%BGP-4-VPNV4NH_MASK : Nexthop [IP_address] may not be reachable from neigbor [IP_address] – not /32 mask

While practicing MPLS using OSPF as the IGP in the P/PE network, encounter a peculiar results when iBGP received routes from PE but CE are not able to reach the other CE. After some research, apparently the connectivity is broken due to the loopback ip address that was configured.

VPNv4 route is being sent to the iBGP neighbor. The address of the next hop is a loopback interface that does not have a /32 mask defined. OSPF is being used on this loopback interface, and the OSPF network type of this interface is LOOPBACK. OSPF advertises this IP address as a host route (with mask /32), regardless of what mask is configured. This advertising conflicts with TDP, which uses configured masks, so the TDP neighbors may not receive a tag for the route indicated in this error message. This condition could break connectivity between sites that belong to the same VPN.

Solution A : Configure the loopback that is being used as the next-hop loopback to use a 32-bit network mask (/32)
Solution B : Configure the network type to point-to-point by entering the ip ospf network point-to-point command under the loopback interface.

December 7, 2011 | Filed Under Network | Leave a Comment 
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