Cisco CCNP Certification: BGP Attribute Category TutorialPage: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 or Go To our Certification Articles Section
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 origin 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!
About the Author:
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage,
home of free CCNP and CCNA tutorials, The Ultimate CCNA Study
Package, and Ultimate CCNP Study Packages. For a FREE copy
of his latest e-books, “How To Pass The CCNA” and
“How To Pass The CCNP”, just visit the website!
You can also get FREE CCNA and CCNP exam questions every day!
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