Bgp Route Selection - 3Com MSR 50 Series Configuration Manual

832 C 56: BGP C
HAPTER ONFIGURATION

BGP Route Selection

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 AS 20 to AS 10
travels through Router C that is selected according to LOCAL_PREF.
Figure 239 LOCAL_PREF attribute
EBGP
8.0.0.0
Router A
EBGP
AS 10
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, having nothing to do
with 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
for Problems Caused by Large Scale BGP Networks" on page
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.
Route selection rule
The current BGP implementation supports the following route selection rule:
Discard routes with unreachable NEXT_HOP first
■
Select the route with the highest Preferred_value
■
Select the route with the highest LOCAL_PREF
■
Select the route originated by the local router
■
LOCAL_PREF=100
Router B
2.1.1.1
IBGP
NEXT_HOP=2.1.1.1
LOCAL_PREF=100
Router D
IBGP
D=8.0.0.0
NEXT_HOP=3.1.1.1
IBGP
LOCAL_PREF=200
3.1.1.1
AS 20
Router C
LOCAL_PREF=200
"Settlements
835).