First ping probe timeout

Unanswered Question
Nov 5th, 2008

Hello,

May be silly question but...

When I check connectivity between two neighbour Cisco devices (routers and switches) using standart ping command with default parameters, I frequently see, what first ping probe is timeout and next four are successfull

.!!!!

I suppose what on Ethernet links this is due ARP mechanism. But default ping timeout 2s, ARP Requst/Reply roundtrip on 100 Mbit/s Ethernet is ~ 100 us (I have observed with analyzer).

The same situation on serial point-to-point links, where no ARP exists.

Any Idea, why first ping probe is timeout?

Also I have found this question in Cisco BCMSN Course LAB Guide

On some pings, there was one lost packet (.) and then four good packets. You should know why that occurred.

Best Regards,

Tomas

I have this problem too.
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ohassairi Thu, 11/06/2008 - 00:45

theorically, the router can make a ping only after it gets the mac address of remote host.

so theorically, there should not be any loss.

that's why i think the first ping is not a real ping: how can the rtr make ping without it knows the destination mac address!

Richard Burts Thu, 11/06/2008 - 09:40

Tomas

To understand this behavior I suggest that you start with show arp and look for the destination that you will ping. Then run debug arp and debug ip icmp. Then try the ping. This should help to clarify what the router does if the destination is not in the arp table and how that impacts the first ping.

HTH

Rick

1pipantom2 Thu, 11/06/2008 - 15:10

Hello all,

Thank You. You just confirmed to me, what first ping timeout is due ARP functionality.

Best Regards,

Tomas.

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