OSPF sham link configuration example using Cisco IOS XR

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:

PE1:

interface Loopback1
 vrf VRF_OSPF
 ipv4 address 172.0.1.4 255.255.255.255    ---->Sham link source IP

!

router ospf VRF_OSPF
 vrf VRF_OSPF
  redistribute bgp 65545
  area 0
   sham-link 172.0.1.4 172.0.1.5
   !
   interface GigabitEthernet0/0/0/6     ---> Link to CE1
   !
  !
 !

!

router bgp 65545
 vrf VRF_OSPF
  rd 65545:3
  address-family ipv4 unicast
   redistribute connected
  !
 !

!


PE2:

interface Loopback1
 vrf VRF_OSPF
 ipv4 address 172.0.1.5 255.255.255.255    ---->Sham link source IP

!


router ospf VRF_OSPF
 vrf VRF_OSPF
  redistribute bgp 65545
  area 0
   sham-link 172.0.1.5 172.0.1.4
   !
   interface GigabitEthernet0/0/0/6     ---> Link to CE1
   !
  !

 !

router bgp 65545
 vrf VRF_OSPF
  rd 65545:3
  address-family ipv4 unicast
   redistribute connected
  !
 !
!

CE1:

router ospf VRF_OSPF
 redistribute connected
 address-family ipv4 unicast
 area 0
  interface GigabitEthernet0/0/0/6    ---->link to PE1
  !
  interface GigabitEthernet0/0/0/8    --->backdoor link
  !
 !

!

CE2:

router ospf VRF_OSPF
 redistribute connected
 address-family ipv4 unicast
 area 0
  interface GigabitEthernet0/0/0/6  ---->link to PE2
  !
  interface GigabitEthernet0/0/0/8  --->backdoor link
  !
 !

!

Check the result:

RP/0/0/CPU0:Edge1#show ospf vrf VRF_OSPF sham-links 
Sat Apr 29 08:48:27.436 UTC

Sham Links for OSPF VRF_OSPF, VRF VRF_OSPF

Sham Link OSPF_SL0 to address 172.0.1.5 is up
Area 0, source address 172.0.1.4
IfIndex = 12
  Run as demand circuit
  DoNotAge LSA allowed., Cost of using 1
  Transmit Delay is 1 sec, State POINT_TO_POINT,
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
    Hello due in 00:00:06:427
    Adjacency State FULL (Hello suppressed)
    Number of DBD retrans during last exchange 0
    Index 2/2, retransmission queue length 0, number of retransmission 0
    First 0(0)/0(0) Next 0(0)/0(0)
    Last retransmission scan length is 0, maximum is 0

    Last retransmission scan time is 0 msec, maximum is 0 msec




RP/0/0/CPU0:Edge1#show ospf vrf VRF_OSPF neighbor 
Sat Apr 29 08:49:35.922 UTC

* Indicates MADJ interface
# Indicates Neighbor awaiting BFD session up

Neighbors for OSPF VRF_OSPF, VRF VRF_OSPF

Neighbor ID     Pri   State           Dead Time   Address         Interface
10.127.224.2    1     FULL/  -           -        172.0.1.5       OSPF_SL0
    Neighbor is up for 00:44:16
30.0.0.0        1     FULL/BDR        00:00:35    172.0.0.3       GigabitEthernet0/0/0/6
    Neighbor is up for 03:12:02


Total neighbor count: 2



Before configuring sham link, the routes from CE2 on PE1 points to CE1 and go pass backdoor link

RP/0/0/CPU0:Edge1# show route vrf VRF_OSPF 30.0.1.186/32 
Sat Apr 29 09:03:12.326 UTC

Routing entry for 30.0.1.186/32
  Known via "ospf VRF_OSPF", distance 110, metric 20, type extern 2
  Installed Apr 29 09:03:08.046 for 00:00:04
  Routing Descriptor Blocks
    172.0.0.3, from 172.0.0.5, via GigabitEthernet0/0/0/6
      Route metric is 20
  No advertising protos. 

After configuring sham link, the routes points to PE2 via MPLS network

RP/0/0/CPU0:Edge1# show route vrf VRF_OSPF 30.0.1.186/32 
Sat Apr 29 09:06:30.362 UTC

Routing entry for 30.0.1.186/32
  Known via "ospf VRF_OSPF", distance 254, metric 20, type extern 2
  Installed Apr 29 09:06:26.493 for 00:00:03
  Routing Descriptor Blocks
    172.0.1.5, from 172.0.0.5
      Route metric is 20

  No advertising protos.

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.