Cisco IOS XR Ospf Sham-link Configuration Example

To provide the backdoor link of CE routers wait as a backup link we can enable sham-link on PE routers.Here is the configuration on a IOS XR PE router:

router ospf 100

vrf test

redistribute bgp 1

address-family ipv4 unicast

area 0

sham-link 1.1.1.1 2.2.2.2 ==>1.1.1.1 is a local loopback ,2.2.2.2 is a remote loopback of PE routers

!

interface GigabitEthernet0/1/0/1.1

router bgp 1

vrf test

rd 1.1.1.1:100

address-family ipv4 unicast

redistribute connected ==>redistribute connected or network command for shamlink loopbacks

redistribute static

redistribute ospf 100

RP/0/RSP0/CPU0:TESTROUTER#show ospf vrf test neighbor

Neighbors for OSPF 100, VRF test

Neighbor ID     Pri   State           Dead Time   Address         Interface

2.2.2.2       0     FULL/  -        00:00:36    2.2.2.2       OSPF_SL0

Neighbor is up for 00:19:28

1.1.1.10        1     FULL/DR         00:00:31    10.10.10.2      GigabitEthernet0/1/0/1.1

Neighbor is up for 01:02:03

Total neighbor count: 2

CPE configuration:

router ospf 100

router-id 1.1.1.10

network 1.1.1.10 0.0.0.0 area 0     è lo0 ip address of CPE router

network 10.10.10.2 0.0.0.0 area 0 è connection to PE-1(Vlan100)

network 10.10.11.1 0.0.0.0 area 0 è backdoor link of CPE routers

10.10.15.0/24 is a connected interface`s IP Prefix on CPE-2 router which is attached to area 0.

When the backdoor link is active and Ospf neighborship is established between CPE routers, CPE-1 sees this ip route through the backdoor link.

Lets close the backdoor link...

IP routing table entry for 10.10.15.0/24 on CPE router before the sham-link is active on PE routers:

CPE-1#show ip route ospf

O IA    10.10.15.0/24 [110/50] via 10.10.10.1, 00:00:02, Vlan100

Intra area routes come as inter area routes when they are redistributed from BGP to OSPF.

After sham-link configuration:

CPE-1#show ip route ospf

10.10.15.0/24 [110/2] via 10.5.18.5, 00:12:00, Vlan100

We see that, after we had configured shamlink between PE-1 and PE-2, previously coming inter-area routes became intra area routes as they were originated.This can prevent choosing backdoor link instead of Mpls VPN network.