MPLS - CE-PE routing : OSPF Sham Link

Change to this network topology.

I have added backup link between R4 and R5 (R4:int f0/0.45 - int f0/0/45:R5), and then  included into OSPF Area 0.
By default, OSPF networks prefers the network that comes from same area which is area 0. Therefore, routes of R4 and R5 are follow. Even if the bandwidth between R4 and R5 through MPLS VPN are faster(100Mbps), R4 and R5 get advertized through 10Mbps link which is backup link. 

Configs for this network. are from here...

R1#
router ospf 1
network 1.1.14.1 0.0.0.0 area 0
no area 1 virtual-link 1.1.4.4
!

R2#
router ospf 1
network 1.1.25.2 0.0.0.0 area 0
!

R4#
router ospf 1
network 1.1.4.4 0.0.0.0 area 0
network 1.1.14.4 0.0.0.0 area 0
no area 1 virtual-link 1.1.1.1
exit
int f0/0.45
encap dot1q 45
ip add 1.1.45.4 255.255.255.0
bandw 10000
exit
router ospf 1
network 1.1.45.4 0.0.0.0 area 0
exit
!

R5#
router ospf 1
network 1.1.5.5 0.0.0.0 area 0
network 1.1.25.5 0.0.0.0 area 0
exit
int f0/0.45
encap dot1q 45
ip add 1.1.45.5 255.255.255.0
bandw 10000
exit
router ospf 1
network 1.1.45.5 0.0.0.0 area 0
!

SW1#
vlan 45
exit
!

--------------------------------------------------

To solve this problem, use the Sham Link.

The sham link works like that when OSPF advertises through MPSL VPN, the LSA type becomesType 1 instead of Type 3.

First, before creating sham link, configure the ip address on router R1, R2 and R3.

          The ip addresses must be complied followings.

• The ip addresses must be included into VRF.
• The ip addresses must not be included into OSPF.
• The ip addresses must be advertised through BGP.

Configuration

R1#

int lo1

ip vrf forwarding company1

ip add 2.2.1.1 255.255.255.255

exit

router bgp 1

address-family ipv4 vrf company1

network 2.2.1.1 mask 255.255.255.255

exit

R2#

R1#

int lo1

ip vrf forwarding company1

ip add 2.2.1.2 255.255.255.255

exit

router bgp 1

address-family ipv4 vrf company1

network 2.2.1.2 mask 255.255.255.255

exit

Second, configure sham link

R1#

router ospf 1 vrf company1

area 0 sham-link 2.2.1.1 2.2.1.2

R2#

router ospf 1 vrf company1

area 0 sham-link 2.2.1.2 2.2.1.1

verify the existence of sham link and the changed route from R1 to R4

Test for backup link.

Shutdown the interface f0/0.25 in R5, and traceroute to 1.1.5.5 from R4.

The route to R5 has been changed from MPLS VPN route to backup link.