3com S7906E 설치 설명서

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

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:

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 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 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:

Discard routes with unreachable NEXT_HOPs 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

Select the route with the shortest AS-PATH

Select IGP, EGP, Incomplete routes in turn

Select the route with the lowest MED value

Select routes learned from eBGP, confederation, iBGP in turn