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BGP Path Selection !

Hi all, referring to this doc

http://www.cisco.com/en/US/docs/ios/iproute/command/reference/irp_bgp1.html#wp1012961

under bgp deterministic med... it said that the order in which paths are recieved also affect the selection process, i didnt understand how ?, suppose a bgp router receives a path to destination with local pref of 100. then after some time it recieves path to same destination from a different neighbour with local pref of 200, so wont it change the best path to new one ?

Can you kindly guide me in this !

Thanks

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Accepted Solutions
Blue

Re: BGP Path Selection !

Hey, Rocky:

I can see your confusion if you depend on only that document to clear things up for you. This is why I am providing you with the following supplement. See below.

In a nutshell, what they are saying is that the order in which the paths for the same prefix are received by a router can effect the best-path selection for MED-BASED path selection.

In other words, they are referring to the situation in which none of the higher precedence BGP attributes is configured. Or the default values exist. Therefore, the best path selection will boil down to executing a MED comparison and the path with the lowest MED will be selected as the best path. And it is in that SPECIFIC circumstance that the order in which prefxies are received can alter the best path selection.

Lastly, the reason why the order in which the routes are received effects the MED comparison is explained in the document below.

Makes sense?

Read this:

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094925.shtml

HTH

Victor

4 REPLIES

Re: BGP Path Selection !

Hi Ovais,

suppose a bgp router receives a path to destination with local pref of 100. then after some time it recieves path to same destination from a different neighbour with local pref of 200, so wont it change the best path to new one ?

Seems you need to know what is BGP Best Path Selection Algorithm.

Please check this link out!

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

In short if the factor in your case is only the local-preference then it should use the new one(local pref = 200) to be installed in the ip routing table.

Hopes this helps

Thot

New Member

Re: BGP Path Selection !

Dear Thot, i am aware of the selection process, thats why i got confused that why the order in which paths are recieved matters at all ?, can you kindly look the doc i pasted and see under bgp deterministic med and tell me why does the best path changes ?

Blue

Re: BGP Path Selection !

Hey, Rocky:

I can see your confusion if you depend on only that document to clear things up for you. This is why I am providing you with the following supplement. See below.

In a nutshell, what they are saying is that the order in which the paths for the same prefix are received by a router can effect the best-path selection for MED-BASED path selection.

In other words, they are referring to the situation in which none of the higher precedence BGP attributes is configured. Or the default values exist. Therefore, the best path selection will boil down to executing a MED comparison and the path with the lowest MED will be selected as the best path. And it is in that SPECIFIC circumstance that the order in which prefxies are received can alter the best path selection.

Lastly, the reason why the order in which the routes are received effects the MED comparison is explained in the document below.

Makes sense?

Read this:

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094925.shtml

HTH

Victor

New Member

Re: BGP Path Selection !

Hi Victor, as you know that there are lot of commands in BGP, and there is no single book that references to the maximum of these commands( i have read BGP4 command and config but didnt helped me much ), is there any resource that you guys will recommend that gives a good insite to all of BGP Commands ?

Thanks

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