Solved: ISR : Behaviour to Ping packets

gauravprakash · ‎11-06-2008

Hi,

Do Cisco ISR routers give low priority to ICMP packets (request/reply)? Is this inbuilt in IOS ?

Rgds,

Gaurav

Richard Burts · ‎11-07-2008

I think that there are at least two ways to understand the question from Gaurav. Thomas and Joseph have understood it in terms of priority in forwarding the traffic. But I wonder if Gaurav was asking about the processing of ICMP (when the ICMP destination is the router itself). And the answer to that is that yes the IOS treats processing ICMP packets and generating responses as a lower priority task. And that is built into IOS.

HTH

Rick

HTH

Rick

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tcordier · ‎11-07-2008

No. The default configuration for a Cisco router is a single FiFo (first-in, first-out) queue. All packets are send in the order in which they arrive. Only when you configure QoS or queuing other that FiFo (e.g. Priority queueing), the router will apply different priorities to packets.

HTH, Thomas

Joseph W. Doherty · ‎11-07-2008

Not 100% positive what ISR defaults are for all interface types, but defaults usually were WFQ for E1 and slower interfaces and FIFO for faster interfaces.

If WFQ is active, it treats flows equally if they have the same IP Precence marking. If the markings differ, better marked packets are provided a higher proportion of bandwidth. Usually, ping packets will have a BE marking, but the marking can be increased (for instance using a router's extended ping command).

Marking would not have any impact with FIFO unless WRED is also enabled. (WRED can often also be used with WFQ, but defaults should be WRED not active.)

tcordier · ‎11-07-2008

You are right, I was talking gibberish. For most serial interface below E1 the router will use WFQ. Going back to the question at hand, I would think that WFQ should give more priority to ICMP packets, rather than less, as WFQ is designed to favor low-volume traffic. The observed performance for the ping packets should not be caused by a default router behavior.

- Thomas

Richard Burts · ‎11-07-2008

I think that there are at least two ways to understand the question from Gaurav. Thomas and Joseph have understood it in terms of priority in forwarding the traffic. But I wonder if Gaurav was asking about the processing of ICMP (when the ICMP destination is the router itself). And the answer to that is that yes the IOS treats processing ICMP packets and generating responses as a lower priority task. And that is built into IOS.

HTH

Rick

HTH

Rick

Joseph W. Doherty · ‎11-07-2008

Good point Rick!

For the Gaurav, if you need more accurate router ping response, using SLA pings with a SLA responder can be used to offset the issue of a router's low priority processing of normal ping requests.

tcordier · ‎11-09-2008

I was unaware of that. Is that documented somewhere? I could not find anything related to that.

Thanks, Thomas

Joseph W. Doherty · ‎11-07-2008

NB: I know Cisco describes WFQ as favoring light bandwidth flows, but from actually watching how it queues flows and weighs them, along with the theory how WFQ should function, I'm not 100% certain "favor" is truly descriptive except in the sense that heavy bandwidth flows might not adversely impact the light bandwidth flows, as they're likely to do when performing FIFO queuing.