BGP Cost Community and Point of Insertion

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Dec 1st, 2008
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Hi

Looking at the BGP Cost Community, I keep reading about how this influences the BGP best path selection process at the point of insertion (POI). Does this mean that the decision is taken immediately after step 8 in the Best Path Algorithm - ie Prefer the path with the lowest IGP metric to the BGP next hop?

For some reason I cannot get my head round this POI stuff.

Thanks


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Harold Ritter Mon, 12/01/2008 - 11:22
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  • Cisco Employee,

Walter,


The POI indicates when, in the BGP best path selection process, the cost community is taken in consideration. There is only two choices for the value of the POI:


Router(config-route-map)#set extcommunity cost ?

<0-255> Community ID

igp Compare following IGP cost comparison

pre-bestpath Compare before all other steps in bestpath calculation


Router(config-route-map)#set extcommunity cost



So as you can see it can be either compared before everything else (pre-bestpath) or right after the IGP cost comparison (igp).


Regards

Robin Martinez Thu, 10/21/2010 - 14:33
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When I look at the options for this command it appears to me that there are actually 3 different scenarios: I can select IGP, pre-bestpath or I can not specify the POI at all.




The question is, if I don't specify, which is chosen? The documentation leads me to believe that it would be IGP by default but the fact that there is a specific keyword for IGP makes me doubt myself.



Any thoughts?







Edit:


Nevermind, I figured this out the easy way, just try it. If you don't specify either option it does just default IGP and actually adds that keyword into the config.




IR0(config)#route-map test-cost-comm

IR0(config-route-map)#set extcommunity cost 1 100


IR0#show run | sec route-map test-cost-comm

route-map test-cost-comm permit 10

set extcommunity cost igp 1 100


IR0#show route-map test-cost-comm

route-map test-cost-comm, permit, sequence 10

  Match clauses:

  Set clauses:

    extended community Cost:igp:1:100

  Policy routing matches: 0 packets, 0 bytes




Message was edited by: Robin Martinez

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