Mpls vpn跨域optionB有两种方式：
第一种方式为ASBR之间通过单跳MP-EBGP，ASBR之间必须直联
第二种方式为ASBR之间通过多跳MP-EBGP，ASBR之间必须采用tunnel方式，本文只介绍这种方式。Mpls vpn over gre！
对于mpls vpn over gre的这种方式，有两种方法可以使VPNv4的下一跳地址在接收路由更新的AS中可达：
（1） 将ASBR之间的tunnel互联地址重分布到IGP；
（2） ASBR在向PE（或RR）传递从另一个AS学到的VPNv4的路由时，下一跳为自已，
也就是next-hop-self
实验拓扑图如下：
方法一：
默认情况下，R6从AS1学到VPNv4的路由再通告给R10（RR）时，下一跳地址为R4的F0/0的地址，但是R10学到VPNv4路由的下一跳地址不可达，所以在R4和R6上分别将tunnel的互联地址重新分发到IGP中，这样R10才通将VPNv4的路由传递给R9。
在标签交换方面，下一跳地址不改变，就不会重新分配标签。如R9学到的VPNv4的路由分配out标签是由R4分配的。 R9# show ip cef vrf test 11.11.11.11
11.11.11.11/32, version 34, epoch 0, cached adjacency 192.168.2.9 0 packets, 0 bytes
tag information set, all rewrites owned local tag: VPN route head
fast tag rewrite with Fa0/0, 192.168.2.9, tags imposed {840 427} via 192.168.0.5, 0 dependencies, recursive
next hop 192.168.2.9, FastEthernet0/0 via 192.168.0.5/32 (Default) valid cached adjacency
tag rewrite with Fa0/0, 192.168.2.9, tags imposed {840 427}
R11#traceroute 12.12.12.12
Type escape sequence to abort.
Tracing the route to 12.12.12.12
1 10.0.0.1 60 msec 140 msec 168 msec
2 192.168.1.2 [MPLS: Labels 232/643 Exp 0] 1008 msec 1028 msec 936 msec 3 192.168.1.6 [MPLS: Labels 332/643 Exp 0] 840 msec 1004 msec 816 msec 4 192.168.1.10 [MPLS: Labels 426/643 Exp 0] 888 msec 836 msec 768 msec
5 192.168.0.6 [MPLS: Label 643 Exp 0] 768 msec 812 msec 1176 msec //只有单层标签 6 192.168.2.2 [MPLS: Labels 717/939 Exp 0] 888 msec 932 msec 936 msec 7 192.168.2.6 [MPLS: Labels 813/939 Exp 0] 792 msec 740 msec 816 msec 8 10.0.0.5 [AS 20] [MPLS: Label 939 Exp 0] 796 msec 688 msec 984 msec 9 10.0.0.6 [AS 20] 1032 msec 884 msec 984 msec R11#
R4和R6上只保存本AS内的VPNv4的路由。 R4#show mpls forwarding-table
Local Outgoing Prefix Bytes tag Outgoing Next Hop tag tag or VC or Tunnel Id switched interface 416 Untagged 6.6.6.6/32 0 Fa0/0 192.168.0.2 417 110 1:1:10.0.0.0/30 610 Fa1/1 192.168.1.9 418 Pop tag 192.168.1.16/30 0 Fa1/1 192.168.1.9 419 Pop tag 192.168.1.4/30 0 Fa1/1 192.168.1.9 420 303 192.168.1.12/30 0 Fa1/1 192.168.1.9 421 300 192.168.1.0/30 0 Fa1/1 192.168.1.9 422 301 1.1.1.1/32 15844 Fa1/1 192.168.1.9 423 302 2.2.2.2/32 0 Fa1/1 192.168.1.9 424 Pop tag 3.3.3.3/32 0 Fa1/1 192.168.1.9 425 328 5.5.5.5/32 3573 Fa1/1 192.168.1.9 426 Pop tag 192.168.0.6/32 640 Tu0 point2point 427 138 1:1:11.11.11.11/32 \\
5068 Fa1/1 192.168.1.9 428 137 1:1:192.168.10.0/24 \\
0 Fa1/1 192.168.1.9
R6#show mpls forwarding-table
Local Outgoing Prefix Bytes tag Outgoing Next Hop tag tag or VC or Tunnel Id switched interface 608 Untagged 4.4.4.4/32 0 Fa0/0 192.168.0.1 609 Pop tag 192.168.0.1/32 0 Fa0/0 192.168.0.1 610 Pop tag 192.168.2.12/30 0 Fa1/0 192.168.2.2 611 Pop tag 192.168.2.4/30 0 Fa1/0 192.168.2.2 612 714 192.168.2.16/30 0 Fa1/0 192.168.2.2 613 716 192.168.2.8/30 0 Fa1/0 192.168.2.2 614 Pop tag 7.7.7.7/32 0 Fa1/0 192.168.2.2
615 715 8.8.8.8/32 0 Fa1/0 192.168.2.2 640 717 9.9.9.9/32 0 Fa1/0 192.168.2.2 641 713 10.10.10.10/32 1216 Fa1/0 192.168.2.2 642 Pop tag 192.168.0.5/32 640 Tu0 point2point 643 939 1:1:12.12.12.12/32 \\
610 Fa1/0 192.168.2.2 644 938 1:1:192.168.20.0/24 \\
0 Fa1/0 192.168.2.2
R4的配置： hostname R4 !
boot-start-marker boot-end-marker ! !
ip subnet-zero ! ! ip cef
no ip domain-lookup mpls label range 400 499 mpls label protocol ldp ! !
!
interface Loopback0
ip address 4.4.4.4 255.255.255.255 no ip directed-broadcast !
interface Tunnel0
ip address 192.168.0.5 255.255.255.252 no ip directed-broadcast mpls bgp forwarding tunnel source Loopback0 tunnel destination 6.6.6.6 !
interface FastEthernet0/0
ip address 192.168.0.1 255.255.255.252 no ip directed-broadcast mpls bgp forwarding !
interface FastEthernet1/0 no ip address
no ip directed-broadcast shutdown duplex half speed auto !
interface FastEthernet1/1
ip address 192.168.1.10 255.255.255.252 no ip directed-broadcast duplex half speed auto
tag-switching ip !
router ospf 1
log-adjacency-changes
redistribute connected subnets network 4.4.4.4 0.0.0.0 area 0 network 192.168.1.8 0.0.0.3 area 0 !
router bgp 1
no bgp default route-target filter bgp log-neighbor-changes neighbor 5.5.5.5 remote-as 1
neighbor 5.5.5.5 update-source Loopback0 neighbor 192.168.0.6 remote-as 2 !
address-family ipv4 neighbor 5.5.5.5 activate
no neighbor 192.168.0.6 activate no auto-summary no synchronization exit-address-family !
address-family vpnv4 neighbor 5.5.5.5 activate
neighbor 5.5.5.5 send-community both neighbor 192.168.0.6 activate
neighbor 192.168.0.6 send-community both exit-address-family !
ip classless
ip route 6.6.6.6 255.255.255.255 192.168.0.2 ! ! !
control-plane ! !
line con 0
exec-timeout 0 0 stopbits 1 line aux 0 stopbits 1 line vty 0 4 login !
no cns aaa enable
R6的配置： hostname R6 !
boot-start-marker boot-end-marker ! !
ip subnet-zero ! ! ip cef
no ip domain-lookup mpls label range 600 699 mpls label protocol ldp ! !
!
interface Loopback0
ip address 6.6.6.6 255.255.255.255 no ip directed-broadcast !
interface Tunnel0
ip address 192.168.0.6 255.255.255.252 no ip directed-broadcast mpls bgp forwarding tunnel source Loopback0 tunnel destination 4.4.4.4 !
interface FastEthernet0/0
ip address 192.168.0.2 255.255.255.252 no ip directed-broadcast
Mpls vpn跨域optionB有两种方式
