This read-write value, in seconds, sets how long a dynamically learned RIPng route stays valid in the IPv6 routing table without receiving a refreshing update, defaulting to 180 seconds. If no update for a route arrives within this window it is treated as stale and aged out, so tuning it balances dropping a route to a dead neighbor quickly versus tolerating a few missed update cycles. For example, with the default 180-second expire time and a 30-second update interval, a route survives roughly six missed updates before being aged out.