VSS just reverts to old version during upgrade

[RahmaSallm]

We experienced something unusual with our VSS pair and I'm wondering if anyone had a similar issue. Trying to sort our if it is HW or SW related.

We have upgraded our C6807-XL VSS many times before without trouble.  This time we ran ISSU and the standby switch (switch2) rebooted and came up with the new version 15.5.1SY10 But when switch1 tried to load the new version, it failed to establish the VSL connection and rebooted about 5 times. Then the entire system just reverted to the old version 15.5.1SY2 with a total interruption of about 10 seconds.

Has anyone experienced anything similar? Is it specific to the C6870 platform?

[marce1000]

 - Review this thread : https://community.cisco.com/t5/switching/vss-synchronizaion-issue-with-c6807-cisco-switch/m-p/4729020 and or you may need  to have an FPGA Upgrade too , 

 M.

[Leo Laohoo]

Post the complete output to the command "dir" and "dir slavebootflash:".