组网需求

如图所示，所有路由器都配置BGP，AR1在AS65008中，AR2和AR3在AS65009中。AR1与AR2、AR3之间运行EBGP，AR2和AR3之间运行IBGP。需求从AS65008到AS65009的流量优先通过AR3。

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AR1和AR2、AR1和AR3之间配置EBGP连接；在AR2和AR3之间配置IBGP连接。
通过使用路由策略增大AR2发送给AR1的MED值，这样可以使AR1选择AR3作为流量发往AS65009的入口设备。

操作步骤

配置IP

AR1

<Huawei>sys
[Huawei]sys AR1

[AR1]int g0/0/0
[AR1-GigabitEthernet0/0/0]ip add 12.12.12.1 24
[AR1-GigabitEthernet0/0/0]q

[AR1]int g0/0/1
[AR1-GigabitEthernet0/0/1]ip add 13.13.13.1 24
[AR1-GigabitEthernet0/0/1]q

AR2

<Huawei>sys
[Huawei]sys AR2

[AR2]int g0/0/0
[AR2-GigabitEthernet0/0/0]ip add 12.12.12.2 24
[AR2-GigabitEthernet0/0/0]q

[AR2]int g0/0/1
[AR2-GigabitEthernet0/0/1]ip add 23.23.23.2 24
[AR2-GigabitEthernet0/0/1]q

AR3

<Huawei>sys
[Huawei]sys AR3

[AR3]int g0/0/0
[AR3-GigabitEthernet0/0/0]ip add 13.13.13.3 24
[AR3-GigabitEthernet0/0/0]q

[AR3]int g0/0/1
[AR3-GigabitEthernet0/0/1]ip add 23.23.23.3 24
[AR3-GigabitEthernet0/0/1]q

配置bgp

AR1

[AR1]bgp 65008
[AR1-bgp]router-id 1.1.1.1
[AR1-bgp]peer 12.12.12.2 as-number 65009
[AR1-bgp]peer 13.13.13.3 as-number 65009
[AR1-bgp]q

AR2

[AR2]bgp 65009
[AR2-bgp]router-id 2.2.2.2 
[AR2-bgp]peer 12.12.12.1 as-number 65008
[AR2-bgp]peer 23.23.23.3 as-number 65009

[AR2-bgp]ipv4-family unicast 
[AR2-bgp-af-ipv4]network 23.23.23.0 255.255.255.0
[AR2-bgp-af-ipv4]q
[AR2-bgp]q

AR3

[AR3]bgp 65009
[AR3-bgp]router-id 3.3.3.3
[AR3-bgp]peer 13.13.13.1 as-number 65008
[AR3-bgp]peer 23.23.23.2 as-number 65009


[AR3-bgp]ipv4-family unicast 
[AR3-bgp-af-ipv4]network 23.23.23.0 255.255.255.0
[AR3-bgp-af-ipv4]q

查看AR1的路由表

[AR1]display bgp routing-table 

 BGP Local router ID is 1.1.1.1 
 Status codes: * - valid, > - best, d - damped,
               h - history,  i - internal, s - suppressed, S - Stale
               Origin : i - IGP, e - EGP, ? - incomplete


 Total Number of Routes: 2
      Network            NextHop        MED        LocPrf    PrefVal Path/Ogn

 *>   23.23.23.0/24      12.12.12.2      0                     0      65009i
 *                       13.13.13.3      0                     0      65009i

[AR1]display bgp routing-table 23.23.23.0 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 23.23.23.0/24:
 From: 12.12.12.2 (2.2.2.2)
 Route Duration: 00h01m53s  
 Direct Out-interface: GigabitEthernet0/0/0
 Original nexthop: 12.12.12.2
 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:
    13.13.13.3
    12.12.12.2
 BGP routing table entry information of 23.23.23.0/24:
 From: 13.13.13.3 (3.3.3.3)
 Route Duration: 00h14m09s  
 Direct Out-interface: GigabitEthernet0/0/1
 Original nexthop: 13.13.13.3
 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 yet

从路由表中可以看出，到目的地址23.23.23.0/24有两条有效路由，其中下一跳为12.12.12.2的路由是最优路由（因为AR2的Router ID要小一些）。

配置MED属性

AR2

#配置路由策略，在路由出口增大开销
[AR2]route-policy AR2 permit node 10
[AR2-route-policy]apply community
[AR2-route-policy]apply cost 100

[AR2]bgp 65009
[AR2-bgp]peer 12.12.12.1 route-policy AR2 export 
[AR2-bgp]q

验证

AR1

<AR1>display bgp routing-table 

 BGP Local router ID is 1.1.1.1 
 Status codes: * - valid, > - best, d - damped,
               h - history,  i - internal, s - suppressed, S - Stale
               Origin : i - IGP, e - EGP, ? - incomplete


 Total Number of Routes: 2
      Network            NextHop        MED        LocPrf    PrefVal Path/Ogn

 *>   23.23.23.0/24      13.13.13.3      0                     0      65009i
 *                       12.12.12.2      100                   0      65009i


<AR1>display bgp routing-table 23.23.23.0 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 23.23.23.0/24:
 From: 13.13.13.3 (3.3.3.3)
 Route Duration: 00h45m33s  
 Direct Out-interface: GigabitEthernet0/0/1
 Original nexthop: 13.13.13.3
 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:
    13.13.13.3
    12.12.12.2
 BGP routing table entry information of 23.23.23.0/24:
 From: 12.12.12.2 (2.2.2.2)
 Route Duration: 00h15m43s  
 Direct Out-interface: GigabitEthernet0/0/0
 Original nexthop: 12.12.12.2
 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

从路由表中可以看出，由于下一跳为12.12.12.2（AR2）的路由MED值为100，而下一跳为13.13.13.3的MED值为0，所以BGP优先选择MED值较小的路由。

目录CONTENT

配置BGP的MED属性控制路由选择