BGP synchronization Issue

Answered Question
Mar 6th, 2009

Hey Guys,

Sync rule says that BGP transit as will never send the route from 1 as to another as untill unless it is learned via IGP, but I have the same routes installed in the IGP table through redistribution, but still my transit AS is not sending the traffic to adjecent AS if I enable sync., why is it so..

Thanks in advance

I have this problem too.
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Correct Answer by Harold Ritter about 7 years 9 months ago

Harish,

The synchronization might be an issue if you use ospf as your IGP and you also use route-reflectors. The reason is that with ospf as an IGP, the BGP synchronization check doesn't only verify that the route as been learnt via the IGP but also that the advertising router for that ospf route is equal to the BGP peer RID, which will not be the case if you use route-reflectors.

In this case the only solution is to disable the synchronization, which should be turned off by default anyway.

Regards

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Istvan_Rabai Fri, 03/06/2009 - 23:48

Hi Harish,

- Are you sure the BGP router receives the same routes from an IBGP peer as well?

- Did you check if the routes don't have one of the following community attributes attached:

no-export, local-as, no-advertise.

Because of lack of information I have no other ideas.

Cheers:

Istvan

harish.ab Sat, 03/07/2009 - 08:25

Hello Istvan

Thanks for your reply.. I m receiving the same route through mu ospf and IBGP since ospf prefers in terms of AD only that is installed in the routing table and when i debug BGP updates i m seeing an error of 'no valid route towards x.x.x.x network .

johnlloyd_13 Sat, 03/07/2009 - 20:22

do a show ip bgp and check whether that network is valid and chosen as best. also, do trace route on the network you're having errors. check if the command next hop self is on that neighbor.

harish.ab Sat, 03/07/2009 - 21:45

in show ip bgp , i can see that it is learned as internal route and valid and I am able to do a ping and trace also..i have manipulated next hope and its taking as the expected one.. but still. If i disable synchronization, my router is sending that update to another AS.. It is very strange. !

Harold Ritter Sun, 03/08/2009 - 09:59

Harish,

Could you please attach the "show ip bgp " from the router where you expect the prefix to be advertised to the eBGP peer.

Regards

Mohamed Sobair Sat, 03/07/2009 - 01:33

Hi,

Sync rule states: " ABGP speaker will not advertise any IBGP learned route to any EBGP peer unless the route is validated by the IGP".

If your EBGP neighbor doesnt recieve any updates from the Synch enabled router, then possible cause could be a filteration issue. Check out your filtering. also check if the routes has been installed in the BGP table.

HTH

Mohamed

Correct Answer
Harold Ritter Sat, 03/07/2009 - 07:34

Harish,

The synchronization might be an issue if you use ospf as your IGP and you also use route-reflectors. The reason is that with ospf as an IGP, the BGP synchronization check doesn't only verify that the route as been learnt via the IGP but also that the advertising router for that ospf route is equal to the BGP peer RID, which will not be the case if you use route-reflectors.

In this case the only solution is to disable the synchronization, which should be turned off by default anyway.

Regards

harish.ab Sun, 03/08/2009 - 10:32

Hello HAROLD,

Great.. that was the issue.. Thank you very much for the support..

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