H3C Technologies H3C S12500-X Series Switches - Manual (Page 192)

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Figure 49 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
4-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 "
1059H
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.
• Extended community attribute
To meet new demands, BGP defines the extended community attribute. The extended community
attribute has the following advantages over the COMMUNITY attribute:
{
Provides more attribute values by extending the attribute length to eight bytes.
{
Allows for using different types of extended community attributes in different scenarios to
enhance route filtering and control and simplify configuration and management.