Operation Manual – IPv4 Routing
H3C S5500-EI Series Ethernet Switches
Chapter 5 BGP Configuration
5-9
In general, BGP compares MEDs of routes to the same AS only.
Note:
You can use the
compare-different-as-med
command to force BGP to compare MED
values of routes to different ASs.
5) LOCAL_PREF
This attribute is exchanged between IBGP peers only, thus 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 5-9
LOCAL_PREF attribute
6) COMMUNITY
The COMMUNITY attribute is used to simplify routing policy usage and ease
management and maintenance. It is 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 include:
z
Internet: By default, all routes belong to the Internet community. Routes with this
attribute can be advertised to all BGP peers.
z
No_Export: After received, routes with this attribute cannot be advertised out the
local AS or out the local confederation but can be advertised to other sub-ASs in
the confederation (for confederation information, refer to
Settlements for Problems
Caused by Large Scale BGP Networks
).