To replace AS number for its private eBGP peers

Unanswered Question
Jun 24th, 2008

Hi,

I have a question regarding BGP. Currently my sites configured with eBGP between them. In few months time, we are going to connect one of the site to ISP MPLS cloud.

My question is, for the site that are going to connect to ISP MPLS cloud, can it advertised all routes from its eBGP peer to ISP but only with its own AS number? Meaning to replace other eBGP peers AS number with the one connected to ISP once advertised out to ISP cloud

So from the ISP, they only see one AS number instead of many.

Thanks.

I have this problem too.
0 votes
  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 3 (1 ratings)
Loading.
srimural Tue, 06/24/2008 - 20:15

Hi,

As niranjan suggested we can use as-override for the same. But make sure that if you have a multihomed site(2 CE at the same location connecting to two different PE) make sure that you have SOO configured, else loops will be there.

Regards,

Srinath.M

Cable&Wireless

+91 99809 30364

exxonmiidris Tue, 06/24/2008 - 21:16

Thanks All. Appreciate the solution suggested.

By the way, the override should be configured on CE router right?

srimural Tue, 06/24/2008 - 21:23

Hi,

no it has to be configured PE which receives routes from you.

The use of as-override is

if you have sites with similar AS number accross the globe, for eg, UK and HK and AS number is 1.

if you send a prefix from HK to MPLS Cloud and UK BGP checks and finds that the orgin AS is 1 and matching with local AS. So it will discard the route to prevent loops. So inorder to override this behaviour we use override. But as i mentioned, when u override the loop prevention in eBGP, we need to make use of Site of Orgin attribute to help.

I hope i helped you a bit.

Thanks and Regards,

Srinath.M

Cable&Wireless

+91 99809 30364

n.nandrekar Tue, 06/24/2008 - 21:48

hi!

PE router is where you actually configure it for the normal scenarios wherein you are connecting 2 same AS sites over mpls vpn.

But in your scenario i guess the requirement is different, and that you want to advertise only 1 AS to the ISP. so in this case, it would be CE device. Casically the router on which you configure, will replace the AS numbers in the AS path with its own AS number while advertising the routes to the specific neighbor.

Regards,

Niranjan

srimural Tue, 06/24/2008 - 21:54

Hi Niranjan,

May be you would have got me wrong with my words. Sorry for that.

What i mean to convey is

Consider you are customer connecting to MPLS-VPN and you are using the same AS number for all your sites. In this scenario, if you do not configure as override on the PE you will not be able to receive routes for the other regions in your domain. BGP will discard routes.

Regards,

Srinath.M

Cable&Wireless

+91 99809 30364

srimural Tue, 06/24/2008 - 22:20

Hi,

Here you want AS50 to be advertised to US from RU , Please correct if I am wrong.

You can use AS Overide on the PE or use Allow-as in in the PE router.

If you want to rewrite AS50 from the AS path, may need to disable iBGP on RU. Redistribute it to a IGP and again advertise using network statements in eBGP.

Regards,

Srinath.M

n.nandrekar Tue, 06/24/2008 - 22:37

hi!

The question of where to configure it depends on what is the aim you want to achieve.

If you just want to mak the scenario work, (which currently might not work as AS 50 routes wont pass the aspath check), then you can ust configure AS-override on the PEs. It will replace the aspath of all the outes coming from PE to CE with AS 100 in the as path. The solution will work.

Also allow as-in wouldnot be required as there is no muli-homing / hub spoke scenario.

But for some other reason, you want to replace the the AS-50,60 with AS70 when advertising to the PE, then you would have to configure as-override on the CE. The first solution is the normally used one. Havent seen anyone configuring override on the CE but it can achieve your aim.

Hope this makes things clearer.

Regards,

Niranjan

srimural Tue, 06/24/2008 - 22:41

Hi,

I also suggest the configuration of as-override on the PE. Thats a standard solution rather than tweaking something.

Regards,

Srinath

exxonmiidris Tue, 06/24/2008 - 23:05

Guys,

I don't think the ISP want to do it on PE. So i need to tweak the CE in order to get this achieved.

router bgp 70

neighbor (PE ip address) remote-as 100

neighbor (PE ip address) as-override

is it correct?

Actions

This Discussion