A system for determining reachablity of a neighboring node from a node. A packet is received over a network by node. The packet has a destination address of a node other than the receiving node. The node reads a source address from the packet. The node then determines whether the source address is for a neighboring node in which the node has an interest. The node then determines whether the packet indicates a forward link from the node to the neighboring node. Reachablity information for the neighboring node is then stored in a memory responsive to a determination of the forward link.