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I have a Cisco 3750 stack. When I try and ping a device connected to one of the interfaces, the output packets counters on the interface do not increase. The input packets counter on the interface is increasing however (irrespective of whether I'm pinging the device). None of the error counters are increasing, and I am getting both an ARP entry and a mac-address-table entry. The IP I'm pinging from is in the right VLAN and subnet to ping the device. Any ideas what I can do to troubleshoot this error?

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1) Set up some access-lists matching what traffic you're looking for. probably something like ip icmp and perhaps a hardware address. 2) set up ip packet debugging, on some interfaces, corresponding to that access list 3) ping away, and see whether you can follow the flow of the traffic. – Tom O'Connor Apr 22 '11 at 11:05
@Cian, are you sure that the OS (likely windows) hasn't got the firewall enabled, thus blocking ICMP? What caused you to look at this workstation to begin with? – Mike Pennington Apr 22 '11 at 11:24
Are you looking at the counters for the source port or the destination port? – joeqwerty Apr 22 '11 at 13:25
1. Do source and dest ports belong the same VLAN or different ones? 2. Is IP Routing enabled on this stack? – DmitryK Apr 23 '11 at 15:52
Can you post the interface config for both ports? sh vlan and sh int status would also be useful. – HampusLi Apr 24 '11 at 18:14
up vote 1 down vote accepted

I wouldn't be terribly concerned about the output counters not incrementing - cisco always has "interesting" counter bugs. Try unshutting VLAN1, sometimes it is shutdown by default.

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I would echo this comment re: counter bugs. Counters on vlan interfaces don't work properly on 3750's because the packet is switched in hardware, so the CPU doesn't see it and as a consequence the counters do not increment. – paulos Sep 16 '11 at 23:54

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