Monday, April 17, 2006


Strangeness in the backup delay command.

The backup delay command that specifies how long the router will wait before activating a backup link upon the primary link going down and how long it will wait to deactivate a backup link already up upon the primary link coming back up has an option that doesn't quite make sense. The syntax for this interface sub-command is

backup delay {enable-delay | never} {disable-delay | never}

The enable-delay and disable-delay parameters range from 0 to 4294967294 and presumably never is meant to be infinity. The semantics for the never keyword in this command for the activation case is to never activate the backup and in the disable case is to never deactivate an activated backup link.

The latter case makes sense because one could imagine a user wanting a backup link that came up to stay up until some manual intervention is done to later bring it down. However the former case where the activation delay is infinity and the backup never comes up seems to be simply gratuitous. Under what conditions would you go to the trouble of configuring a backup interface but then specify that it should never come up?

I am guessing that this is the result of some engineer's attempt to make the command symmetrical for the enable and disable cases. Symmetry usually indicates elegance but it seems to me that in this case the command would have been best left asymmetrical with the never option only for the disable case.

Am I missing something? Is there indeed some situation where configuring something like

backup delay never 20

might make sense? I wonder.

I think you have truly stumbled across a worthless command option - I can't think of any reason for this.
I think you can use the Never option to not activate the backup if at some point you are testing stuff on the primary and bringing it down a lot and you don't want the backup to come up. Also if it's a dialup backup it might be technically good but economically prohibitive at some times to have that backup activate. So using the never leaves all the configuration there , just sort of disabled.
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