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Cisco CCNP Certification: The Local Preference BGP Attribute
By Chris Bryant
Expert Author
Article Date: 2006-01-18
When studying for your BSCI exam for the CCNP, you get your first taste of BGP.
One of the major differences between BGP and the other protocols you´ve studied to date is that BGP uses attributes to describe paths, and to influence the selection of one path over the other.
In this free tutorial, we´re going to take a look at the Local Preference attribute and compare it to the Cisco-proprietary BGP attribute "weight".
The Local Preference (LOCAL_PREF) attribute is used to influence how traffic will flow from one Autonomous System (AS) to another when multiple paths exist. For example, if AS 100 has two different paths to a destination network in AS 200, the LOCAL_PREF attribute can be used to influence the path selection.
The major difference between the Weight and LOCAL_PREF attributes is that when the LOCAL_PREF attribute is changed, that change is reflected throughout the AS. The new LOCAL_PREF value will be advertised to all other routers in the AS, as compared to the Weight attribute, which is locally significant only. If you change the Weight for a path on one router in an AS, the other routers in the AS will not learn of the change.
A route-map can be used to change a local preference value. For example, if you want to change the local preference value to 200 for the path advertisement 10.2.2.0/24 coming in from neighbor 10.1.1.1, there are three steps involved. First, write an ACL matching the remote network you want to change the local preference for.
R1(config)#access-list 5 permit 10.2.2.0 0.0.0.255
Second, write a route-map setting the local preference to 200. This will double the default value of 100, and the path with the highest local preference will be the preferred path.
R1(config)#route-map PREFER_PATH permit 10
R1(config-route-map)#match ip address 5
R1(config-route-map)#set local-pref 200
Finally, apply the route-map to routes that are being received from 10.1.1.1.
R1(config)#router bgp 100
R1(config-router)#network 10.1.1.1 route-map PREFER_PATH in
R1 will then advertise this new local preference value to all other routers in AS 100 - all of its iBGP neighbors.
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 daily exam questions, as well as The Ultimate CCNA and CCNP Study Packages.
For a FREE copy of his latest e-books, "How To Pass The CCNA" or "How To Pass The CCNP", and for free daily exam question, visit the website and download your copies!
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