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.

Protocol dependencies

  • Ethernet: Typically, Loop uses Ethernet as its transport protocol. The well known Ethernet type for Loop is 0x9000.

Example traffic

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: ethloopdata
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

Preference Settings


Example capture file


Display Filter

A complete list of Loop display filter fields can be found in the display filter reference

Show only the Loop based traffic:


Capture Filter

You cannot directly filter Loop protocols while capturing. However, if you know the Ethernet type used (see above), you can filter on that one.

External links


Imported from on 2020-08-11 23:16:04 UTC