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New Member

Intermittent High Ping Latency

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?

5 REPLIES

Re: Intermittent High Ping Latency

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.

HTH

Sundar

New Member

Re: Intermittent High Ping Latency

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.

Thanks

Re: Intermittent High Ping Latency

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.

HTH

Sundar

New Member

Re: Intermittent High Ping Latency

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)?

New Member

Re: Intermittent High Ping Latency

Bit of digging - I see that this is true - http://ioshints.blogspot.com/2007/01/cef-punted-packets.html

CEF will punt packets destined for the switch/router. As the switch/router often has instances where CPU is busy (i.e. recalculating FIB tables) this will delay the packet being generated.

Thanks for you your help!

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