Tag :: PE-CE

The EOS implementation of OSPF uses an alternate Area Border Router (ABR) behavior. This is implemented as an optimization over the standard OSPF so that the packets would not be dropped when a router loses Active backbone connection which could otherwise be successfully forwarded. As per this new behavior, when an ABR loses active backbone connection, it is allowed to consider summary-lsa from non-backbone area during SPF calculation and the subsequent route installation process thus ensuring improved connectivity. The EOS implementation of OSPFv3 also inherits the same behavior.

This feature adds the support for OSPFv3 multi-site domains (currently this feature is added for IPv6 address family only) described in RFC6565 (OSPFv3 as a Provider to Customer Edge Protocol for BGP/MPLS IP Virtual Private Networks (VPNs) ) and enables routes BGP VPN routes to retain their original route type if they are in the same OSPFv3 domain. Two sites are considered to be in the same OSPFv3 domain if it is intended that routes from one site to the other be considered intra-network routes.

Configuring OSPF as PE-CE protocol enables us to distinguish between the “real external routes” and intra network routes between the sites that are stretched across VPN.  But the problem arises when VPN sites are in the same area and have a backdoor connection. With OSPF as PE-CE protocol redistribution, CE routers end up getting inter-area routes(assuming the VRFs on the PE devices that connect the CE sites, are configured with the same OSPF domain id) that actually belong to the same area and just happen to be multihomed to the backbone. 

Configuring OSPF as PE-CE protocol enables us to distinguish between the “real external routes” and intra network routes between the sites that are stretched across VPN.  But the problem arises when VPN sites are in the same area and have a backdoor connection. With OSPFv3 as PE-CE protocol redistribution, CE routers end up getting inter-area routes (assuming the VRFs on the PE devices that connect the CE sites, are configured with the same OSPFv3 domain id) that actually belong to the same area and just happen to be multihomed to the backbone.