bgp communities

nel

hey,

Im just busy reading up BGP communities as i have to do some config stuff for these at work and have a question. Am i right in thinking bgp communities are used to tag/mark routes which then can have a routing policy applied to them? For example, all routes marked with X can have a local pref of 150 set, all routes marked with Y can have a local pref of 100?

For some reason its keep throwing me when i see the sytax, i think its because i dont understand it properly. can someone explain to me where you derive the command values from?

For example, the following source shows the AS100 being marked with the following command: Using BGP Community Values to Control Routing Policy in Upstream Provider Network  [IP Routing] - Cisco Systems

set community 100:250

I take it the 100 value indicates the AS but What i dont understand is what is the 250 value for and how do you get that value?

Later on in the output you can see that the community is matched using a route map and that the local pref is defined.

i would appreciate it if someone could clear the values up as no matter what i read or where i read it i cant seem to see where you get them from . Any explanations are much appreciated.

Thanks

SteveO86

You have a route map matching to communities and setting the desired metric. Like in the link you have.

To left of the colon is the AS Number, the numbers to the right can be configured by you as long as their is a route map that matches the right community you are sending then you can set whatever metric you want. Although it would be easier (and best practice) to keep the community somewhat relevant to it's purpose but it's not necessary.

Hope this helps.

Extract from Cisco's Site:

A community is a group of destinations that share some common attribute. Each destination can belong to multiple communities. Autonomous system administrators can define to which communities a destination belongs. By default, all destinations belong to the general Internet community. The community is carried as the communities attribute.The communities attribute is an optional, transitive, global attribute in the numerical range from 1 to 4,294,967,200