AS_PATH vs. LOCAL_PREF in BGP

Unanswered Question
Feb 6th, 2007

I am preparing for my CCIE lab and I am working on understanding BGP in depth. I am reading the Routing TCP/IP (Cisco press) vol2 by Doyle. I am trying to understand the difference in AS_PATH vs. LOCAL_PREF in BGP. I am having a hard time understanding the difference in them (how they work and when to use each) Can some one give me more info on these 2 att., or point me in the direction to figure out how to best use these 2 att.

Thanks,

Chris

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sundar.palaniappan Tue, 02/06/2007 - 17:08

Chris,

As you may know, BGP uses a process to choose the best path when multiple paths to the same destination exist. The rules list govern the path selection process. If there is a tie in any rule then the next rule is applied and it keeps going till the best path is determined.

Local Preference ranks higher than AS Path rule. Routes with a higher local preference are preferred. Shorter AS path, equivalent of hops in IGP, is preferred. There's plenty of documentation on CCO to enlighten yourself on this topic.

In a real world situation, AS_Path is widely used to influence incoming traffic from another AS and local preference is used to influence outbound routes when multiple paths exist. Local preference is applicable only within an AS whereas AS Path is applicable between ASs.

Here's a couple of links that you may find useful.

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094431.shtml

http://www.cisco.com/en/US/tech/tk365/tech_configuration_examples_list.html

HTH

Sundar

ChrisCopley_2 Tue, 02/06/2007 - 17:53

Sundar,

Thank you for the detail explanation and the links. After reading your post and going over the link on CCO it is all crystal clear now. I went back and re-read the Doyle information and it makes sense what he is saying now. Greatly appreciated!!!

Thanks,

Chris

sundar.palaniappan Tue, 02/06/2007 - 18:02

Chris,

Glad that helped!!

Thanks for the rating. If you could please resolve the thread and that may serve as a useful reference for others.

HTH

Sundar

ChrisCopley_2 Tue, 02/06/2007 - 18:25

Sundar,

I am not sure how to mark it as resolved on the site. Can you enlighten me on how to do so?

Chris

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