The LOCAL_PREF attribute is exchanged between iBGP peers only, and therefore is not advertised to any other
AS. It indicates the priority of a BGP router. LOCAL_PREF is used to determine the best route for traffic leaving
the local AS. When a BGP router obtains from several iBGP peers multiple routes to the same destination but
with different next hops, it considers the route with the highest LOCAL_PREF value as the best route. As shown
below, traffic from AS20 to AS10 travels through Router C that is selected according to LOCAL_PREF.
Figure 61: 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.
Chapter 17 Border Gateway Protocol (BGP)
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