Huawei Quidway NetEngine80E Configuration Manual page 972

HUAWEI NetEngine80E/40E Router
Configuration Guide - IP Routing
selected as the optimal route. To change the route selection result on Router B, run the bestroute
med-none-as-maximum command on Router B. Detailed configurations are as follows:
[RouterB] display bgp routing-table
BGP Local router ID is 10.1.2.1
Status codes: * - valid, > - best, d - damped,
Total Number of Routes: 2
*>
*
[RouterB] display bgp routing-table 1.1.1.9
BGP local router ID : 10.1.2.1
Local AS number : 200
Paths:
BGP routing table entry information of 1.1.1.9/32:
From: 10.1.4.2 (10.1.1.2)
Route Duration: 00h08m42s
Direct Out-interface: GigabitEthernet0/0/2
Original nexthop: 10.1.4.2
Qos information : 0x0
AS-path 65001, origin igp, MED 100, pref-val 0, valid, external, best, select,
active, pre 255
Advertised to such 2 peers:
BGP routing table entry information of 1.1.1.9/32:
From: 10.1.2.2 (10.1.2.2)
Route Duration: 16h33m10s
Direct Out-interface: GigabitEthernet0/0/0
Original nexthop: 10.1.2.2
Qos information : 0x0
AS-path 65001, origin igp, pref-val 0, valid, external, pre 255, not preferred for
MED
Not advertised to any peer yet
The preceding command output shows that two routes 1.1.1.9/32 are available in the BGP routing
table of Router B. The MED of the route with the next hop address 10.1.4.2 is 100, and the MED
of the route with the next hop address 10.1.2.2 is considered as 4294967295 because it carries
no MED. Therefore, the route with the next hop address 10.1.4.2 is selected as the optimal route.
In addition, BGP selects routes in the same sequence they are received. Therefore, the route
selection result is relevant to the sequence in which the routes are received. For example, the
following three BGP routes are available on a device:
l
l
l
If the compare-different-as-med (BGP) command is run, route B is the optimal route,
regardless of the sequence in which the routes are received. If the compare-different-as-med
(BGP) command is not configured, BGP does not compare the MED values of routes learned
from different ASs. The route selection is described in the following cases:
l
Issue 02 (2014-09-30)
h - history,
Origin : i - IGP, e - EGP, ? - incomplete
Network
1.1.1.9/32
2 available, 1 best, 1 select
10.1.2.2
10.1.4.2
Route A1: AS_Path { 65001 200 }, med 100, igp cost 13, internal, Router id 4.4.4.4
Route A2: AS_Path { 65001 100 }, med 150, igp cost 11, internal, Router id 5.5.5.5
Route B: AS_Path { 65002 300 }, med 0, igp cost 12, internal, Router id 6.6.6.6
Case 1: Route A1 is received first, followed by route B, and then route A2.
– BGP first compares route A1 and route B. The leftmost AS number in the AS_Path of
route A1 is 65001, which is different from its counterpart in route B (65002). Therefore,
Huawei Proprietary and Confidential
Copyright © Huawei Technologies Co., Ltd.
i - internal, s - suppressed, S - Stale
NextHop MED
10.1.4.2 100
10.1.2.2
8 BGP Configuration
LocPrf PrefVal Path/Ogn
0 65001i
0 65001i
947