[c-nsp] loop-testing 10G with cisco?

Phil Mayers p.mayers at imperial.ac.uk
Thu Dec 11 07:43:38 EST 2008

Gert Doering wrote:
> Hi,
> I'm facing a new challenge and hope that one of you has a nice trick for
> me...
> We are working on taking a 10G wavelength into service, and it has packet
> errors (CRC errors showing up on one side).  3 different fiber providers
> involved, plus local in-house cabling.  Difficult.
> For "classic" lines (T1 to STM-<whatever> serials), the normal approach to
> diagnosing this is to loop back the fiber at certain points of the path,
> and see which part is the problematic one.
> This works with 10G fiber as well, but I can't find a way to ping-test 
> the loop from the Cisco.
> Here's the setup:
> Cisco 6506, Sup720-10GE  =======<fiber path>======) loop cable
> I can *see* the loop ("show cdp neighbor" shows myself, and if UDLD is
> enabled, the interface goes to errdisable).  
> I just can't ping-test, as the router is clever(?) enough to locally 
> loop the packets - self-ping won't leave the box.  On a serial interface, 
> pinging myself will make the router send out the packet over the serial, 
> so I can nicely ping-test the loop.
> So - what's the trick for testing such things with a 10GE LAN interface?

WAG, but maybe something like:

int Te1/1
  ip unnumbered Lo1
ip route Te1/1

The un-numbered command ought to stop the router ARPing for next-hop, 
and you can then:


Obviously you won't get any replies because the cisco won't receive on but the counters should increment.

I'd be interested to know if this works...

The other option might be to use two 10gig ports and a kind of Y-cable 
arrangement for the patch lead, using a port for TX, and a port for RX, 
and put either the TX or RX port into a vrf.

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