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Cisco CCNP Certification: The BGP Weight Attribute |
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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:
Article by Chris Bryant.
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