372 CHAPTER 30: BGP CONFIGURATION
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 123 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:
■ 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
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” on page
375).
■ 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 the local AS or other ASs in the local confederation.
BGP Route Selection Route selection rules
BGP supports the following route selection rules:
■ Discard routes with unreachable NEXT_HOP first
■ Select the route with the highest Preferred_value
■ Select the route with the highest LOCAL_PREF
EBGP
Router B
Router A
Router C
Router D
D=8.0.0.0
NEXT_HOP=3.1.1.1
LOCAL_PREF=200
IBGP
IBGP
IBGP
EBGP
2.1.1.1
8.0.0.0
LOCAL_PREF=100
NEXT_HOP=2.1.1.1
LOCAL_PREF=100
LOCAL_PREF=200
3.1.1.1
AS 20
AS 10
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