The value of tmnxMsdpSACacheLifetime specifies lifetime given to
SA cache entries in seconds when created or refreshed.
This object does not measure time per se; instead, it is the
delta from the time at which an SA message is received at
which it should be expired if not refreshed. (i.e., it is
the value of tmnxMsdpSrcActiveExpiryTime immediately after
receiving an SA message applying to that row.)