KB.15.15

Figure 59 LOCAL_PREF attribute

COMMUNITY

The COMMUNITY attribute is used to simplify routing policy usage, and to ease management and

maintenance. It identifies a collection of destination addresses having identical attributes, without

physical boundaries in between, and having nothing to do with the local AS. Well known community

attributes involve:

Internet By default, all routes belong to the Internet community. Routes with

this attribute can be advertised to all BGP peers.

No_Export After being received, routes with this attribute cannot be advertised

out the local AS.

No_Advertise After being received, routes with this attribute cannot be advertised

to other BGP peers.

No_Export_Subconfed After being received, routes with this attribute cannot be advertised

out the local AS.

BGP route selection

Route selection rules

The current BGP implementation supports the following route selection sequence:

• Prefer the route with the lowest Administrative Distance.

• Prefer the route with the larger weight.

• Prefer the route with the highest LOCAL_PREF value.

• Prefer the path that was locally originated via a network or through redistribution from an

IGP.

• Prefer the route with the shortest path, excluding confederation segments.

• Prefer the route with the “best” ORIGIN. IGP is better than EGP, which is better than Incomplete.

• If bgp always-compare-med is not configured, prefer any routes that do not have an inferior

MED. If bgp always-compare-med has been configured, prefer the route with the lowest MED.

306 Border Gateway Protocol (BGP)