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If the failed gateway is not the current default, the IP layer can immediately switch to a default gateway. If it is the current default that failed, the IP layer MUST select a different default gateway (assuming more than one default is known) for the failed route and for establishing new routes.


When a gateway does fail, the other gateways on the connected network will learn of the failure through some inter-gateway routing protocol. However, this will not happen instantaneously, since gateway routing protocols typically have a settling time of 30-60 seconds. If the host switches to an alternative gateway before the gateways have agreed on the failure, the new target gateway will probably forward the datagram to the failed gateway and send a Redirect back to the host pointing to the failed gateway (!). The result is likely to be a rapid oscillation in the contents of the host's route cache during the gateway settling period. It has been proposed that the dead- gateway logic should include some hysteresis mechanism to prevent such oscillations. However, experience has not shown any harm from such oscillations, since service cannot be restored to the host until the gateways' routing information does settle down.


One implementation technique for choosing a new default gateway is to simply round-robin among the default gateways in the host's list. Another is to rank the gateways in priority order, and when the current default gateway is not the highest priority one, to "ping" the higher-priority gateways slowly to detect when they return to service. This pinging can be at a very low rate, e.g., 0.005 per second.

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Connected: An Internet Encyclopedia New Gateway Selection


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