Ask the Experts - Config eBGP: two neighbour with the same AS

[chuongnh]

Dear Experts,

We have a problem about eBGP configuration.

Now my customer need config 2 physical circiut to our network.

The first circiut use for internet traffic, other circiut use for peering between IXPs in our country.

We think, we create 2 neighbour with the same customer's AS, and separate policies between 2 neighbour.

The example below:

neighbor 202.78.224.41 remote-as 24085

neighbor 202.78.224.41 ebgp-multihop 255

neighbor 202.78.224.41 version 4

neighbor 202.78.224.41 remove-private-AS

neighbor 202.78.224.41 route-map CUSTOMER_IN in

neighbor 202.78.224.41 route-map CUSTOMER-OUT out

neighbor 203.113.158.158 remote-as 24085

neighbor 203.113.158.158 version 4

neighbor 203.113.158.158 remove-private-AS

neighbor 203.113.158.158 route-map CUSTOMER-PEER-IN in

neighbor 203.113.158.158 route-map CUSTOMER-PEER-OUT in

Could you tell me this config can be done or not ??

Beacause i check in ietf, i see only draft about multisession BGP. I'm not sure about it and want to ask the Experts about this problem.

Thanks for your support!

[pkhatri]

Hi,

It is perfectly legal to have two or more BGP session with neighbors in the same remote AS.

There should be absolutely no problems with using such a config. You'll just have to be mindful that if you advertise any routes you learn over one of the sessions over the other session, the router at the other end will drop them since it will see its own AS in the AS Path ...

Pls do remember to rate posts.

Paresh

[chuongnh]

Thanks for your information.

According to your comment, i'm still not sure about this problem. Beacause i have not never seen this config before.

When i search this problem in internet, no documents deal with it except draft of ietf.

About routing information, i'm sure about separate routes between two sessions.

Notice: this is an eBGP configuration. Our AS is 7552

router bgp 7552

neighbor 202.78.224.41 remote-as 24085

neighbor 202.78.224.41 ebgp-multihop 255

neighbor 202.78.224.41 version 4

neighbor 202.78.224.41 remove-private-AS

neighbor 202.78.224.41 route-map CUSTOMER_IN in

neighbor 202.78.224.41 route-map CUSTOMER-OUT out

neighbor 203.113.158.158 remote-as 24085

neighbor 203.113.158.158 version 4

neighbor 203.113.158.158 remove-private-AS

neighbor 203.113.158.158 route-map CUSTOMER-PEER-IN in

neighbor 203.113.158.158 route-map CUSTOMER-PEER-OUT in

[pkhatri]

Maybe I'm really understanding the issue here...

Are you asking whether or not it is possible for a single router to have more than EBGP session to the same AS ? Is that the question ?

Paresh

[chuongnh]

In my case, this is a configuration in single router.

This single router need config two separate sessions to the same AS on separate physical circiut.

[pkhatri]

Ok, so I did understand you correctly. So the comments I made in my first post are still valid.

You won't find this in any RFCs/drafts because there is nothing really special about this.

Here's a simple example of dual-homing to a single AS:

http://www.cisco.com/warp/public/459/40.html#conf2

Pls do remember to rate posts.

Paresh

[chuongnh]

Hi pkhatri,

You send me the URL about load sharing. Could you see in session: Load Sharing When Dual-Homed to One Internet Service Provider (ISP) Through a Single Local Router . I think you understand me like this case.

But in my case, RA =====RB

- RA in AS 7552, RB in AS 24085.

- RA have 2 physical link connect to RB.

And i have to config like my example, Please notice that, only one router in AS 7552 and only one router in AS 24085.

In your URL, RA in AS11 connect to 2 routers RB n RC in AS 10. It's different with my case.

[pkhatri]

Ok, the example I gave you was slightiy different. However, there is no problem with running 2 EBGP sessions between the same two routers (as long as each of the sessions peers to a different address). However, there is not much point in doing so. Even if you enforce different policies over the two sessions, all routes learned via the two eBGP sessions will end up in the same BGP routing table ... So I don't see much point in doing this.

Pls do remember to rate posts.

Paresh

[chuongnh]

Hi,

This problem with your experience is ok. But i still not sure about multisession. Can you so me some example about this case or some documents practise about it.

Thanks so much !

[pkhatri]

Hi,

I'm afraid you will not find too many examples of this since people don't use it much in practice.

However, there is one example of this in the book 'Internet Routing Architectures'... it does not have the config but has a figure and outlines the reasons why you would not use it.

Pls do remember to rate posts.

Paresh

[mheusinger]

Hello,

So you have eBGP between two routers RA and RB connected with two physical interfaces.

From BGP update behaviour you will get the same updates from a single peer, no matter how many BGP sessions you setup, because only the best path for a prefix is sent. And your router will only have one BGP table. So the question is: what do you want to gain from having two eBGP sessions between RA and RB?

The only technical answer I can think off is to direct traffic to certain destinations over a specific physical link (correct me if I am wrong). The same can be achieved however by using eBGP multi-hop between RA and RB loopbacks and an inbound route-map setting the BGP next-hop to the desired physical link interface IP.

This setup would reduce number of BGP updates and is well known with example configs readily available.

Hope this helps! Please rate all posts.

Regards, Martin

[chuongnh]

Hi,

About routing update, i think like this:

With incoming traffic from our neighbour:

- With the first circiut, we will advertise prefix-list A to the first neighbour.

- With the second circiut, we will advertise prefix-list B to the second neighbour.

- A different B.

With ougoing traffic to our neighbour:

- With the first circiut, we will receicve routing information from the first neighbour normally

- With the second circiut, we will set an access-list extended with source address and destination address. After that, apply this access-list to route-map. This route-map will apply on the second circiut with next-hop ip of second physical service. Every destination we want to control go through the second receiving will be done.

If you see any problem, please post your reply.

[chuongnh]

Hi pkhatri ,

Could you show me which page description like you said. I will have information quickly.

Let me check and send you a real config we will apply.

Many thanks.

[rtaulton]

It is not entirely clear to me what you are trying to do. Are you looking for a method to distribute load? If so you can receive the routes you need via both paths and adjust bgp maximum-path parameter. By default BGP will only populate the routing table with one path regardless of the number of possible paths even if they have the same costs. Use bgp maximum-path n (where n is the number of paths you want to load balance) to be able to load-balance the paths. If you are looking to route some routes via one interface and other routes via the other interface that is a different issue. Depending on the number of routes you want to specify there are a number of methods for doing this. It would seem to me that if you are connecting to just one remote router from one local device you are either concerned about load or want to prioritize some traffic over other traffic. Again it is hard to tell exactly what you want to do.

[pkhatri]

Hi,

It is on page 213 of the original edition.

Paresh