at the specific scenario in a future lesson, feel free to make a suggestion at the Lesson Ideas page of the Network Lessons forum:

Here you can suggest additional topics to be covered in the future.

I hope this has been helpful!


(Andy K) #11

Doesn’t using the Allow_AS_IN option set yourself up for a routing loop? What if CE2 received an update that included its own AS# and that update actually originated on that router? It just seems like the safest option would be to configure as-override on the PE router. Am I missing something?

(Lazaros Agapides) #12

Hello Andy

Take a look at @andrew’s response to a similar question below:

If you have further questions, don’t hesitate to ask!

I hope this has been helpful!


(Andy K) #13

Thanks Lazaros. Makes sense now.