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功能简介：

伪链路（sham-link）不是一条真正的链路，它是指在两台PE上的VRF间建立的一条“虚拟链路”。伪链路和正常的OSPF 链路一样，有自己的OSPF 接口，可以发送OSPF 协议报文，建立邻居，发送LSA ，当LSA 在伪链路中进行泛洪时，所有的OSPF 路由类型不会改变。

不同PE的VRF OSPF实例建立伪链路的目的主要有以下两点：

1）通过MP-IBGP携带私网路由的方式，只是传递路由，到达对端PE后的还原工作也只是尽力而为式的引入原有OSPF 路由信息，并不能真正使OSPF 的拓扑信息得到沟通。通过伪链路在建立一条OSPF 链路，从而使各个站点内的OSPF 实例能够真正连通，并建立完整的拓扑信息。

2）同一VPN内的不同站点通过MPLS 骨干网交互信息，但是这些VPN站点内部又连接一条链路，其目的是在MPLS 骨干网无法使用时，VPN站点间仍然可以通过该链路通信，这种链路称为“后门链路”。如果VPN用户的两个站点同属于同一个OSPF 区域，且这两个站点间又连接一条“后门链路”，那么两个站点内的路由即通过MPLS 骨干网交互，也通过“后门链路”交互，由于通过MPLS 骨干网交互后的路由为域间路由，而通过“后门链路”交互后的路由为域内路由，显然后门链路所通告的域内路由优于MPLS 骨干网通告的域间路由，因而这两个站点内的路由转发会优先走后门链路，这不符合 VPN用户连接“后门链路”的意图，因此这种应用也需要使用伪链路。

一、组网需求

SW2与SW3建立sham-link，得到的结果是：

1、SW2从SW3学习到的4.4.4.4/32路由为OSPF域内路由；

2、SW1由于从SW2学习到的是域内路由，优于从SW3学习到的VPN路由，因此SW1去往4.4.4.4/32的路径是 SW1-->SW5-->SW2-->SW3-->SW4。

二、组网拓扑

三、配置要点

1、设备基础配置，参考”OSPF VPN案例剖析“章节

2、新建LOOP接口，将该LOOP接口添加进VRF：

3、将LOOP接口通告进L3VPN：

4、配置sham-link

四、配置步骤

1、新建LOOP接口，将该LOOP接口添加进VRF：

SW2:

interface Loopback 1

ip vrf forwarding ruijie

ip address 22.22.22.22 255.255.255.255

SW3:

interface Loopback 1

ip vrf forwarding ruijie

ip address 33.33.33.33 255.255.255.255

2、将LOOP接口通告进L3VPN：

SW2:

router bgp 1

address-family ipv4 vrf ruijie

network 22.22.22.22 mask 255.255.255.255

SW3:

router bgp 1

address-family ipv4 vrf ruijie

network 33.33.33.33 mask 255.255.255.255

3、配置sham-link

SW2:

router ospf 1 vrf ruijie

area 0 sham-link 22.22.22.22 33.33.33.33

SW3:

router ospf 1 vrf ruijie

area 0 sham-link 33.33.33.33 22.22.22.22

----->技巧：

sham-link必须使用LOOP地址建立，且LOOP接口必须通告入OSPF，否则sham-link无法建立。如果配置错误，设备会自动告警，例如：

SW1(config-router)#area 0 sham-link 10.10.15.1 20.20.34.3

SW1(config-router)#*Mar 18 12:21:56: %7: Warning: The source address of the sham-link must be a 32-bit loopback address bound to local vrf ruijie，please re-configure it.*Mar 18 12:21:56: %7: Warning: The source address of the sham-link should not be advertised by OSPF.

五、功能验证

1、配置SW2与SW3的sham-link功能，确保sham-link邻居建立：

SW2#sho ip ospf sham-links

Sham Link SLINK0 to address 33.33.33.33 is up

Area 0.0.0.0 source address 22.22.22.22, Cost: 1

Output interface is GigabitEthernet 0/1

Nexthop address 3.3.3.3

Transmit Delay is 1 sec, State Point-To-Point,

Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5

Hello due in 00:00:07

Adjacency state Full

2、通过show ip route vrf ruijie查看SW1/SW2的路由表，确认4.4.4.4的路由为O而不是B或者O E2

SW1#sho ip rout vrf ruijie

Routing Table: ruijie

Codes: C - connected, S - static, R - RIP, B - BGP

O - OSPF, IA - OSPF inter area

N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2

E1 - OSPF external type 1, E2 - OSPF external type 2

i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2

ia - IS-IS inter area, * - candidate default

Gateway of last resort is no set

O 4.4.4.4/32 [110/4] via 10.10.15.5, 00:01:02, GigabitEthernet 0/1

O 5.5.5.5/32 [110/101] via 10.10.15.5, 04:55:06, GigabitEthernet 0/1

C 10.10.15.0/24 is directly connected, GigabitEthernet 0/1

C 10.10.15.1/32 is local host.

O 10.10.25.0/24 [110/2] via 10.10.15.5, 02:04:46, GigabitEthernet 0/1

O 20.20.34.0/24 [110/4] via 10.10.15.5, 00:01:02, GigabitEthernet 0/1

B 33.33.33.33/32 [200/0] via 3.3.3.3, 00:06:02

SW2#sho ip route vrf ruijie

Routing Table: ruijie

Codes: C - connected, S - static, R - RIP, B - BGP

O - OSPF, IA - OSPF inter area

N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2

E1 - OSPF external type 1, E2 - OSPF external type 2

i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2

ia - IS-IS inter area, * - candidate default

Gateway of last resort is no set

O 4.4.4.4/32 [110/2] via 3.3.3.3, 00:01:20

O 5.5.5.5/32 [110/101] via 10.10.25.5, 02:04:15, GigabitEthernet 0/1

O 10.10.15.0/24 [110/2] via 10.10.25.5, 02:04:15, GigabitEthernet 0/1

C 10.10.25.0/24 is directly connected, GigabitEthernet 0/1

C 10.10.25.2/32 is local host.

O 20.20.34.0/24 [110/2] via 3.3.3.3, 00:01:20

C 22.22.22.22/32 is local host.

B 33.33.33.33/32 [200/0] via 3.3.3.3, 00:01:38

3、通过show ip route 查看SW5/SW4路由表，确认学习到的私网路由为O

SW4#sho ip route

Codes: C - connected, S - static, R - RIP, B - BGP

O - OSPF, IA - OSPF inter area

N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2

E1 - OSPF external type 1, E2 - OSPF external type 2

i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2

ia - IS-IS inter area, * - candidate default

Gateway of last resort is no set

C 4.4.4.4/32 is local host.

O 5.5.5.5/32 [110/103] via 20.20.34.3, 00:02:05, GigabitEthernet 0/1

O 10.10.15.0/24 [110/4] via 20.20.34.3, 00:02:05, GigabitEthernet 0/1

O 10.10.25.0/24 [110/3] via 20.20.34.3, 00:02:05, GigabitEthernet 0/1

C 20.20.34.0/24 is directly connected, GigabitEthernet 0/1

C 20.20.34.4/32 is local host.

O E2 22.22.22.22/32 [110/1] via 20.20.34.3, 00:02:24, GigabitEthernet 0/1

O E2 33.33.33.33/32 [110/1] via 20.20.34.3, 00:02:04, GigabitEthernet 0/1

SW5#sho ip route

Codes: C - connected, S - static, R - RIP, B - BGP

O - OSPF, IA - OSPF inter area

N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2

E1 - OSPF external type 1, E2 - OSPF external type 2

i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2

ia - IS-IS inter area, * - candidate default

Gateway of last resort is no set

O 4.4.4.4/32 [110/3] via 10.10.25.2, 00:02:32, GigabitEthernet 0/2

C 5.5.5.5/32 is local host.

C 10.10.15.0/24 is directly connected, GigabitEthernet 0/1

C 10.10.15.5/32 is local host.

C 10.10.25.0/24 is directly connected, GigabitEthernet 0/2

C 10.10.25.5/32 is local host.

O 20.20.34.0/24 [110/3] via 10.10.25.2, 00:02:32, GigabitEthernet 0/2

O E2 22.22.22.22/32 [110/1] via 10.10.25.2, 00:02:31, GigabitEthernet 0/2

O E2 33.33.33.33/32 [110/1] via 10.10.15.1, 00:02:48, GigabitEthernet 0/1

[110/1] via 10.10.25.2, 00:02:48, GigabitEthernet 0/2

4、通过show ip ospf da 查看SW1/SW2的OSPF数据库，确认私网4.4.4.4的1类LSA

SW1# sho ip os da router ----->这里截取了部分

LS age: 871

Options: 0x2 (-|-|-|-|-|-|E|-) ----->注意到，这里DN位没有置位

Flags: 0x0

LS Type: router-LSA

Link State ID: 20.20.34.4

Advertising Router: 20.20.34.4

LS Seq Number: 80000014

Checksum: 0xb82e

Length: 48

Number of Links: 2

Link connected to: a Transit Network

(Link ID) Designated Router address: 20.20.34.4

(Link Data) Router Interface address: 20.20.34.4

Number of TOS metrics: 0

TOS 0 Metric: 1

Link connected to: Stub Network

(Link ID) Network/subnet number: 4.4.4.4

(Link Data) Network Mask: 255.255.255.255

Number of TOS metrics: 0

TOS 0 Metric: 0

通过以上信息验证，可以确认sham-link功能正常；也可学习到，sham-link功能启用后，OSPF路由在路由表的存在形式。