Neighbor discovery cache

The neighbor discovery cache lists information about neighbors in the network and can contain the following types of neighbors:

The following table describes the states in the neighbor cache.

Table 1. Neighbor cache states

State

Description

Incomplete

Address resolution is in progress and the system has not yet determined the link-layer address of the neighbor.

The neighbor cache may also enter the Incomplete state when the switch cannot confirm subsequent reachability during the ND process for router neighbors. By contrast, the system deletes host neighbors, rather than enter the Incomplete state, if ND fails to confirm reachability.

Tip:

Router neighbors: when the R bit is set in the received neighbor advertisement

Host neighbors: when the R bit is not set in the received neighbor advertisement

Reachable

A node receives positive confirmation within the last reachable time period.

Stale

Reachability of the neighbor is unknown.

Until the system sends traffic to the neighbor, make no attempt to verify its reachability.

Delay

A time period longer than the reachable time period passes since the node received the last positive confirmation, and a packet was sent within the last DELAY_FIRST_PROBE_TIME period.

If no reachability confirmation is received within the DELAY_FIRST_PROBE_TIME period after entering the DELAY state, neighbor solicitation is sent and the state changes to probe.

Probe

Reachability confirmation is sought from the device every retransmit timer period.

The following events involve Layer 2 and Layer 3 interaction during processing and affect the neighbor cache:

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