Sunday, December 5, 2010

Salient features of the BGP routing protocol

* Border gateway protocol - as the name implies it is a protocol used among gateway nodes of the ASs

* Focuses on reachability (loop free) rather than optimality. - why? - it is hard to come across a single routing metric since transit ASs have different internal routing protocols and routing metrics.

* Trust is important in BGP - because it relies solely on route advertisements from peers

* Does not fall into distance vector or link state routing - advertizes complete path as a list of ASs

* Looping within BGP is avoided by advertising complete path AS prefixes - if an AS already sees itself in the list, it determines that the path is no longer useful.



* AS - numbers in BGP-4 are 16bit which is good for non-stub ASs which need to be unique. Stub ASs can have duplicate identifiers.

* BGP uses reliable messaging by using TCP as the transport protocol.

* Supports negative advertisements in the form of withdrawn route - when failure occurs.

* Route selection at every BGP speaker (gateway) is done based on a local decision - the decision to transit or not is also made here.

* BGP needs to scale only to the number of ASs - 65K (with a 16bit identifier). Intra-domain routing takes care of the routing within domains - hence we achieve a  scalable solution to internet routing.

* Route injection is done by a border router to let the inter-domain routing protocol know about the gateway to the remaining network. This can be done by the border router for specific prefixes or the default route.

* The iBGP routing protocol is used for scalable route injections within backbone networks - to accommodate the large number of BGP prefixes within every packet  broadcast by a border router to the routers within its domain - iBGP works in 2 parts - it provides every router in the backbone AS with the information on which border router to use to reach a particular prefix, and it also maintains a conventional intra-domain routing protocol to figure out how to reach the border routers.

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