So after quite a lot of testing we arrived at these conclusions:
Problem seen - ap-224 dual link sometimes (seemingly randomly) fails to form on start-up, power allocation problems suspected.
Tested on Cisco 2960-X and HP 2920, both PoE+, no midspan injectors used.
We think that this problem arises when power is allocated to Eth1 rather than Eth0. You can test the theory by connecting Eth1 first, waiting a few seconds, and then connecting Eth0. This will result in the AP running in power saving mode with a single link. Reversing this process we have found that the AP comes up as it should do with a functioning dual link.
This seems to be because the AP will always bring Eth0 Up, even if Eth1 is supplying the power. The LLDP negotation that would normally take place for increased power allocation cannot take place in this configuration (a packet capture shows that the AP is sending requests for increased power but that none is being allocated).
Solutions - there are a couple of ways that we found to deal with this:
a) Allocate PoE statically on the switchports
Cisco: 'power inline consumption <value>' eg '25000')
HP: 'poe-allocate-by power'
: 'poe-value 25'
b) Turn PoE off on the port connected to Eth1
Cisco: 'power inline never'
HP: 'no power-over-ethernet'
Solution a) is less than ideal because the switch will take the allocated value out of its available budget, even if the AP isn't actually drawing that much power, much better to use LLDP if possible. So our choice is b), though it's not great having to do this either.
Whether this behaviour is as per PoE standards or not we're not sure, but it certainly seems undesirable so (if there isn't one already) we'll initiate a bug report for this...
#AP225