>What you'd do is bump a generation number when a back-end is registered
>or unregistered.  Then you consult that generation number when assigning
>a back-end to a request -- if it's newer than what the session has cached,
>then the session needs to refresh its notion of which back-ends exist.

jsut be sure to use a large enough type for this generation number
such that it can't conceivably roll over within the uptime of the
machine.  the ifindex is a little fragile because of this (repeatedly
cloning and destroying interfaces can easily overflow the 16 bits that
are used in some places).

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