EBGP-Multihop

[sharma.neeraj]

Hi,

Is ther anybody who can tell me about pros & cons using EBG-Multihop.

Specially in MBGP environment with RR deployment.

There is a case in which we are getting AS_Path at CE router as moer than 255 Limit....hence CE sends a message to PE to reset the Connection..this somewhat as AS_Path loop condition....BGP command of AS-override is also used...

[mheusinger]

Hello,

eBGP-multihop allows to loadshare through IGP between two external BGP peers (Pro). Another usage is with InterAS MBGP peering, where one option is to peer between the RR of the two AS.

You have to make sure however that there are no IP routing issues like routing loops, because of the additional routing entries needed in the eBGP peer.

With respect to your specific problem: Can you tell more about your setup, possibly configs and error messages. From what I understand about your problem:

Using AS-override implies the usage of SOO to prevent loops. A possible PE config would look like this:

router bgp 65000

address-family ipv4 vrf A

neighbor 10.1.1.1 remote-as 65001

neighbor 10.1.1.1 as-override

neighbor 10.1.1.1 route-map A-SOO in

exit-address-family

route-map A-SOO permit 10

set extcommunity soo 65000:1234

Make sure to apply this route-map to every BGP neighbor connected to the same customer site.

Hope this helps! Please rate all posts.

Regards, Martin

[sharma.neeraj]

Hi Martin,

Thanks for Response.

Moreover this type of error message appears on Router.I'm interested wat culd be reason..

Jan 25 11:03:41.369 UTC: %BGP-6-ASPATH: Long AS path 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 2874 received from 195.17.6.98: Has more than 255 AS

Jan 25 11:03:41.369 UTC: %BGP-5-ADJCHANGE: neighbor 195.17.6.98 vpn vrf vvreadnet-gtwy Down BGP Notification sent

Jan 25 11:03:41.369 UTC: %BGP-3-NOTIFICATION: sent to neighbor 195.17.6.98 3/11 (invalid or corrupt AS path) 516 bytes 50020200 02FF0B3A 0B3A0B3A 0B3A0B3A 0B

Jan 25 11:04:19.853 UTC: %BGP-5-ADJCHANGE: neighbor 195.17.6.98 vpn vrf vvrdnet-gtwy Up

bst618

Jan 25 12:03:41.370 MET: %BGP-3-NOTIFICATION: received from neighbor 195.17.6.97 3/11 (invalid or corrupt AS path) 516 bytes 50020200 02FF0B3A 0B3A0B3A 0B3A0B3A 0B

Jan 25 12:03:41.370 MET: %BGP-5-ADJCHANGE: neighbor 195.17.6.97 vpn vrf vvrdnet-gtwy Down BGP Notification received

Jan 25 12:04:19.851 MET: %BGP-5-ADJCHANGE: neighbor 195.17.6.97 vpn vrf vvrdnet-gtwy Up

[pkhatri]

Hi Neeraj,

The issue here is that the received AS-PATH has more than 255 AS numbers in it... Obviously, some router along the path is pre-pending the AS 2874 like crazy... I can't see any scenario where this would be required.

So what is happening is that the receiving router receives this ridiculously long AS PATH and decides to close the session by sending a NOTIFICATION message.

I would be interested in seeing your BGP config on both routers if possible...

A temporary fix to this problem is to use the following command:

'bgp maxas-limit 2000'

That will allow an AS-PATH of length 2000.

Hope that helps - pls rate the post if it does.

Paresh

[sharma.neeraj]

Hi Paresh..

Thanks,but this is what i also wanted to know how we this is accumulating like hell..

Since we used SOO also with AS-Overside..then y it's creating Loop.

Wat we did for the solution is resduced the

neighbor EBGP multihop Remote -As 255

we reduced the 255 to 5 .

After this problem doesn't appear..

but even i'm interested is how this happens.