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New Member

Ping Duplicate (DUP!)

Hi,

I tried to ping an ethernet interface in router in another site and I got the following:

router# ping x.x.x.x

PING x.x.x.x (x.x.x.x): 56 data bytes

64 bytes from x.x.x.x: icmp_seq=0 ttl=254 time=10.352 ms

64 bytes from x.x.x.x: icmp_seq=0 ttl=253 time=10.878 ms (DUP!)

64 bytes from x.x.x.x: icmp_seq=1 ttl=254 time=10.314 ms

64 bytes from x.x.x.x: icmp_seq=1 ttl=253 time=10.876 ms (DUP!)

do you have any idea about the cause of this problem.

Regards,

6 REPLIES

Re: Ping Duplicate (DUP!)

Most likely, this means what it implies: the same ip address is configured in two places.

The ttl differs 1 hop but without knowing the topology this isn't of much help.

regards,

Leo

Cisco Employee

Re: Ping Duplicate (DUP!)

Hi Leo,

How would the unicast ICMP Echo Request packet sent by the ping be received by two different hosts with the same IP?

One more question to the original poster: What IP address were you pinging? Could it be a broadcast address on a link?

Regards, Martin

New Member

Re: Ping Duplicate (DUP!)

Hi,

The IP address is not a broadcast, it is 10.0.0.1/25. Also there is no any other device has the same IP.

Regards,

Re: Ping Duplicate (DUP!)

In my opinion, this is very well possible. It could be something like a router-pair that was configured for HSRP but has wrongly been assigned the same interface IP.

Regards,

Leo

Cisco Employee

Re: Ping Duplicate (DUP!)

Hi,

The problem with duplicate IPs is the MAC address. Depending on the ARP cache/CAM table of the sending device only one station would get the IP packet. So imho there is more to it than just a duplicate IP.

What is the ARP cache/CAM table in the 10.0.0.0/25 attached devices?

Regards, Martin

New Member

in my experience this happens

in my experience this happens with Wireless devices. We have seen it when there is another stronger radio/microwave near by in the LoS pathway that is sitting on or very near the same frequency as the AP. for example we have an AP sitting on a hilltop for our various departments to connect to. Verizon came in with a microwave shot off the same hill, supposed to use 6GHZ but didnt get their FCC licensing done in time so they just turned it up on 5.8Ghz... well as soon as they did I noticed my network go to crap. when i would ping the AP on the hill i would get 50-100 Dup! packets inside of 60 seconds of running a ping. not good. its like the microwave is "stealing" the packets and replying as well. so essentially my AP and their MW are both responding to my ping. its a huge mess, verizon says they should be @6 in 3 months... this is the only time I've ever seen this in my 20 years.

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