1. Topology
CE1------[R1(PE1)---R2(P)---R3(ASBR1)]------[R4(ASBR2)----R5(PE2)]----------CE2
[AS 100] [AS 200]
2. 基本原理
PE与ASBR之间不需要建立IBGP邻居，只是ASBR之间建立EBGP，PE之间建立MP-EBGP邻居。PE之间的可达性，是将ASBR将BGP路由重新分发到IGP中来保证的。
3. 标签分配过程
×××私网标签由PE1直接分配给PE2；
到 PE1 Loopback接口地址的公网标签逐跳分发。需要说明的是，公网标签在域内是由LDP来分发的，在域间是由MP-EBGP来分发的。
4. 标签转发过程
非常简单，和MP-IBGP MPLS ×××的转发过程一样
5. 特点
ASBR 上不需要感知×××的存在，只需要PE来维护×××路由和私网标签转发表。
6. 各LSR上的相关表项
- R1 （PE1 in AS 100）
C3600-R1#show ip bgp ***v4 all labels
Network Next Hop In label/Out label
Route Distinguisher: 100:1 (***-a)
10.16.1.0/24 0.0.0.0 20/aggregate(***-a)
C3600-R1#show mpls forwarding-table
Local Outgoing Prefix Bytes tag Outgoing Next Hop
tag tag or VC or Tunnel Id switched interface
20 Aggregate 10.16.1.0/24[V] 5752
C3600-R1#show ip cef vrf ***-a 10.16.1.0 255.255.255.0
10.16.1.0/24, version 7, epoch 0, attached, connected
0 packets, 0 bytes
tag information set
local tag: 20
via FastEthernet1/0, 0 dependencies
valid glean adjacency
tag rewrite with , , tags imposed: {}
- R2 （P in AS 100）
P路由器感知不到××× LSP，所以只需要关注公网LSP
C3600-R2#sho mpls forwarding-table
Local Outgoing Prefix Bytes tag Outgoing Next Hop
tag tag or VC or Tunnel Id switched interface
16 Pop tag 1.1.1.1/32 31006 Fa0/0 172.16.12.1
- R3 （ASBR1 in AS 100）
ASBR不感知×××的存在
实际上，这个公网标签都是由LDP分配的，但 BGP会使用该标签继续分配给EBGP邻居
C3600-R3#show mpls forwarding-table
Local Outgoing Prefix Bytes tag Outgoing Next Hop
tag tag or VC or Tunnel Id switched interface
17 16 1.1.1.1/32 3984 Fa0/0 172.16.23.2
可以看出，BGP分配的这个公网标签是来自LDP 的
C3600-R3#show ip bgp labels
Network Next Hop In Label/Out Label
1.1.1.1/32 172.16.23.2 17/nolabel
C3600-R3#show ip bgp 1.1.1.1
BGP routing table entry for 1.1.1.1/32, version 5
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Advertised to update-groups:
1
Local
172.16.23.2 from 0.0.0.0 (3.3.3.3)
Origin IGP, metric 3, localpref 100, weight 32768, valid, sourced, local,
best
mpls labels in/out 17(from LDP)/nolabel
- R4 （ASBR2 in AS 200）
这个公网标签都是由EBGP分配的
C3600-R4#show mpls forwarding-table
Local Outgoing Prefix Bytes tag Outgoing Next Hop
tag tag or VC or Tunnel Id switched interface
18 17 1.1.1.1/32 4395 Fa0/0 172.16.34.3
C3600-R4#show ip bgp labels
Network Next Hop In Label/Out Label
1.1.1.1/32 172.16.34.3 18/17
C3600-R4#show ip bgp 1.1.1.1
BGP routing table entry for 1.1.1.1/32, version 4
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Not advertised to any peer
100
172.16.34.3 from 172.16.34.3 (3.3.3.3)
Origin IGP, metric 3, localpref 100, valid, external, best
mpls labels in/out 18/17
- R5 （PE2 in AS 200）
C3600-R5#show mpls forwarding-table
Local Outgoing Prefix Bytes tag Outgoing Next Hop
tag tag or VC or Tunnel Id switched interface
20 18 1.1.1.1/32 0 Fa0/0 172.16.45.4
C3600-R5#show ip bgp ***v4 all labels
Network Next Hop In label/Out label
Route Distinguisher: 100:1 (***-a)
10.16.1.0/24 1.1.1.1 nolabel/20
C3600-R5#show ip cef vrf ***-a 10.16.1.0 255.255.255.0
10.16.1.0/24, version 8, epoch 0, cached adjacency 172.16.45.4
0 packets, 0 bytes
tag information set
local tag: ×××-route-head
fast tag rewrite with Fa0/0, 172.16.45.4, tags imposed: {18 20}
via 1.1.1.1, 0 dependencies, recursive
next hop 172.16.45.4, FastEthernet0/0 via 1.1.1.1/32
valid cached adjacency
tag rewrite with Fa0/0, 172.16.45.4, tags imposed: {18 20}
C3600-R5#traceroute vrf ***-a 10.16.1.1
Type escape sequence to abort.
Tracing the route to 10.16.1.1
1 172.16.45.4 [MPLS: Labels 18/20 Exp 0] 180 msec 244 msec 308 msec
2 172.16.34.3 [MPLS: Labels 17/20 Exp 0] 220 msec 500 msec 252 msec
3 172.16.23.2 [MPLS: Labels 16/20 Exp 0] 216 msec 224 msec 244 msec
4 10.16.1.1 [MPLS: Label 20 Exp 0] 280 msec 356 msec *
7. 额外配置
在实现Option C时，需要在ASBR上做一些额外的配置，让EBGP邻居之间发布公网标签
neighbor 172.16.34.4 route-map Rec_200 in
neighbor 172.16.34.4 route-map Adv_200 out
neighbor 172.16.34.4 send-label
route-map Rec_200 permit 10
match ip address prefix-list Rec_200
match mpls-label
route-map Adv_200 permit 10
match ip address prefix-list Adv_200
set mpls-label
ip prefix-list Adv_200 seq 10 permit 1.1.1.1/32
ip prefix-list Rec_200 seq 10 permit 5.5.5.5/32
如果要为所有的公网地址都发送标签，可以不用配置route-map，只需要一条”neighbor 172.16.34.4 send-label“就搞定了。
CE1------[R1(PE1)---R2(P)---R3(ASBR1)]------[R4(ASBR2)----R5(PE2)]----------CE2
[AS 100] [AS 200]
2. 基本原理
PE与ASBR之间不需要建立IBGP邻居，只是ASBR之间建立EBGP，PE之间建立MP-EBGP邻居。PE之间的可达性，是将ASBR将BGP路由重新分发到IGP中来保证的。
3. 标签分配过程
×××私网标签由PE1直接分配给PE2；
到 PE1 Loopback接口地址的公网标签逐跳分发。需要说明的是，公网标签在域内是由LDP来分发的，在域间是由MP-EBGP来分发的。
4. 标签转发过程
非常简单，和MP-IBGP MPLS ×××的转发过程一样
5. 特点
ASBR 上不需要感知×××的存在，只需要PE来维护×××路由和私网标签转发表。
6. 各LSR上的相关表项
- R1 （PE1 in AS 100）
C3600-R1#show ip bgp ***v4 all labels
Network Next Hop In label/Out label
Route Distinguisher: 100:1 (***-a)
10.16.1.0/24 0.0.0.0 20/aggregate(***-a)
C3600-R1#show mpls forwarding-table
Local Outgoing Prefix Bytes tag Outgoing Next Hop
tag tag or VC or Tunnel Id switched interface
20 Aggregate 10.16.1.0/24[V] 5752
C3600-R1#show ip cef vrf ***-a 10.16.1.0 255.255.255.0
10.16.1.0/24, version 7, epoch 0, attached, connected
0 packets, 0 bytes
tag information set
local tag: 20
via FastEthernet1/0, 0 dependencies
valid glean adjacency
tag rewrite with , , tags imposed: {}
- R2 （P in AS 100）
P路由器感知不到××× LSP，所以只需要关注公网LSP
C3600-R2#sho mpls forwarding-table
Local Outgoing Prefix Bytes tag Outgoing Next Hop
tag tag or VC or Tunnel Id switched interface
16 Pop tag 1.1.1.1/32 31006 Fa0/0 172.16.12.1
- R3 （ASBR1 in AS 100）
ASBR不感知×××的存在
实际上，这个公网标签都是由LDP分配的，但 BGP会使用该标签继续分配给EBGP邻居
C3600-R3#show mpls forwarding-table
Local Outgoing Prefix Bytes tag Outgoing Next Hop
tag tag or VC or Tunnel Id switched interface
17 16 1.1.1.1/32 3984 Fa0/0 172.16.23.2
可以看出，BGP分配的这个公网标签是来自LDP 的
C3600-R3#show ip bgp labels
Network Next Hop In Label/Out Label
1.1.1.1/32 172.16.23.2 17/nolabel
C3600-R3#show ip bgp 1.1.1.1
BGP routing table entry for 1.1.1.1/32, version 5
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Advertised to update-groups:
1
Local
172.16.23.2 from 0.0.0.0 (3.3.3.3)
Origin IGP, metric 3, localpref 100, weight 32768, valid, sourced, local,
best
mpls labels in/out 17(from LDP)/nolabel
- R4 （ASBR2 in AS 200）
这个公网标签都是由EBGP分配的
C3600-R4#show mpls forwarding-table
Local Outgoing Prefix Bytes tag Outgoing Next Hop
tag tag or VC or Tunnel Id switched interface
18 17 1.1.1.1/32 4395 Fa0/0 172.16.34.3
C3600-R4#show ip bgp labels
Network Next Hop In Label/Out Label
1.1.1.1/32 172.16.34.3 18/17
C3600-R4#show ip bgp 1.1.1.1
BGP routing table entry for 1.1.1.1/32, version 4
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Not advertised to any peer
100
172.16.34.3 from 172.16.34.3 (3.3.3.3)
Origin IGP, metric 3, localpref 100, valid, external, best
mpls labels in/out 18/17
- R5 （PE2 in AS 200）
C3600-R5#show mpls forwarding-table
Local Outgoing Prefix Bytes tag Outgoing Next Hop
tag tag or VC or Tunnel Id switched interface
20 18 1.1.1.1/32 0 Fa0/0 172.16.45.4
C3600-R5#show ip bgp ***v4 all labels
Network Next Hop In label/Out label
Route Distinguisher: 100:1 (***-a)
10.16.1.0/24 1.1.1.1 nolabel/20
C3600-R5#show ip cef vrf ***-a 10.16.1.0 255.255.255.0
10.16.1.0/24, version 8, epoch 0, cached adjacency 172.16.45.4
0 packets, 0 bytes
tag information set
local tag: ×××-route-head
fast tag rewrite with Fa0/0, 172.16.45.4, tags imposed: {18 20}
via 1.1.1.1, 0 dependencies, recursive
next hop 172.16.45.4, FastEthernet0/0 via 1.1.1.1/32
valid cached adjacency
tag rewrite with Fa0/0, 172.16.45.4, tags imposed: {18 20}
C3600-R5#traceroute vrf ***-a 10.16.1.1
Type escape sequence to abort.
Tracing the route to 10.16.1.1
1 172.16.45.4 [MPLS: Labels 18/20 Exp 0] 180 msec 244 msec 308 msec
2 172.16.34.3 [MPLS: Labels 17/20 Exp 0] 220 msec 500 msec 252 msec
3 172.16.23.2 [MPLS: Labels 16/20 Exp 0] 216 msec 224 msec 244 msec
4 10.16.1.1 [MPLS: Label 20 Exp 0] 280 msec 356 msec *
7. 额外配置
在实现Option C时，需要在ASBR上做一些额外的配置，让EBGP邻居之间发布公网标签
neighbor 172.16.34.4 route-map Rec_200 in
neighbor 172.16.34.4 route-map Adv_200 out
neighbor 172.16.34.4 send-label
route-map Rec_200 permit 10
match ip address prefix-list Rec_200
match mpls-label
route-map Adv_200 permit 10
match ip address prefix-list Adv_200
set mpls-label
ip prefix-list Adv_200 seq 10 permit 1.1.1.1/32
ip prefix-list Rec_200 seq 10 permit 5.5.5.5/32
如果要为所有的公网地址都发送标签，可以不用配置route-map，只需要一条”neighbor 172.16.34.4 send-label“就搞定了。