Check/verify loop detection

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 .

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

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