TABLE OF CONTENTS The reachability state of the neighbor is an important piece of information that is part of each Neighbor Cache entry. The reachability state of a neighbor can be one of five values: INCOMPLETE, REACHABLE, STALE, DELAY, or PROBE.
The reachability state of the neighbor is in the INCOMPLETE state when the cache entry is newly created. The INCOMPLETE state represents address resolution for the link-layer address of the neighbor being in progress. Packets destined to the neighbor are queued while the reachability state of the neighbor is INCOMPLETE.
The neighbor becomes REACHABLE when the address resolution completes successfully. At this state queued packets are transmitted immediately.
The STALE state indicates the neighbor is not known to be reachable. The state remains STALE until traffic is generated to the neighbor and at that point probe packets are sent to verify the reachability of that neighbor.
A node begins reachability probe when traffic is generated to a neighbor in the STALE state. The node will delay probe packet transmission for a short period to give upper layer protocols a chance to provide neighbor reachability information. This delay causes the neighbor reachability state to transit to the DELAY state. The neighbor in the DELAY state is considered no longer known to be reachable.
Probe packets are sent to neighbor once the delay period expires. Subsequently the neighbor reachability state transits into the PROBE state. The probe packets are unicast Neighbor Solicitation packets. The neighbor in the PROBE state...
