H3C Technologies H3C S12500 Series Switches - Manual (Page 212)

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Figure 76 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). The receiving router processes the route (for example, determining whether to
advertise the route and the scope for advertising the route) based on the COMMUNITY attribute
values rather than complex filtering rules (for example, ACLs). This simplifies routing policy usage
and facilitates 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—After received, 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—After received, routes with this attribute cannot be advertised to other BGP
peers.
{
NO_EXPORT_SUBCONFED—After received, routes with this attribute cannot be advertised out
of the local AS or other sub-ASs in the local confederation.
You can define community lists based on the BGP community attribute to filter BGP routes.
• Extended community attribute
To meet increasing user demands, BGP defines a new attribute—extended community attribute.
The extended community attribute has the following advantages over the community attribute:
{
The extended community attribute has an eight-byte length.
{
The extended community attribute supports various types.