HP VPN Firewall Appliances Appendix Protocol Reference

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Figure 26 LOCAL_PREF attribute
• COMMUNITY
The COMMUNITY attribute identifies the community of BGP routes. A BGP community is a group
of routes with the same characteristics. It has no geographical boundaries. Routes of different ASs
can belong to the same community.
A route can carry one or more COMMUNITY attribute values (each of which is represented by a
four-byte integer). A router uses the COMMUNITY attribute to determine whether to advertise the
route and the advertising scope without using complex filters, such as ACLs. This mechanism
simplifies routing policy configuration, management, and maintenance.
Well-known community attributes involve the following:
{ INTERNET—By default, all routes belong to the Internet community. Routes with this attribute can
be advertised to all BGP peers.
{ NO_EXPORT—Routes with this attribute cannot be advertised out of the local AS or out of the
local confederation, but can be advertised to other sub-ASs in the confederation. For
confederation information, see "Settlements for problems in large-scale BGP networks."
{ NO_ADVERTISE—Routes with this attribute cannot be advertised to other BGP peers.
{ NO_EXPORT_SUBCONFED—Routes with this attribute cannot be advertised out of the local AS
or other sub-ASs in the local confederation.
You can configure BGP community lists to filter BGP routes based on the BGP COMMUNITY
attribute.
BGP route selection
BGP discards routes with unreachable NEXT_HOPs. If multiple routes to the same destination are
available, BGP selects the best route in the following sequence:
1. Highest Preferred_value
2. Highest LOCAL_PREF