Following up on my question to the roundtable at Tuesday's meeting, I did some more troubleshooting yesterday, and traced the problem down to the LACP link between the switch and the backup appliance. Running packet captures on both the backup appliance and the windows client, I can see that ARP packets are being lost on the LACP link. When I ping from the windows client to the appliance, I see the arp request and reply at the appliance, but the reply never makes it to the windows client. Pinging the other direction, appliance to windows client I see the arp requests at the appliance, but they never make it to the windows client and go unanswered. The backup appliance only does LACP with XOR source+dest mac, so I configured the switch to match. If I change the load balance algorithm to anything else on the switch the link goes down entirely, so it appears to be the right choice. Could this be just a config problem with LACP?
In any case, I gave up on LACP specifically and switched the appliance to use round-robin and left the ports un-bonded on the switch which is working fine.
-- Wyatt Zacharias