BGP Route Determination and the BGP Decision Process
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Limitations on BGP's Ability to Select Efficient Routes
When considering the route selection process, it's very important to remember that BGP is a routing protocol that operates at the inter-autonomous-system level. Thus, routes are chosen between ASes and not at the level of individual routers within an AS.So, for example, when BGP stores information about the path to a network, it stores it as a sequence of autonomous systems, not a sequence of specific routers. BGP cannot deal with individual routers in an AS because by definition, the details of what happens within an AS are supposed to be hidden from the outside world. It doesn't know the structure of ASes outside its own.
However, this has an important implication for how BGP selects routes: BGP cannot guarantee that it will pick the fastest, lowest-cost route to every network. It can select a route that minimizes the number of ASes that lie between itself and a particular network, but of course ASes are not all the same. Some ASes are large and consist of many slow links, while others are small and fast. Choosing a route through two of the latter type of AS will be better than choosing a route through one of the former, but BGP can't know that. Policies can be used to influence AS selection to some extent, but in general, since BGP doesn't know what happens in an AS, it cannot guarantee the efficiency of a route overall. (Incidentally, this is the reason why there is no general cost or distance path attribute in BGP.)
Naturally, a facility exists to allow BGP speakers to originate new routes to networks. A BGP speaker may obtain knowledge about a new route from an interior routing protocol on an AS to which it is directly attached and choose to share this information with other ASes. It will create a new entry in its RIB for this network and then send information about it out to other BGP peers.
BGP also includes a mechanism for not only advertising routes it can reach, but those it cannot. The latter are called unfeasible or withdrawn routes, and are mentioned in Update messages as needed to indicate that a router can no longer reach the specific network.
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