You have to master the details on BGP to pass the BSCI exam and to earn your CCNP, but BGP is an entirely new world from the protocols you studied to earn your CCNA. BGP paths contain attributes, while no protocol you studied for the CCNA carried. BGP Attributes are used to choose the best path when multiple loop-free paths exist, as well as give you other specific information about the paths. This additional information includes the autonomous systems that are along the path to a given destination, what the next-hop IP address is, and much more.
Before we examine the specific attributes, we need to understand the categories used to differentiate BGP attributes. Some attributes are required, some aren't some attributes will be carried between routers, where others will not.
The first category is the well-known mandatory attribute. As you'd expect, these attributes are required and will be understood by all BGP speakers. Mandatory attributes include the origiin code, AS_Path, and next-hop.
Well-known discretionary attributes don't have to be present, but if they are , all BGP speakers will understand their meaning. BGP attributes that fall into this category are the MED, local preference, and atomic aggregate.
Optional transitive attributes may not be fully understood by all BGP speakers, but the attributes are sent between routers as paths are exchanged. The aggregator and community attributes fall into this category.
Finally, we have the optional nontransitive attribute. If a BGP speaker does not understand this attribute, the speaker will not forward the attribute. The Originator ID and Cluster ID are optional nontransitive attributes.
There's one important BGP attribute that was left out of this list indeed, if you're working in an all-Cisco environment, it may be the most important attribute of all. The weight attribute is Cisco-proprietary, so if you're working in a multivendor environment, this attribute is of limited value. However, the weight attribute is the first attribute considered when BGP is deciding between valid, loop-free paths, so it's an attribute we have to keep in mind. The weight attribute doesn't really fit in any of the four BGP classes we talked about earlier in the article.
If you don't know what these attributes do yet, that's okay. We'll examine each of these attributes in more detail in the next part of this free BGP tutorial. Keep studying!
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Article added to SearchWarp.com on Friday, December 16, 2005 View other articles written by Chris Bryant CCIE 12933(13,724)
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