3Com S7906E - Switch, Configuration Manual

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1-8
The LOCAL_PREF attribute is exchanged between iBGP peers only, and thus is 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 1-9 LOCAL_PREF attribute
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
6) COMMUNITY
The COMMUNITY attribute is used to simplify routing policy usage and ease management and
maintenance. It identifies 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 involve:
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 in Large Scale BGP Networks ).
z
No_Advertise: After received, routes with this attribute cannot be advertised to other BGP peers.
z
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
The current BGP implementation supports the following route selection sequence:
z
Discard routes with unreachable NEXT_HOPs first
z
Select the route with the highest Preferred_value
z
Select the route with the highest LOCAL_PREF
z
Select the route originated by the local router
z
Select the route with the shortest AS-PATH
z
Select IGP, EGP, Incomplete routes in turn
z
Select the route with the lowest MED value
z
Select routes learned from eBGP, confederation, iBGP in turn