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OSPF E2 Routes In PE-CE

This document will show when customer run OSPF with PE in that case the routes advertised from one CE to another CE travelling MPLS backbone should be IA(Inter Area) routes but in some cases the routes could be external. MPLS backbone is working as trasparent to the customer, so for customer every route should be inter area route. But may be a day you get a call from your customer which is getting all its routes as external routes instead of inter area routes.

When VPNV4 routes can be external?
With respect to a particular OSPF instance associated with a VRF, a  VPN-IPv4 route that is installed in the VRF and then selected as the preferred route is treated as an External Route if one of the   following conditions holds:

a)OSPF route type should be external.
b)Route is from different domain of OSPF instance.

Option a) is very much clear and I will be showing the simulation for option b). Every good administator don't want external routes in the network becasue they occupy more space than normal route. Option a) & b) is cleary defined in RFC4577 section

Main Topology

Design Topology
CE2 is connected to PE2 as OSPF as routing protocol in area 0; Same is used between CE1 & PE1. OSPF neighborship are up among the clients. At PE end OSPF routes are redistributed in MP-BGP vrf instance and MP-BGP vrf routes are redistributed in OSPF. 

 Configuration PE1
 router ospf 2 vrf TEST
 redistribute bgp 1 metric 10 subnets
 router bgp 1
 address-family ipv4 vrf TEST
 redistribute connected
 redistribute ospf 2 vrf TEST metric 10
 no synchronization

Configuration PE2
 router ospf 10 vrf TEST
 redistribute bgp 1 metric 10 subnets
 router bgp 1
 address-family ipv4 vrf TEST
 redistribute connected
 redistribute ospf 10 vrf TEST metric 10
 no synchronization

Figure 1

Figure 2

Note:- Check the superbackbone in figure 1 & figure 2. From this it is also conforms that service provider backbone works as superbackbone. So no need for area 0 for CE vpn.

On CE1 a route is advertised in OSPF and let see how the route looks in CE2 routing table.
Figure 3

Stumble to see the route looks as external route but it should come as inter area route. Everything seems fine routes are coming but whats the reason for E2 route.

Turning Point
Change the OSPF process id to 10 for vrf TEST on PE1.
Figure 4

See the route now appears as IA route. Weired Problem solved.

As per RFC4577 section, If we use the different OSPF instance id on PE routers for same VPN the route type changes from inter area route to external route.

Shivlu Jain