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Command to see if IP or MAC is getting to Cisco Router

Posted on 2015-01-16
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Last Modified: 2016-10-14
I have a 2821 router and I'm trying to see if a specific IP or MAC is passing the WAN and hitting this router.  What is the command to monitor for this?
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Question by:bergquistcompany
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by:Spartan_1337
ID: 40554156
you could try: show mac-address-table address H.H.H

What is it you are trying to do?
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John-Charles-Herzberg earned 168 total points
ID: 40554163
I would use Using the ping or trace route commands.  More information on how do do this is attached in the PDF from cisco
13730-ext-ping-trace.pdf
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by:Spartan_1337
ID: 40554166
Or you can use: show arp
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by:bergquistcompany
ID: 40554171
I have a device on one side of my WAN that isn't talking to the device on the other side of the WAN so I want to see if the request is getting across the WAN (not blocked)  by seeing if I can see any trace of that IP or MAC registering with the router on the other side of the WAN
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by:giltjr
ID: 40554189
John-Charles-Herzberg suggestion of using tracert is a good place to start.  This assumes that all the routing equipment is configured to allow ICMP packets through.

Depending on the type of routers used, you could look at the ACL's and/or do packet captures of traffic flowing through them.
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by:Predrag Jovic
Predrag Jovic earned 166 total points
ID: 40554223
Aside of traceroute, the easiest way is  to turn on debuging on router you are trying to ping #debug ip icmp so if you ping router with debug on -  you will see dubug messages.
Something like:
*Jun  12 17:52:50.311: ICMP: echo reply sent, src 10.1.12.8, dst 10.0.11.100
If it is production router don't forget to turn off debugging.
There is also packet capture function, more complex, but more suitable for use on production router.

MAC addresses are locally significant, so MAC address never passes out of LAN.
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by:Craig Beck
Craig Beck earned 166 total points
ID: 40556320
You can't monitor MAC addresses over layer-3, so ARP is out of the question, as is looking at the CAM table..  You would have to monitor using IP instead.  Ping or traceroute won't help either really.

Just create an ACL on the distant router which permits the IP you want to monitor, and use the debug ip packet command to see what traffic hits the router.
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