5.3.12.1 When a Router Ceases Forwarding

When a router ceases forwarding it MUST stop advertising all routes, except for third party routes. It MAY continue to receive and use routes from other routers in its routing domains. If the forwarding database is retained, the router MUST NOT cease timing the routes in the forwarding database. If routes that have been received from other routers are remembered, the router MUST NOT cease timing the routes that it has remembered. It MUST discard any routes whose timers expire while forwarding is disabled, just as it would do if forwarding were enabled.

DISCUSSION

When a router ceases forwarding, it essentially ceases being a router. It is still a host, and must follow all of the requirements of Host Requirements [INTRO:2]. The router may still be a passive member of one or more routing domains, however. As such, it is allowed to maintain its forwarding database by listening to other routers in its routing domain. It may not, however, advertise any of the routes in its forwarding database, since it itself is doing no forwarding. The only exception to this rule is when the router is advertising a route that uses only some other router, but which this router has been asked to advertise.

A router MAY send ICMP destination unreachable (host unreachable) messages to the senders of packets that it is unable to forward. It SHOULD NOT send ICMP redirect messages.

DISCUSSION

Note that sending an ICMP destination unreachable (host unreachable) is a router action. This message should not be sent by hosts. This exception to the rules for hosts is allowed so that packets may be rerouted in the shortest possible time, and so that black holes are avoided.