华为AR2220 配置BGP的MED属性控制路由选择
一、组网需求
1、如图所示，所有路由器都配置BGP，RouterA在AS65008中，RouterB和RouterC在AS65009中。RouterA与RouterB、RouterC之间运行EBGP，RouterB和RouterC之间运行IBGP。需求从AS65008到AS65009的流量优先通过RouterC
2、网络拓扑
3、配置思路
采用如下的思路配置BGP的MED属性控制路由选择：
在RouterA和RouterB、RouterA和RouterC之间配置EBGP连接；在RouterB和RouterC之间配置IBGP连接。
通过使用路由策略增大RouterB发送给RouterA的MED值，这样可以使RouterA选择RouterC作为流量发往AS65009的入口设备。
二、操作步骤
1、配置各接口IP地址
system-view
[Huawei]sysname RouterA
[RouterA]interface GigabitEthernet 0/0/1
[RouterA-GigabitEthernet0/0/1]ip address 200.1.1.2 24
[RouterA-GigabitEthernet0/0/1]q
[RouterA]interface GigabitEthernet 0/0/2
[RouterA-GigabitEthernet0/0/2]ip address 200.1.2.2 24
[RouterA-GigabitEthernet0/0/2]q
system-view
[Huawei]sysname RouterB
[RouterB]interface GigabitEthernet 0/0/1
[RouterB-GigabitEthernet0/0/1]ip address 200.1.1.1 24
[RouterB-GigabitEthernet0/0/1]q
[RouterB]interface GigabitEthernet 0/0/0
[RouterB-GigabitEthernet0/0/0]ip address 9.1.1.1 24
[RouterB-GigabitEthernet0/0/0]q
system-view
[Huawei]sysname RouterC
[RouterC]interface GigabitEthernet 0/0/0
[RouterC-GigabitEthernet0/0/0]ip address 9.1.1.2 24
[RouterC-GigabitEthernet0/0/0]q
[RouterC]interface GigabitEthernet 0/0/2
[RouterC-GigabitEthernet0/0/2]ip address 200.1.2.1 24
[RouterC-GigabitEthernet0/0/2]q 2、配置BGP连接
[RouterA]bgp 65008
[RouterA-bgp]router-id 1.1.1.1
[RouterA-bgp]peer 200.1.1.1 as-number 65009
[RouterA-bgp]peer 200.1.2.1 as-number 65009
[RouterA-bgp]q
[RouterB]bgp 65009
[RouterB-bgp]router-id 2.2.2.2
[RouterB-bgp]peer 200.1.1.2 as-number 65008
[RouterB-bgp]peer 9.1.1.2 as-number 65009
[RouterB-bgp]ipv4-family unicast
[RouterB-bgp-af-ipv4]network 9.1.1.0 24
[RouterB-bgp-af-ipv4]q
[RouterB-bgp]q
[RouterC]bgp 65009
[RouterC-bgp]router-id 3.3.3.3
[RouterC-bgp]peer 200.1.2.2 as-number 65008
[RouterC-bgp]peer 9.1.1.1 as-number 65009
[RouterC-bgp]ipv4-family unicast
[RouterC-bgp-af-ipv4]network 9.1.1.0 24
[RouterC-bgp-af-ipv4]q
[RouterC-bgp]q3、查看RouterA的路由表，从路由表中可以看出，到目的地址9.1.1.0/24有两条有效路由，其中下一跳为200.1.1.1的路由是最优路由（因为RouterB的Router ID要小一些）。
[RouterA]display bgp routing-table 9.1.1.0 24
BGP local router ID : 1.1.1.1
Local AS number : 65008
Paths: 2 available, 1 best, 1 select
BGP routing table entry information of 9.1.1.0/24:
From: 200.1.1.1 (2.2.2.2)
Route Duration: 00h02m01s
Direct Out-interface: GigabitEthernet0/0/1
Original nexthop: 200.1.1.1
Qos information : 0x0
AS-path 65009, origin igp, MED 0, pref-val 0, valid, external, best, select, ac
tive, pre 255
Advertised to such 2 peers:
200.1.1.1
200.1.2.1
BGP routing table entry information of 9.1.1.0/24:
From: 200.1.2.1 (3.3.3.3)
Route Duration: 00h00m27s
Direct Out-interface: GigabitEthernet0/0/2
Original nexthop: 200.1.2.1
Qos information : 0x0
AS-path 65009, origin igp, MED 0, pref-val 0, valid, external, pre 255, not pre
ferred for router ID
Not advertised to any peer yet4、配置MED属性
[RouterB]route-policy policy10 permit node 10
[RouterB-route-policy]apply cost 100
[RouterB-route-policy]q
[RouterB]bgp 65009
[RouterB-bgp]peer 200.1.1.2 route-policy policy10 export
[RouterB-bgp]q5、查看RouterA路由表，从路由表中可以看出，由于下一跳为200.1.1.1（RouterB）的路由MED值为100，而下一跳为200.1.2.1的MED值为0，所以BGP优先选择MED值较小的路由。
[RouterA]display bgp routing-table 9.1.1.0 24
BGP local router ID : 1.1.1.1
Local AS number : 65008
Paths: 2 available, 1 best, 1 select
BGP routing table entry information of 9.1.1.0/24:
From: 200.1.2.1 (3.3.3.3)
Route Duration: 00h02m56s
Direct Out-interface: GigabitEthernet0/0/2
Original nexthop: 200.1.2.1
Qos information : 0x0
AS-path 65009, origin igp, MED 0, pref-val 0, valid, external, best, select, ac
tive, pre 255
Advertised to such 2 peers:
200.1.1.1
200.1.2.1
BGP routing table entry information of 9.1.1.0/24:
From: 200.1.1.1 (2.2.2.2)
Route Duration: 00h00m29s
Direct Out-interface: GigabitEthernet0/0/1
Original nexthop: 200.1.1.1
Qos information : 0x0
AS-path 65009, origin igp, MED 100, pref-val 0, valid, external, pre 255, not p
referred for MED
Not advertised to any peer yet