iBGP - What happens in the real world?

Unanswered Question
Feb 5th, 2008

Hi All,


As far as i can see with iBGP there are 2 options :-


1. synchronization is enable and redistribution of BGP into the IGP


2. no synchronization + every router (not just iBGP routers) part of a full mesh iBGP


If you take option 1 - the argument is one of scalability because you are redistributing BGP into OSPF or whatever IGp you have. Lots of routes here. Too many probably.


If you take option 2 - you still have to let transit routers in the AS know about the external routes. They must run BGP? If they don't, then the iBGP peers will enter all the BGP routes in the routing table (that's fine), but when they start forwarding to each other, the transit routers must also know about these routes or they will be dropped and black holed.


I think my summary is correct - if not, please let me know.


Question - what happens in the real world? What do ISP's and the like do? Which option?


Leave aside things like Route Reflectors for now - they just indicate option 2 to me - mesh.

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Danilo Dy Tue, 02/05/2008 - 04:50

Hi,


Explanation regarding BGP synchronization http://www.cisco.com/univercd/cc/td/doc/cisintwk/ics/icsbgp4.htm#wp19382


Well, in the real world, try not to have Figure 12-6 (non-BGP routers) because BGP will have to wait for IGP to catch up before it can advertise the route learned.


If you can't avoid non-BGP routers, try putting a IP GRE Tunnel between BGP routers (Router A & B in Fiugre 12-6).


IMHO, BGP routers are core and should be in optimum performance to route and switch as fast as possible.


Regards,

Dandy

fgleeson Tue, 02/05/2008 - 05:56

Thanks Dandy.


Sounds like the iBGP peers should be directly connected - over copper or fibre or whatever is required.


If you have a situation like 12-6, then you must :-

1. redistribute BGP into IGP

2. run BGP on the transit router also


Sounds like there is NO escaping it. It absolutely has to be one or the other.


Teh only other option is the one i mentioned earlier - directly connect the iBGP peers. Not always possible.


Thanks for the reply and the debate!!

attrgautam Tue, 02/05/2008 - 06:09

One of the options Service Providers have to enable a BGP free Core is do to enable MPLS on the Core. Enabling MPLS allows all BGP traffic from edge router to edge router to mapped to an LSP and the core need not be aware of BGP and just forward the traffic.

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