So we're now going to move on and have a look at BGP's attributes. At first, by way of introduction, we're going to look at what an attribute actually is. It's part of the BGP update, so when a BGP router sends routing information to its neighbor, not only does it send the prefix, it sends lots of other pieces as well and these pieces are really the characteristics of the prefix or "the attributes". So it would be things like, the origin, the Autonomous System (AS) path, next hop, metric, community, and so forth, just listing some of these attributes. And this is really, little bits of useful information for the neighboring AS, or for other Autonomous Systems around the network. Some of these attributes are transitive, in other words, they are passed from Autonomous System to Autonomous System. Other of the attributes are non-transitive, whether they exist just within the AS, or only used between two Autonomous Systems. Some of the attributes have to be set, some of them are entirely optional. The operator needs them then they're used, if the operator doesn't need them then they're left alone. So the attributes we're going to look at in this section include, the AS-PATH, the NEXT-HOP, the ORIGIN, the AGGREGATOR, LOCAL_PREFERENCE, Multi-Exit Discriminator, and the COMMUNITY. We've included weight in the list. Now weight is not strictly a BGP attribute, in that it's not passed within the BGP update information, it's only local to the router. But it is part of the specification and has been implemented by several vendors as well.
© Produced by Philip Smith and the Network Startup Resource Center, through the University of Oregon.
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