Using Selective ACK on Virtual Address

We have a 2 tier system whereby Tier 1 needs to know quickly if Tier 2 services have stopped, be that VS or the pools or the box itself.

 

To that end we've setup an ICMP monitor at tier 1 to ping the VS at Tier 2 and we've configured the ICMP option within Virtual Address to be Selective (ie don't respond if VS is red). This works well most of the time.

 

However every now and again we'll see that a service has been marked down at Tier 1 and the ICMP monitor is the one saying it had marked the service down. You go to Tier 2 and the VS is green (it has been down but come back up) and a TCPDUMP shows that the ICMP request is getting to Tier 2 but Tier 2 is NOT sending an ICMP response.

 

Typically the only fix is to reboot the Tier 2 box - which is not ideal in a production environment.

 

I have opened a case with F5 for this but wondered if anybody else had come across something similar etc.