L3 MPLS VPN InterAS Option B： MP-eBGP between ASBRs
Option B的优势在于取消了的VRF相关的配置，减少了很多表项，减轻了ASBR的负载，通过MP-eBGP在ASBR之间交换vpnv4的路由，由于取消了VRF，就需要在ASBRs上关掉route-target的过滤：no bgp default route-target filter 。
每改变一次下一跳，标签就会被重新分配。
R4和R5之间的标签通过BGP来分配。neighbor 45.1.1.5 send-label，在ASBR互联的接口上自动启用mpls bgp forwarding 。R4可以对R2进行next-hop-self，也可以把互联接口的IP重分发进IGP。使得R2的下一跳可达。在ASBR上配置：no bgp default route-target filter
配置步骤：
AS 100 和 AS 200 的IGP和LDP
PE-CE protocol
AS100 和AS200 内的MP-BGP邻居
R4和R7的MP-BGP邻居
ASBRs之间的MP-eBGP，激活之后自动产生mpls bgp forwarding，跟IOS版本有关。有的不需要额外配置。
R2的配置：
ip vrf A
rd 100:2
route-target export 100:2
route-target import 100:2
route-target import 200:9
mpls label range 200 299
!
interface Loopback0
ip address 2.2.2.2 255.255.255.255
!
interface FastEthernet0/0
ip address 23.1.1.2 255.255.255.0
mpls ip
!
interface FastEthernet0/1
ip vrf forwarding A
ip address 12.1.1.2 255.255.255.0
router eigrp 90
network 2.2.2.2 0.0.0.0
network 23.1.1.0 0.0.0.255
no auto-summary
!
router ospf 110 vrf A
router-id 2.2.2.2
log-adjacency-changes
redistribute bgp 100 subnets
network 12.1.1.0 0.0.0.255 area 0
!
router bgp 100
no bgp default ipv4-unicast
bgp log-neighbor-changes
neighbor 4.4.4.4 remote-as 100
neighbor 4.4.4.4 update-source Loopback0
!
address-family vpnv4
neighbor 4.4.4.4 activate
neighbor 4.4.4.4 send-community extended
exit-address-family
!
address-family ipv4 vrf A
redistribute ospf 110 vrf A match internal external 1 external 2 nssa-external 1 nssa-external 2
no synchronization
exit-address-family
!
R4的配置：
!
mpls label range 400 499
!
interface Loopback0
ip address 4.4.4.4 255.255.255.255
!
interface FastEthernet0/1
ip address 34.1.1.4 255.255.255.0
mpls ip
!
interface Serial1/0
ip address 47.1.1.4 255.255.255.0
!
router eigrp 90
network 4.4.4.4 0.0.0.0
network 34.1.1.0 0.0.0.255
no auto-summary
!
router bgp 100
no bgp default ipv4-unicast
no bgp default route-target filter
bgp log-neighbor-changes
neighbor 2.2.2.2 remote-as 100
neighbor 2.2.2.2 update-source Loopback0
neighbor 47.1.1.7 remote-as 200
!
address-family vpnv4
neighbor 2.2.2.2 activate
neighbor 2.2.2.2 send-community extended
neighbor 2.2.2.2 next-hop-self
neighbor 47.1.1.7 activate
neighbor 47.1.1.7 send-community extended
exit-address-family
!
!
R1#traceroute 10.1.1.1 source l0
Type escape sequence to abort.
Tracing the route to 10.1.1.1
1 12.1.1.2 120 msec 44 msec 28 msec
2 23.1.1.3 [MPLS: Labels 300/407 Exp 0] 236 msec 228 msec 296 msec
3 34.1.1.4 [MPLS: Label 407 Exp 0] 252 msec 260 msec 104 msec
4 47.1.1.7 [MPLS: Label 704 Exp 0] 284 msec 152 msec 216 msec
5 78.1.1.8 [MPLS: Labels 802/905 Exp 0] 232 msec 184 msec 272 msec
6 19.1.1.9 [MPLS: Label 905 Exp 0] 148 msec 140 msec 228 msec
7 19.1.1.10 164 msec * 304 msec
R1#
R2#show bgp vpnv4 un all labels
Network Next Hop In label/Out label
Route Distinguisher: 100:2 (A)
1.1.1.1/32 12.1.1.1 203/nolabel
10.1.1.1/32 4.4.4.4 nolabel/407
12.1.1.0/24 0.0.0.0 204/aggregate(A)
19.1.1.0/24 4.4.4.4 nolabel/406
Route Distinguisher: 200:9
10.1.1.1/32 4.4.4.4 nolabel/407
19.1.1.0/24 4.4.4.4 nolabel/406
R4#sh bgp vpnv4 un all labels
Network Next Hop In label/Out label
Route Distinguisher: 100:2
1.1.1.1/32 2.2.2.2 404/203
12.1.1.0/24 2.2.2.2 405/204
Route Distinguisher: 200:9
10.1.1.1/32 47.1.1.7 407/704
19.1.1.0/24 47.1.1.7 406/705
R9#sh bgp vpnv4 un all labels
Network Next Hop In label/Out label
Route Distinguisher: 100:2
1.1.1.1/32 47.1.1.4 nolabel/404
12.1.1.0/24 47.1.1.4 nolabel/405
Route Distinguisher: 200:9 (A)
1.1.1.1/32 47.1.1.4 nolabel/404
10.1.1.1/32 19.1.1.10 905/nolabel
12.1.1.0/24 47.1.1.4 nolabel/405
19.1.1.0/24 0.0.0.0 906/aggregate(A)
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