DUAL Message Types

DUAL messages are sent in one of three message types: queries, updates, and replies. Queries are sent when a destination becomes unreachable. Updates are sent to indicate a change of metric to advertise a new destination. Replies are sent in response to the queries from a neighbor.

If a query is received when no feasible successor is found, the query is propagated. If a feasible successor is found, the query is not propagated, and a reply is sent to the neighbor. Conditions for sending queries are as follows:

• When a direct connect interface is down

• When a query has been received

• When the metric has changed

• When an update has been received

Conditions for sending replies are as follows:

• When a feasible successor is present

• When a query is received from an active route

• When the route state changes from active to passive

Conditions for sending updates are as follows:

• When the metric has changed

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