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When would you use internal BGP

Could someone explain and provide an example of when and why you would use internal BGP.  If someone could explain in their own words I would greatly appreciate it.
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denver218
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denver218
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Garry GlendownConsulting and Network/Security SpecialistCommented:
Typical "depends" question ... for one, when using things like MPLS VPN, BGP is more or less a "must" in order to supply the correct routing/forwarding information. For regular internal routing, protocols like OSPF, EIGRP or ISIS are most likely more efficient, as long as the amount of routes isn't too large. In our backbone (with ~10 locations, currently ~250 internal routes and ~50 MPLS VRFs), all public Internet as well as infrastructure routing is done using OSPF, mainly for low latency of routing changes etc., with iBGP doing just MPLS prefix distribution via Route Reflectors. This structure has been in place for almost 15 years now and has proven to be rather stable and relatively low maintenance, while at the same time allowing good flexibility for stuff like redundant connections to customer sites (via leased lines and DSL connections). Border routers of course distribute the external Internet prefixes via iBGP between each other, but do not redistribute into the backbone infrastructure; while our new backbone routers would be able to carry the full table, our structure of outside connections does not warrant the extra complexity of implementing this (YMMV - if you have multiple uplinks at different locations in your backbone, distributing outgoing traffic to the closest/best path to the outside may be better and/or required ...)
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denver218Author Commented:
Thanks.
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