Huawei Quidway NetEngine80E Configuration Manual page 958

HUAWEI NetEngine80E/40E Router
Configuration Guide - IP Routing
external, pre 255, not preferred for AS-Path
Not advertised to any peer yet
The preceding command output shows that the AS_Path length of the route learned from
Router B is shorter than that of the route learned from Router C and that the route learned from
Router B is selected as the optimal route.
routes 172.16.1.0 learned from Router B and Router C.
Table 8-25 Attribute comparison of the routes 172.16.1.0 learned from Router B and Router C
Route Attribute
PrefVal
Local_Pref
Route type
AIGP
AS_Path
AS numbers can be added to the AS_Path as required. However, if an AS number is added to
the AS_Path of a route, the route cannot be received by devices in this AS. Therefore, the local
AS number is added in most cases. For example in
the AS_Path of a route before advertising the route to Router A, Router A will discard the route
upon receipt because the route carries Router A's AS number.
Replacing AS Numbers
When the apply as-path command is configured, if overwrite is specified in the command, the
device will replace the AS numbers in the original AS_Path attribute to achieve the following
goals:
l
l
l
AS number replacement can also be used for the purpose of load balancing. For example in
Figure
Router A to replace the AS_Path of the route learned from Router C so that the route has the
same AS_Path as that of the route learned from Router B, and the two routes are used to load-
balance traffic. Detailed configurations on Router A are as follows:
#
bgp 65001
#
ipv4-family unicast
undo synchronization
Issue 02 (2014-09-30)
Route Learned
from Router B
0
-
Learned from a peer
-
65002 65004 65005
Hide the actual path information.
Prevent a route from being discarded by replacing the AS_Path of the route with a shorter
one if the as-path-limit command is configured on the device that receives this route.
Reduce the AS_Path length.
8-20, the apply as-path 65002 65004 65005 overwrite command can be configured on
peer 10.1.3.2 route-policy replace_asn import
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Table 8-25 shows the attribute comparison of the
Route Learned
from Router C
0
-
Learned from a peer
-
65003 65003 65003
65003 65005
Figure 8-20, if Router C adds AS 65001 to
//Apply export policy named
8 BGP Configuration
Comparison
The same.
The same.
The same.
The same.
The route learned
from Router B is
optimal.
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