I hav a small query regarding ibgp session, possibly an awkard question but i got to clear this doubt from all the champs out there.
Y is ibpg peering required between 2 internal routers connected to ISP's(say 2 isp's).in case one of the ebgp path is inaccessibe,the first router needs to use the second internal router to move thru second isp via second ebgp link...but then wouldnt an internal routing protocol other than ibgp suffice for this?
No an internal routing protocol would not suffice for this. Your 2 routers running BGP need to exchange information with each other that can not be included in an internal routing protocol (for example they need to share prefixes that they have learned with the AS path advertised with the prefix - how do you send AS path using an internal routing protocol?).
I totally agree with you that BGP would be required if his routers are receiving the full/partial internet routing table and using the AS-path info to route to the internet
But If the user is using BGP only to advertise his subnets and just uses a default route for routing to the internet then an IGP can be used between his routers. (for eg ospf can be configured between his routers to send the default information)
Narayan, only saw your post after my post. I too had thought about usage of default routes and an IGP, but since the OP asked about "in case one of the ebgp path is inaccessibe", I think we need to assume we're working with full route tables. Otherwise a default could forward a packet to a next hop that has the missing path. (Of course, why one ISP has a path/router that the other doesn't, is a different issue.)
Re-reading the OP, if we're talking about the physical path is lost to the one ISP, then the default route might save the need to pass one router's routes to the other, which could be done via an IGP, although I still wonder whether is would be advisable not to have the two internal routers in BGP communication (even if they filter their routes and pass a default).
If I'm not clear, I read the original post as "in case one of the ebgp path is inaccessibe", meaning one external path (route) is lost from one ISP, but Narayan's implied interpretation, that the path on one ISP is loss, I now think is more likely correct.
Non-BGP internal routing alone might suffice, but it would be inadvisable, especially when dealing with the Internet. For instance, both eBGP routers could redistribute into an IGP, then they would "know" of all routes. However, first, most IGPs wouldn't be happy with full Internet route tables, second, except for Internet routes known to only one router, the other router wouldn't, by default, know of shorter AS paths. Third, you might bump into some BGP issues if your internal routers within the same AS don't communicate with each other via iBGP or if they are in different ASs but advertise to other ASs the same network(s).
Beyond thinking in theory what might be done, I can't think of any advantage of trying to use an IGP in lieu of iBGP.
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This is actually a pretty cool feature, i didn't even know it existed until I was looking for a solution to advertise a subnet (prefix in BGP talk), only if a certain condition existed. This is exactly what conditional advertisements does
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