Multihoming concept.

Hello, BGP is one of EGP protocols EIGRP , OSPF, RIP are one of IGP protocols

So You need to consider of using BGP or EIGRP or OSPF (EGp and IGP are jus type of protocol) To beter understending of EGP and IGP protocols behavior consult:

i know wat EGP, IGP are. thanx

You need to use BGP if you are connected to two (or more) different providers. If all your connections are to the same AS then you are effectively part of their AS and could use their IGP. Any books that say that BGP will produce different routing are simply wrong. In fact, BGP normally relies on the IGP for the detailed routing to the next-hop that BGP advertises.

you may be right on one part that igp does not provide you with suboptimal routing but when you use igp you are limited by the fact that they all depend on metric .lets say you have 2 isp and you get a route to xyz.com from 1 isp with a metric of 20 . and from isp 2 you get it with a metric of 30 you will choose isp 1 to go out . but what if for some reason you get a route for www.com with a lower metric from an isp2 then you will choose isp2 to go out to www.com . we must never forget traffic flows out of our network depend on metric advertised by our isp . traffic enters our network through metrics advertised by us. in short you can control how traffic can come in but if you want to control traffic to go out bgp is much more simple to implement. static route on the other hand is the most simple way of going out but again has his own issues .

Thanx man!! U cleared the concept how IGP chooses sub-optimal path but how BGP overcomes this drawback?

The concept of multihoming may be quite clear to you, but what you are calling multihoming is not at all clear. Unfortunately, multihoming is one of those networking terms which can have different meanings depending upon context, ranging from a single PC having two LAN connections (to the same LAN & subnet) to an international organization serviced by multiple, independent ISPs. As you can imagine, the optimal solution will depend upon the context.

Keep in mind that when implementing redundancy, there are two separate issues which must be addressed: How do you detect loss of a communications path and how do you compensate for that loss. Compensating may require propagating the knowledge of the loss well beyond the bounds of the router (or other device) which detects the loss. Also don't forget the third leg of the redundancy tripod, ensuring that the alternate path will actually be functional when needed.

If you are just getting into the realm of multihoming with two (or more) ISPs, you may find the white paper on the subject on my web site helpful in sorting out the issues.

Good luck and have fun!

Thanks

In bgp you can use local prefrence . cisco has some case studies that can help you with bgp .