【理论准备】
balance number
number：进行负载分担的BGP路由条数，当number取值为1时，相当于不进行负载分担。不同型号的设备支持的取值范围不同，请以设备的实际情况为准。
balance命令用来配置BGP负载分担的路由条数。undo balance命令用来取消BGP路由负载分担功能。
缺省情况下，没有进行路由负载分担。
与IGP路由的负载分担不同，BGP没有明确的度量值决定是否对路由进行负载分担。BGP的负载分担是通过改变BGP选路规则来实现的。
【实验拓扑】
本文实验采用的交换机是H3C模拟器，下载地址如下：http://forum.h3c.com/forum.php?mod=viewthread&tid=109740&highlight=H3C%E6%A8%A1%E6%8B%9F%E5%99%A8 有兴趣的朋友可以在论坛上去下载。
【实验过程】
按如上拓扑配置接口IP和BGP AS
R4和R5建立OSPF邻居
R4配置：
#
area 0.0.0.0
network 1.1.1.2 0.0.0.0
network 10.1.3.0 0.0.0.255
#
R5配置：
#
ospf 1
area 0.0.0.0
network 1.1.1.3 0.0.0.0
network 10.1.3.0 0.0.0.255
[R5] dis ospf peer
OSPF Process 1 with Router ID 1.1.1.3
Neighbor Brief Information
Area: 0.0.0.0
Router ID Address Pri Dead-Time Interface State
1.1.1.2 10.1.3.1 1 32 S0/6/2 Full/ -
[R5]dis ip ro
Routing Tables: Public
Destinations : 10 Routes : 10
Destination/Mask Proto Pre Cost NextHop Interface
1.1.1.2/32 OSPF 10 1562 10.1.3.1 S0/6/2
[R4]dis ip ro
Routing Tables: Public
Destinations : 10 Routes : 10
Destination/Mask Proto Pre Cost NextHop Interface
1.1.1.3/32 OSPF 10 1562 10.1.3.2 S0/6/2
【BGP配置】
R3:
bgp 100
network 10.0.0.1 255.255.255.255
undo synchronization
peer 10.1.1.2 as-number 200
peer 10.1.2.2 as-number 200
group ex external
peer 10.1.1.2 group ex
peer 10.1.2.2 group ex
R4:
bgp 200
network 10.1.1.0 255.255.255.0
network 10.1.3.0 255.255.255.0
undo synchronization
peer 10.1.1.1 as-number 100
group ex external
peer 10.1.1.1 group ex
group in internal
peer 1.1.1.3 group in
peer 1.1.1.3 connect-interface LoopBack0
R5:
bgp 200
network 10.1.2.0 255.255.255.0
network 10.1.3.0 255.255.255.0
undo synchronization
peer 10.1.2.1 as-number 100
group in internal
peer 1.1.1.2 group in
peer 1.1.1.2 connect-interface LoopBack0
group ex external
peer 10.1.2.1 group ex
#
[R3]dis bgp peer
BGP local router ID : 10.0.0.1
Local AS number : 100
Total number of peers : 2 Peers in established state : 2
Peer V AS MsgRcvd MsgSent OutQ PrefRcv Up/Down State
10.1.1.2 4 200 5 7 0 0 00:03:36 Established
10.1.2.2 4 200 2 5 0 0 00:00:17 Established
[R3]
[R4]dis bgp peer
BGP local router ID : 1.1.1.2
Local AS number : 200
Total number of peers : 2 Peers in established state : 2
Peer V AS MsgRcvd MsgSent OutQ PrefRcv Up/Down State
10.1.1.1 4 100 7 6 0 1 00:03:58 Established
1.1.1.3 4 200 3 3 0 1 00:00:20 Established
BGP配置完成，我们来看R3上的路由表：
R3]dis bgp ro
Total Number of Routes: 7
BGP Local router ID is 10.0.0.1
Status codes: * - valid, > - best, d - damped,
h - history, i - internal, s - suppressed, S - Stale
Origin : i - IGP, e - EGP, ? - incomplete
Network NextHop MED LocPrf PrefVal Path/Ogn
*> 10.0.0.1/32 0.0.0.0 0 0 i
* 10.1.1.0/24 10.1.1.2 0 0 200i
* 10.1.2.2 0 200i
* 10.1.2.0/24 10.1.2.2 0 0 200i
* 10.1.1.2 0 200i
*> 10.1.3.0/24 10.1.1.2 0 0 200i
* 10.1.2.2 0 0 200i
[R3]
[R3]
[R3]dis ip ro
Routing Tables: Public
Destinations : 11 Routes : 11
Destination/Mask Proto Pre Cost NextHop Interface
1.1.1.1/32 Direct 0 0 127.0.0.1 InLoop0
10.0.0.1/32 Direct 0 0 127.0.0.1 InLoop0
10.1.1.0/24 Direct 0 0 10.1.1.1 S0/6/0
10.1.1.1/32 Direct 0 0 127.0.0.1 InLoop0
10.1.1.2/32 Direct 0 0 10.1.1.2 S0/6/0
10.1.2.0/24 Direct 0 0 10.1.2.1 S0/6/2
10.1.2.1/32 Direct 0 0 127.0.0.1 InLoop0
10.1.2.2/32 Direct 0 0 10.1.2.2 S0/6/2
10.1.3.0/24 BGP 255 0 10.1.1.2 S0/6/0
127.0.0.0/8 Direct 0 0 127.0.0.1 InLoop0
127.0.0.1/32 Direct 0 0 127.0.0.1 InLoop0
由上可见路由表中到10.1.3.0/24的下一跳是10.1.1.2，10.1.2.2存在于BGP路由表中，但是并没有存在于路由表中，是合法的，但不是最佳的。
加入如下命令后我们再来观察：
[R3-bgp]balance 2
[R3-bgp]dis ip ro
Routing Tables: Public
Destinations : 11 Routes : 12
Destination/Mask Proto Pre Cost NextHop Interface
1.1.1.1/32 Direct 0 0 127.0.0.1 InLoop0
10.0.0.1/32 Direct 0 0 127.0.0.1 InLoop0
10.1.1.0/24 Direct 0 0 10.1.1.1 S0/6/0
10.1.1.1/32 Direct 0 0 127.0.0.1 InLoop0
10.1.1.2/32 Direct 0 0 10.1.1.2 S0/6/0
10.1.2.0/24 Direct 0 0 10.1.2.1 S0/6/2
10.1.2.1/32 Direct 0 0 127.0.0.1 InLoop0
10.1.2.2/32 Direct 0 0 10.1.2.2 S0/6/2
10.1.3.0/24 BGP 255 0 10.1.1.2 S0/6/0
BGP 255 0 10.1.2.2 S0/6/2
127.0.0.0/8 Direct 0 0 127.0.0.1 InLoop0
127.0.0.1/32 Direct 0 0 127.0.0.1 InLoop0
[R3-bgp]dis bgp ro
Total Number of Routes: 7
BGP Local router ID is 10.0.0.1
Status codes: * - valid, > - best, d - damped,
h - history, i - internal, s - suppressed, S - Stale
Origin : i - IGP, e - EGP, ? - incomplete
Network NextHop MED LocPrf PrefVal Path/Ogn
*> 10.0.0.1/32 0.0.0.0 0 0 i
* 10.1.1.0/24 10.1.1.2 0 0 200i
* 10.1.2.2 0 200i
* 10.1.2.0/24 10.1.2.2 0 0 200i
* 10.1.1.2 0 200i
*> 10.1.3.0/24 10.1.1.2 0 0 200i
*> 10.1.2.2 0 0 200i
[R3-bgp]
加了命令后可见，在路由表中有两个下一跳，在BGP路由表中两个下一跳都是合法的和最佳的。