BGP Cost Community and Point of Insertion

Unanswered Question
Dec 1st, 2008

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

I have this problem too.
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Harold Ritter Mon, 12/01/2008 - 11:22

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

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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