Hi,
I don't believe there are any issues and what you're seeing is perfectly normal and expected.
You need to remember that IOS is a multi-tasking operating system that has many processes to handle. The way those processes are assigned resource within the CPU is controlled by the scheduler based upon the priority of the process.
From Inside Cisco IOS Software Architecture:
"IOS employs a priority scheme to schedule processes on the CPU. At creation time, every process is assigned one of four priorities based on the process’ purpose. The priorities are static; that is, they’re assigned when a process is created and never changed. The IOS process priorities are:
- Critical - Reserved for essential system processes that resolve resource allocation problems.
- High - Assigned to processes that provide a quick response time, such as a process that receives packets directly from a network interface.
- Medium - The default priority used by most IOS processes.
- Low - Assigned to processes providing periodic background tasks, such as logging messages."
The way the scheduler works is to check and service all processes in the critical priority queue first, then move on to high, medium (or normal) and low priority queues in order. In between each scheduling of processes at some lower level, the next higher level queue is checked again to see if there are now any processes that need servicing at that higher level.
The priorities of each process can be seen with the show processes <pid> command. If you send ICMP traffic to a router then that traffic is handled by the IP Input process, which as seen below, has a Normal priority.
c1kv-1#sh proc 150
Process ID 150 [IP Input], TTY 0
Memory usage [in bytes]
Holding: 53864, Maximum: 53864, Allocated: 0, Freed: 0
Getbufs: 0, Retbufs: 0, Stack: 46192/48000
CPU usage
PC: 2C0D49A, Invoked: 20647, Giveups: 5, uSec: 28
5Sec: 0.00%, 1Min: 0.00%, 5Min: 0.00%, Average: 0.00%
Age: 200895531 msec, Runtime: 594 msec
State: Waiting for Event, Priority: Normal
This means that any ICMP traffic you send to the router will not be given resource on the CPU until there are no critical or high priority processes to be serviced. As such you can't guarantee or expect consistent response times as you have no way of determining which other more important processes are being serviced at any point in time.
The job of a router is to forward traffic and that task, and the processes required to undertake it, will always have priority over tasks such as responding to ICMP packets.
The post High latency issue in LAN.... talks of a similar issue. I've sure if you search this forum further you'll find more items like this raised previously, and the view is typically the same. Don't expect ping responses from a router to be consistent or any variance in response times to be an indication of a problem.
Regards
