This article documents a generic tcpdump restriction related to interface names and provides instructions how to resolve this situation.There are two possible symptoms for this tcpdump issue:
1) The tcpdump command returns an index error:
fw_1 (CBS): ~# tcpdump -ni 20dmz
Tcpdump doesn't support interface names that start with a digit. If the interface argument given to tcpdump starts with a number (e.g. "-i 20dmz"), this number is treated as an index into the list of interfaces as reported by "tcpdump -D". Tcpdump tries to capture traffic on a circuit with that index number and fails if there is no such index.
In order to resolve this issue in an XOS configuration, you can change the device names of the affected circuits so that no device name starts with a digit.