one public AS use in different location-NY and LA with different ISP?

[raymond wang]

Hi

I have one public AS used in NY with one isp. We will bring up another internet link for branch in LA with another ISP. Can I use the same public AS for this new office?

One concern is both offices will advertise out differrent subnets with same AS number. Will the bgp block the traffic since it is AS loop? Maybe ISP will make some changes such as replace-as  to solve this issue?

Thanks

Ray

[Richard Burts]

Ray

It is good to know that you have a public AS number. Do you also have your own IP address space or are you using address space assigned by your ISP?

In general it is not a problem to use the same AS number from 2 different locations, as long as each location is advertising a unique address space. The only place that it will look like a loop is at your NY site and your LA site. They will each reject the advertisement from the other because they will see their AS in the advertised path. But as long as you provide for communication between the sites to not depend on BGP advertisement then you should be fine.

If we knew more about your environment we might be able to get from answers in general to answers for your particular situation.

HTH

Rick

[raymond wang]

Hi Richard,

Thanks for your reply. We use the ip address assigned by ISP and also by our own-2 blocks. Yes that is the concern of the loop. We do have bgp with ISP so that we can advertise our own block out though we can do static, but bgp is prefered. I believe if the ISP side can configure replace-as on their side, that would solve the loop issue. We only accept the default route on both locations. I am not sure how ISP will take care of it-replace-as is just my guess..

Thanks

Ray

[Jon Marshall]

Ray

I'm a bit unclear when you say -

We only accept the default route on both locations

If you only accept the default route then why worry about whether the advertisement is dropped or not because the only place they will be dropped is at the NY and LA routers ie.

NY advertises out subnet1 with AS 10

LA advertises out subnet2 with AS 10

these advertisements will be passed on through the ISPs etc. The only place where the AS PATH is a concern is when NYs advertisment arrives at LA and the reverse. But you can just let both routers drop the advertisement because you don't need to have it as you have a default route anyway. This assumes that traffic between NY and LA uses the same next hop the default route points to.

For your info there is a way of allowing BGP to accept routes with it's own AS PATH in it but unless the next hop for the route is different than the next hop for your default route i can't see the point.

Jon

[Richard Burts]

Ray

Thanks for the additional information.

If you were advertising address space assigned by the ISP there would probably not be any issue at all since the ISP would probably just advertise a summary of their address space and your specific block would not be advertised. But if you have your own provider independent addresses and advertise them with your AS then it is likely that the advertisement from LA would get to NY and be not accepted (and the same when the NY advertisement gets to LA). But as Jon has said, what is the problem with this? If NY uses its default route then its traffic to LA would get routed via the default and would be forwarded to LA. (and the same for traffic from LA to NY)

So my advise is to go ahead with BGP. Advertise your address blocks with your AS. And I do not believe that the ISP needs to do anything.

HTH

Rick

[raymond wang]

Thanks Jon and Richard. I believe if I only receive the bgp default route from the 2 isp in both location, it should be fine. If I want to receive the bgp route for these 2 subnets, It might have issues. My router will have bgp neighbor to the ISP NY router bgp to ISP1 and advertise 1.1.1.0/24. LA router bgp to ISP2 and advertise 2.2.2.0/24. When the 1.1.1.0/24 router goes to LA, my router will drop it since the AS number is the same which means the LA router will not recieve the NY route, but if I only receive the bgp default route from ISP, I believe it should be fine, correct?

Another question is how to receive the specify route 1.1.1.0/24 to LA instead of default route. I would like to know some options. AS-PATH allow in is one, how about replace-AS? I would like to know how ISP normally work this out.

Thanks again

Ray

[Jon Marshall]

Ray

but if I only receive the bgp default route from ISP, I believe it should be fine, correct?

Correct. The only issue with the AS in the path is at your two offices, the rest of the internet will be fine including the ISPs you are connected to.

The allowas-in command is something you configure on your own routers ie. it is not done on the ISP routers so you have conrol over which paths are allowed.

How your ISP handles this depends i guess on the ISP. They can override the AS Path with as-override command or replace it as you say but this is done primarily to remove private AS numbers from the AS Path. You however have a public AS number.

It's worth mentioning again that from the ISP point of view this is not an issue. It is purely an issue when it your sites receive the route and see their own AS.

Jon

[raymond wang]

agree. with the default route we should be fine then.

Thank you guys to confirm this.

[Richard Burts]

Ray

Yes you should be fine if both sites are receiving the default route from their provider.

HTH

Rick

[raymond wang]

Thanks Richard