Configuration Testing Protocol (Loop)
It's basically a layer two "ping" equivalent.
Ethernet configuration testing protocol (CTP) is specified in chapter 8 of the DEC/Intel/Xerox Ethernet v2.0 specification. It doesn't appear in any IEEE 802 specification.
Frame 2 (68 bytes on wire, 68 bytes captured) Arrival Time: Mar 21, 2006 03:02:44.202162000 Time delta from previous packet: 0.000415000 seconds Time since reference or first frame: 0.000415000 seconds Frame Number: 2 Packet Length: 68 bytes Capture Length: 68 bytes Protocols in frame: eth:loop:data Ethernet II, Src: aa:00:04:00:69:04 (aa:00:04:00:69:04), Dst: aa:00:04:00:1d:04 (aa:00:04:00:1d:04) Destination: aa:00:04:00:1d:04 (aa:00:04:00:1d:04) Source: aa:00:04:00:69:04 (aa:00:04:00:69:04) Type: Loopback (0x9000) Configuration Test Protocol (loopback) skipCount: 8 Function: Forward Data (2) Forwarding address: aa:00:04:00:1d:04 (aa:00:04:00:1d:04) Relevant function: Function: Reply (1) Receipt number: 1 Data (40 bytes)
The Loop dissector is fully functional
Example capture file
A complete list of Loop display filter fields can be found in the display filter reference
Show only the Loop based traffic:
You cannot directly filter Loop protocols while capturing. However, if you know the Ethernet type used (see above), you can filter on that one.