check/verify loop detection

[MUHAMMAD MARGOOB MAHFUZ]

when a loop is provided from a non routing device like SDH MUX to a router on POS port, "(LOOPED)" the word is shown from "show controller" command in SONET/POS port. ...... But same can not be found on ethernet/Gethernet port .... so we are unable to find whether loop is actually worked....... is any command or mechanism for that .... my router is cisco ASR9000; IOS-XR .

[mivens]

A simple way is running a neighbour discovery protocol like CDP or LLDP and check if you see yourself as a neighbour.

Ethernet OAM will detect a looped port and generate a L2-ELO-6-MISWIRED alarm.

ISIS will alert if it sees its own hellos.

[MUHAMMAD MARGOOB MAHFUZ]

Thanks for your reply ...... But i was really looking for a command from which i can instantly find out about loop status. ..... like one can easily find from SONET port with "show controller" command .. 

Although i can detect alarm from "sh logging" command but it need OAM configured in the device .    IS OAM configurer by default in cisco IOS-XR ? 
 

[mivens]

Ethernet OAM isn't enabled by default.

The loop detection on a POS interface with HDLC encapsulation works implicitly because the protocol includes a sequence number. If a router sees its own sequence number it knows there is a loop. Ethernet doesn't work the same way.

[xthuijs · Accepted solution]

if I could add a few things to that. looped circuits are more common in sdh like environments such as POS and the like. Ethernet itself doesnt have a loop detection methodology, so we need a L2 or L3 based protocol to help us identify that loop.

POS running HDLC or PPP can easily detect this condition at L2. SDH itself has also loop capabilities in the Sonet overhead section or actually path trace element.

Ethernet doesn't have such POH or SOH and its L2 protocol doesnt naturally have a capability to detect loops. So for ethernet your easiest way to go about it is as Mark suggests to use CDP. OAM requires a bit of configuration and understanding, hence CDP is the easiest to use.

Here is an example how to see a loop:

CPE#show cdp neigh

A9K-BNG          Gig 6/0            121           R       ASR9K Ser Gig 0/0/0/0
CPE              Gig 6/0            164           R       7206VXR   Gig 6/0

in this case you see 2 neighbors, myself and the remote device. What this indicates that the cable is not really looped, but there is a loopback line configured on the peer.

loopback line basically consumes packets and loops them, and still injects packets also.

hence you see the remote neighbor and yourself on this device.

So with CDP you can identify the loop and also the type of loop that is happening.

I was also thinking that ARP can be used for that with duplicate address checking, but I see that this is not reliable. Eventhough when configuring an address we can see our own grat arp being returned, ARP or IP does not generate a dupe addr alert on it because we see the grat arp request from ourselves and not an arp response obviously.

So easiest is to use CDP for this.

regards

xander

[MUHAMMAD MARGOOB MAHFUZ]

Thanks for this elaborate answer for the solution .... this is what i was looking for....

[MUHAMMAD MARGOOB MAHFUZ]

Thank you for this Important information and your valuable time.