Friday, January 22, 2010

BGP Attributes

Today I studied the different BGP Attributes that are used to determine routes. There are Well-Known Attributes and then there are Optional Attributes. Well-Known attributes must be recognized and propagated to BGP neighbors. Optional Attributes may be propagated to neighbors depending on the attributes meaning. Within these two types of attributes, there are sub attributes.

Well-Known Mandatory Attributes (Must be included in BGP Updates):
- AS-path
- Next hop
- Origin

Well-Known Discretionary Attributes (Not mandatory to be included in BGP Updates):
- Local preference
- Atomic aggregate

Optional Transitive Attributes (Must be passed to other AS's even if attribute isn't used):
- Aggregator
- Community

Optional Nontransitive Attibutes: (Doesn't have to be passed to other AS's):
- Multiexit-discriminator (MED)

I've also reviewed how BGP Synchronization works along with the many BGP message types. Synchronization tells BGP to wait until all routers have the same IGP information before updating other AS's with the info when using redistribution of BGP into IGP. From what I've been reading synchronization is outdated as most BGP updates are to large to be redistributed into IGP's anyways (scalability). BGP message types are used for establishing BGP neighbors along with providing keep-alives and BGP router updates

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