Router advertisement messages include a preference level and a lifetime field for each advertised router address.
The preference level specifies the router’s preference to become the default router. When a host chooses a default router address, it chooses the address with the highest preference. You can configure the preference level with the priority statement.
The lifetime field indicates the maximum length of time that the advertised addresses are to be considered valid by hosts in the absence of further advertisements. You can configure the advertising rate with the max-advertisement-interval and min-advertisement-interval statements, and you can configure the lifetime with the lifetime statement.