BGP Synchronization

[suguna]

Hi all,

Could you please clarify me the following:

Cisco documentation on BGP Synchronization says you can disable the Synchronization if one of the following is true:
1) If your AS doesnot pass traffic from one AS to another AS.
2)All the transit routers in your AS run BGP.

My question is as follows:
I agree with the (2) case as i assume they will be fully meshed in that case.

But I am unable to understand the case (1) above. Say for example i have 3 routers A, B and C with only A-C acting as BGP peers where as A-B and B-C are haiving IGP routing protocol running.
Now , if I disable synchoronisation, do nt you think that Router B will drop the packet going to external AS connected to Router A?

Please help me understand this concept.

Regards
Srinivas

 

[plshlpme]

I believe...

if your not passing traffic between two AS's then its an IBGP connection (internal)

Internal bgp connections have to be meshed and it is assumed that all neighbors will all have received the updates.

likely from a route reflector

 

[lathike]

Hi,

Thanks for your prompt reply.

I am still not convinced with your explanation.Let me make my question clear.

The routers A,B and C are in the same AS,say AS1 with only A connected to another external AS router(say router D in AS2).
Now if a traffic orinating from C needs to go to a network in AS2, then i believe that it may drop at router B(running no BGP)if you disable synchronization in the routers of AS1.Do you agree with me??

Thanks

 

[ChrisAC]

There are a couple of reasons for this. Firstly, you don't want your BGP routes being synchronised with your IGP as your IGP won't be designed to handle over 100,000 routes (presuming that you are running a full BGP configuration). This is one reason why sychronisation isn't really used any more.

Also, BGP can be quite processor intensive on your core BGP routes and you don't want BGP flapping to occur everytime a link goes does on your IGP.

Your IGP should advertise a default route so that in your case router C will have a default route for traffic to AS2 and router B will also have a default route via router A which will have a BGP session with AS2.

Chris.

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Chris A.C, CCNA, CCSA
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