23、BGP配置之选路，优选Cluster_List最短

一、实验拓扑

二、实验需求

1、R4不做任何配置，其他配置如图所示

三、实验过程

1、R1的配置

sys
un in en
sys R1
int g0/0/0
ip add 10.0.12.1 24
int g0/0/1
ip add 10.0.13.1 24
int loo0
ip add 1.1.1.1 32

ospf 1 router-id 1.1.1.1
area 0
net 10.0.12.0 0.0.0.255
net 10.0.13.0 0.0.0.255
net 1.1.1.1 0.0.0.0

bgp 200
 router-id 1.1.1.1
 peer 2.2.2.2 as-number 200
 peer 2.2.2.2 connect-interface LoopBack0
 group 1 internal
 peer 3.3.3.3 as-number 200
 peer 3.3.3.3 group 1
 #
 ipv4-family unicast
  undo synchronization
  reflector cluster-id 1
  peer 2.2.2.2 enable
  peer 1 enable
  peer 1 reflect-client
  peer 3.3.3.3 enable
  peer 3.3.3.3 group 1

2、R2的配置

sys
un in en
sys R2
int g0/0/0
ip add 10.0.12.2 24
int g0/0/1
ip add 10.0.24.2 24
int loo0
ip add 2.2.2.2 32

ospf 1 router-id 2.2.2.2
area 0
net 2.2.2.2 0.0.0.0
net 10.0.12.0 0.0.0.255

bgp 200
 router-id 2.2.2.2
 peer 1.1.1.1 as-number 200
 peer 1.1.1.1 connect-interface LoopBack0
 peer 3.3.3.3 as-number 200
 peer 3.3.3.3 connect-interface LoopBack0
 #
 ipv4-family unicast
  undo synchronization
  peer 1.1.1.1 enable
  peer 1.1.1.1 next-hop-local
  peer 3.3.3.3 enable
  peer 3.3.3.3 next-hop-local

3、R3的配置

sys
un in en
sys R3
int g0/0/1
ip add 10.0.13.3 24
int g0/0/0
ip add 10.0.35.3 24
int loo0
ip add 3.3.3.3  32

ospf 1 router-id 3.3.3.3
area 0
net 10.0.13.0 0.0.0.255
net 3.3.3.3 0.0.0.0

bgp 200
 router-id 3.3.3.3
 peer 1.1.1.1 as-number 200
 peer 1.1.1.1 connect-interface LoopBack0
 peer 2.2.2.2 as-number 200
 peer 2.2.2.2 connect-interface LoopBack0
 peer 10.0.35.5 as-number 300
 #
 ipv4-family unicast
  undo synchronization
  peer 1.1.1.1 enable
  peer 1.1.1.1 next-hop-local
  peer 2.2.2.2 enable
  peer 2.2.2.2 next-hop-local
  peer 10.0.35.5 enable

5、R5的配置

sys
un in en
sys R5
int g0/0/0
ip add 10.0.35.5 24
int g0/0/2
ip add 10.0.45.5 24
int g0/0/3
ip add 10.0.54.5 24
int loo0
ip add 5.5.5.5 32


bgp 300
router-id 5.5.5.5
pe 10.0.35.3 as 200
net 10.0.45.0 24
net 10.0.54.0 24

四、实验结果

1、在R2上查看BGP路由表信息

• 从BGP路由表中无法看出优选的是R1反射的BGP路由还是R3通告的BGP路由，我们查看BGP路由明细
  
• 经过R1反射的路由不是最优路由，原因也被标出 not preferred for Cluster list，R3直接通告给R2的BGP路由因为没有经过路由反射器，不存在Cluster_list属性，即被认为CLuster_list长度为0，小于R1反射的BGP路由其Cluster_list长度（1），所有R3通告的BGP路由为优选路由。