Intermittent High Ping Latency

Unanswered Question
Mar 9th, 2007

Hi there,

I have a Cisco 4506/Sup V with approx. 30 VLANs all with HSRP virtual interfaces using rapid spanning tree. S/W revision is 12.2

Pinging the local HSRP interface in a VLAN from a PC with 500ms delays between each ping has good latency response, average is <2ms (over 30000 pings). However the max ping can sometimes be up to 3-500ms RTT.

Should this be expected? I've done a 'show proc cpu' and CPU usage seems fine. Also a 'show platform health' and from what I can tell the values seem reasonable. There are no ACLs used, basic QoS is implemented (expedite TX-queue 3).

Any ideas?

I have this problem too.
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sundar.palaniappan Fri, 03/09/2007 - 13:29

If an odd ping is delayed you probably don't have to worry about it. However, if you are seeing high number of pings delayed then it may need to be investigated.

Moreover, ICMP packets destined to a router/switch itself gets lower priority from the device when compared to the other management traffic or user traffic that has to be switched.



britishlandcco Fri, 03/09/2007 - 13:48

I have seen this on loaded systems, but this system is particularly lightly loaded (less than 5mbps of traffic across it. It would seem about 1-2 packets in about 1-2000 pings will hit between 100-500ms.


sundar.palaniappan Fri, 03/09/2007 - 14:04

I hear you. However, every ping destined to the router/switch has to be process switched and if the processor is busy processing some other traffic at the time the ICMP packet arrived then it would be delayed as it's not considered high priority. In general, the average RTT is a good reflection of the RTT of pings.



britishlandcco Sat, 03/10/2007 - 00:33

I thought CEF (Cisco Express Forwarding) would eliminate the need for process switching or does this not apply to ICMP packets (perhaps as the switch/router itself must generate the reply packet)?


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