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.
