OSPF- SHAM Link

Hi Fellas,

Think about 2 customers want you to merge their network over you MPLS network.
You use Layer 3 VPN but customer is informed you about they use methro-ethernet vs. for backup link for their remote sites (backdoor). You think the interface may remain shutdown, but when it is up, it may **** up everythink.

In this case, we will analyze the backdoor-link ;




CE1 ping CE2's loopback address (5.5.5.5) over MPLS network.
Backdoor link is not active !!!!!!

As you can see, 172.23.55.0 network ( PE2-CE2) is learned over 172.23.54.16 (CE1's next hop)
which is so correct.

O IA is , ospf inter area routes :  

Just in case as a reminder:

just O is in OSPF terminology : Intra Area Routes that; sharing routes in the same OSPF domain !

When we activate the backdoor link;

routing table is become like ;

for CE2;

172.23.54.0 (CE1) is learnt over backdoor link !

It by-passed the MPLS network because..........,

there is some "O" front of it !!!!

O is Intra-Area routes that sharing advertises in same OSPF domain,

that when sharing advertises for OSPF calculation,

OSPF always choose INTRA-AREA routes instead INTER-AREA routes.

So we need to throw out some config :)

PE1;

router ospf 2 vrf asd

 log-adjacency-changes

 area 0 sham-link 31.31.31.31 32.32.32.32

 redistribute bgp 1 subnets

 network 172.23.54.0 0.0.0.255 area 0

PE2;

router ospf 2 vrf asd

 log-adjacency-changes

 area 0 sham-link 32.32.32.32 31.31.31.31

 redistribute bgp 1 subnets

 network 172.23.55.0 0.0.0.255 area 0

Regards !