If you are implementing Cisco's IP Phones, then at some time during the normal course of troubleshooting or testing, you may find yourself pinging one of these phones. If you're pinging from a router, which is pretty common, as you might want to ping from a device closer to the phone than your PC, you may find some interesting results.
The IP Phone will likely miss quite a few pings, often, every other ping. If you've seen this before, it may have caused considerable alarm and confusion. Is your QoS not working correctly? Is it because the router, or the phone, or a switch in the path handles ICMP differently? Is there a load-balancing issue with one path leading to a black hole? Is network utilization too high? Queue depths set correctly?
Fortunately, the answer is probably that there is no problem. Cisco's IP Phones have a special feature that is designed to thwart ICMP-based denial of service attacks. This feature prevents the phone from responding to more than one ping in a 10-ms period. To verify that this is in fact the cause, try pinging with a PC. Cisco IOS routers send pings much faster than PCs, which typically send one ping every second. Each ping from a PC should be successful.
Thomas Alexander Lancaster IV is a consultant and author with over ten years experience in the networking industry, focused on Internet infrastructure.
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