optionB的核心思想是私网路由传递过程是 PE-ASBR1-ASBR2-PE2 在传递过程中私网标签发生了变化（由ASBR重新分配了私网标签），而在数据平面（不考虑PHP机制）当数据包到达ASBR1时标签存在公网标签和私网标签两层，当转发给ASBR2时只有一层由ASBR2分配的私网标签，通过EBGP隧道迭代直接到达ASBR2，ASBR2再把私网标签替换成PE分配的标签并压入公网标签后转发。

　　所以在optionB中私网标签一共有三个，数据包标签最少时只有一层私网标签

　　optionB的缺点是ASBR设备需要维护到用户侧的VPNV4路由，占用大量设备资源，所以现网中如果使用optionB做为跨域方案，建议ASBR设备只做MPLS设备来使用

 

G0/0/0

G0/0/1

loopback0

CE1

10.0.0.1/30

 

192.168.0.1

PE1

12.1.1.1/24

10.0.0.2/30

1.1.1.1

P1

12.1.1.2/24

23.1.1.2/24

2.2.2.2

ASBR1

34.1.1.3/24

23.1.1.3/24

3.3.3.3

ASBR2

34.1.1.4/24

45.1.1.4/24

4.4.4.4

P2

56.1.1.5/24

45.1.1.5/24

5.5.5.5

PE2

56.1.1.6/24

10.10.10.2/30

6.6.6.6

CE1

10.10.10.1/30

 

192.168.1.1

1. 底层互联地址ospf,域内mpls ldp配置略

2. PE配置VPN端口接入CE

3. 查看BGP状态与路由

4. PE与ASBR间建立IBGP-VPNV4邻居关系，传递私网路由，注意ASBR一定要undo policy vpn-target，否则会因为VPN RT值不匹配导致无法安装私网路由

   ASBR
   bgp 100
   peer 1.1.1.1 as-number 100
   peer 1.1.1.1 connect-interface LoopBack0

   ipv4-family unicast
   undo synchronization
   peer 1.1.1.1 enable
   peer 34.1.1.4 enable

   ipv4-family vpnv4
   undo policy vpn-target
   peer 1.1.1.1 enable

   PE

   bgp 100
   peer 3.3.3.3 as-number 100
   peer 3.3.3.3 connect-interface LoopBack0

   ipv4-family unicast
   undo synchronization
   peer 3.3.3.3 enable

   ipv4-family vpnv4
   policy vpn-target
   peer 3.3.3.3 enable
   查看ASBR是否正常接收到PE传递的私网路由

5. ASBR间建立EBGP-VPNV4邻居关系，并配置替换外层VPN标签，使公网LSP完整
   ASBR1

   bgp 100
   peer 1.1.1.1 as-number 100
   peer 1.1.1.1 connect-interface LoopBack0
   peer 34.1.1.4 as-number 200

   ipv4-family unicast
   undo synchronization
   peer 1.1.1.1 enable
   peer 1.1.1.1 route-policy to-pe export
   peer 1.1.1.1 label-route-capability
   peer 34.1.1.4 enable
   peer 34.1.1.4 route-policy to-asbr export
   peer 34.1.1.4 label-route-capability

   ipv4-family vpnv4
   undo policy vpn-target
   peer 1.1.1.1 enable
   peer 34.1.1.4 enable

   route-policy to-asbr permit node 10
   apply mpls-label

   route-policy to-pe permit node 10
   if-match mpls-label
   apply mpls-label

   ASBR2

   bgp 200
   peer 6.6.6.6 as-number 200
   peer 6.6.6.6 connect-interface LoopBack0
   peer 34.1.1.3 as-number 100

   ipv4-family unicast
   undo synchronization
   peer 6.6.6.6 enable
   peer 6.6.6.6 route-policy to-pe export
   peer 6.6.6.6 label-route-capability
   peer 34.1.1.3 enable
   peer 34.1.1.3 route-policy to-asbr export
   peer 34.1.1.3 label-route-capability

   ipv4-family vpnv4
   undo policy vpn-target
   peer 6.6.6.6 enable
   peer 34.1.1.3 enable

   route-policy to-asbr permit node 10
   apply mpls-label

   route-policy to-pe permit node 10
   if-match mpls-label
   apply mpls-label

   查看两端ASBR路由
   

   查看两端ASBR MPLS LSP
   

6. 查看CE设备路由表，并测试连通性