Create an ICMP probe on S-series and add it to the tunnel interface using the command "tunnel probe probe-name <probe>".
This ensures the GRE tunnel interface will change its state to "Down" if the Purview's interface becomes unreachable.
It would be advisable to change the ageing timers in such way ARP entries age out before MAC entries do. The following commands ensure ARP entries age out 5min before the MAC entries do.
set mac agetime 14700 /* 4h5m
arp timeout 14400 /* 4h0m