fails on first ping and works for the rest

Do you have ip inspect commands configured.

There was a posting yesterday from someone having an issue with ip inspect http on an 800 series router.

To troubleshhot, do the following

conf t logging buffer 10000 debug no logging console exit wri mem

clear log debug icmp debug arp

ping ! one for which the first ping fails

show log

Post results of the above testing along with the current config

I am never surprised when the first ping fails and I ignore it.

Long experience tells me that this is not unusual.

Clearly it would be good to understand what is going on and I believe that I do.

that if an IP packet arrives at an interface to be sent and there is no relevant ARP entry then the packet should be dropped. In the meantime, the ARP process continues and when (if?) a subsequent packet arrives it will duly be sent.

Your case seems to be different.

First ping is OK when host is pinged and not when hostname is used.

I have no further explanation.

I have spent a few minutes looking for the ARP/drop behaviour but I can't find it.

I did find;

I am sorry for my late reply. I tried it late last night, and here is what I found :

For every failed ping, the log shows ( sorry I am at work now and has no the exact error string ) two lines :

1. IP sent from external interface IP (WAN ) to the destination IP, and then
2. len=60, encapsulation failed.

I would like to add a comment here : when I ping, I ping from systems on the internal network. I see the NAT shows the translation as expected.

( Turning on debugging on ARP shows a lot of ARP packets from WAN interface ( this is cable modem ) so I have to run it off )

Thanks,

DT

The encapsulation failures means the router does not have an ARP entry for the destination IP address.

How to you get the IP address for the outside interface - via DHCP ?

How is the default route configured ?

Hi Merv,

I get the public IP address by DHCP for FA 4 as follows :

- ip dhcp client update dns

- ip address dhcp

I cleared all the ip inspect and ip address group and still experiece the same problem.

The default route is set by ip route 0.0.0.0 0.0.0.0 fa 4

Thanks,

DT

Change the default route to point to either IP next hop or dhcp if supported.

ip route 0.0.0.0 0.0.0.0

! or better if supported

ip route 0.0.0.0 0.0.0.0 dhcp

Thanks for hints. I am going to try them tonight and post back the result.

BTW the reason to do this is that your router will ARP once for the next hop address instead of having to ARP for every Internet destination you access.

It works !!! I tried the " ip route 0.0.0.0 0.0.0.0 dhcp" and it solved the problem. I do not try the first one because the ip route dhcp shows the same default route of the hardcode version.

I 've never used the ip route with dhcp before. I am glad to learn something new that solves my problem :)

Thanks a lot, Merv.

DT