Router local ping, strange !!

Unanswered Question
Aug 21st, 2009

hi all, i am facing a very strange issue in my remote office. I have 1841 installed with a DXX link terminated on serial interface. IOS is 12.3(11)T5. When i ping any of routers local interface (incl lan and wan IPs), i am getting delays in 800-1500 ms, but at the same time if i ping to any machine on this routers lan, i am getting reply in 100-200 ms. Why is this so ? there is only one link to this router so there is no possibility of assymetric routing. What i am feeling is that router is not responding properly to its local traffic but behaving properly for transient traffic ? what can be the issue. I have checked and cpu utilization is also like 5%-6%. On serial interface reliablity is perfect but i am seeing some CRC but again why only router self packets are suffering ?

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Peter Paluch Fri, 08/21/2009 - 06:46

Hello,

I assume you are pinging the addresses from the router's console. Is my assumption correct?

If you ping the IP address of your serial interface from the router itself, the ping packet will actually be sent out that interface, reach your neighboring router and get forwarded back to you. Therefore, depending on the speed and utilization of your serial link, a higher delay in pings is not unusual. Still, 800 ms is too long.

Can you post your configuration here? It might help us see better what is configured and what might be going on. It will be very helpful.

Best regards,

Peter

rahmanrafatur Fri, 08/21/2009 - 10:10

Hi,

It seems, what you are facing is a desirable behavior.

The reason could lie in the fact that router is a routing device. That means, it is designed to route and switch but not to act as a host.

Now needless to mention that some of the considerate in context to delay in this scenarios are as follows:

1. Packet across the router to the host are cef or fast switched VS packet destined to router which is process switched.

2. ICMP is a reachability test tool, rather than delay calculator. If you are concerned about the delay we may use "IP SLA". Though, I understand you aspiration is to find why delay difference rather then delay computation...

3. In the protocol stack of these networking devices the priority to the ICMP is extremely low. This might also be contributing to the delay. Though I read that your processors utilization is 5 odd %. I need to investigate the idle process is reflected at all in the cpu computation or not and we will have to see the priority of idle process verses ICMP process.

So to summerise the behavior- process switched vs CEF/fast switched. Priority of ICMP process.

therefore I would suggest do a packet capture on telnet session or an HTTP session to your router and look at the delta.

I am pretty confident that you would receive very encouraging values there.

Do kindly let me know if this makes any sense.

Regards..

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