Alcatel Unleashed

Went to an Old Omniswitch OSR site. When pinging between backbone 1 and backbone 2 I receive the normal .... then the occasional T (time out) 1/50 pings. Then I started receiving and X every 10 or 15 pings. What is the X indicate. I know ... is good
T= timeout U=unreachable....but what does an X signify.

thanks,