3.7.d Implement optimize and troubleshoot routing policies

3.7.d [iv] Communities, extended communities

the community attribute by itself is simply a tag to employ some kind of manipulation… it is a means to an end, not the end as the earlier evil routers link would have you believe…

the idea that the low order bits (the second half of the new format) actually prescribe the local preference is incorrect.  one can make an association with that number and a set local preference but it is not automatic… that is the fallacy in that article… ie, you could set the local preference to 200 in a route map AND the lower bits to 200 to be the same, or you could set the local preference to 110 and the lower bits to 200 and there is suddenly no correspondence between the two other than that you set the local preference of 110 to match the community string of 200…

from: http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a00801475b2.shtml

While communities themselves do not alter the BGP decision making process, communities can be used as flags in order to mark a set of routes. Upstream service provider routers can then use these flags to apply specific routing polices (for example, local preference) within their network.

Providers establish a mapping between customer configurable community values and the corresponding local preference values within the provider network. The idea is that customers with specific policies that require the modification of LOCAL_PREF in the provider network set the corresponding community values in their routing updates.

the fact is that while setting the local preference to a corresponding community value is helpful for traffic engineering, the provider end will still have to cooperate… obviously that works great in a lab where you are both the customer and the provider… in a production network you will not be so lucky… the isp will typically supply you with a best path, that is their job; expecting them to cooperate on their end with your attempts at manipulation to extend into their network may not be realistic…

for the sake of the ccie lab exam this is essential to know how to do; the reality is you will probably never get this opportunity…