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03.01.06
Cisco CCNP Certification:
The BGP Weight Attribute
 By
Chris Bryant
When you´re studying for the CCNP certification, especially the BSCI exam, you
must gain a solid understanding of BGP.
BGP isn´t just one of the biggest topics on the BSCI exam, it´s one of the largest.
BGP has a great many details that must be mastered for BSCI success, and those
of you with one eye on the CCIE must learn the fundamentals of BGP now in order
to build on those fundamentals at a later time.
Path attributes are a unique feature of BGP. With interior gateway protocols such
as OSPF and EIGRP, administrative distance is used as a tiebreaker when two routes
to the same destination had different next-hop IP addresses but the same prefix
length. BGP uses path attributes to make this choice.
The first attribute considered by BGP is weight. Weight is a Cisco-proprietary
BGP attribute, so if you´re working in a multivendor environment you should work
with another attribute to influence path selection.
The weight attribute is significant only to the router on which it is changed.
If you set a higher weight for a particular route in order to give it preference
(a higher weight is preferred over a lower one), that weight is not advertised
to other routers.
BGP uses categories such as "transitive", "non-transitive", "mandatory", and "optional"
to classify attributes. Since weight is a locally significant Cisco-proprietary
attribute, it does not all into any of these categories.
The weight can be changed on a single route via a route-map, or it can be set
for a different weight for all routes received from a given neighbor. To change
the weight for all incoming routes, use the "weight" option with the neighbor
command after forming the BGP peer relationships.
R2(config)#router bgp 100
R2(config-router)#neighbor 100.1.1.1 remote-as 10
R2(config-router)#neighbor 100.1.1.1 weight 200
Learning all of the BGP attributes, as well as when to use them, can seem an overwhelming
task when you first start studying for your BSCI and CCNP exams. Break this task
down into small parts, learn one attribute at a time, and soon you´ll have the
BGP attributes mastered.
About the Author:
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage (www.thebryantadvantage.com),
home of FREE CCNA and CCNP tutorials and The Ultimate CCNA Study Package. (CCNP
Study Packages are on the way!) Video courses and training, binary and subnetting
help, FREE tutorials, and corporate training are also available.
For a FREE copy of my latest e-books, "How To Pass The CCNA" or "How To Pass The
CCNP", send a request to chris@thebryantadvantage.com
today ! |
