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New Member

Strange Connectivity Issue

I have a strange conectivity issue and I do not know how to aproach it (e.g. running out of options).

I have a 3620 connected through a T1 to the ISP.Behind the router I have stub network.

Users use Outlook Express to download email from different email servers located at different providers in Internet (different domanin names). Users complain that downloading email from ONLY ONE email server takes a long time 3-5 min. The others are fine. Browsing the Internet works fine also. To test, I disabled the firewall and telnet to the email server at port 110 directly fom the router. It takes the same amount of time (~3 min) to get the server to answer. If I telnet to port 110 to the same server from a totally different location (different ISP involved), it works fine.

The email provider has ICMP disabled so I cannot do too many tests. They said they are not doing any IP blocking on their side. The ISP did a test from their end from the router on the other side of the T1 and works fine. They are totally puzzled of what could be causing this. So, I do not know what else to test or do to fix the issue.

6 REPLIES
VIP Purple

Re: Strange Connectivity Issue

Hello,

this could be a DNS-related problem. Check if your router resolves the name of the mail server correctly. It could also be a problem related to high CPU utilization, issue the show proc cpu command to see what the utilization is.

Regards,

Georg

New Member

Re: Strange Connectivity Issue

Thanks for your answer.

CPU utilization is 5sec: 1%, 1 min: 0%, 5 min: 1%.

In my testing, by telnetting from the router, right to the IP address of the email server, I've took DNS-related problems out of the equation.

Bronze

Re: Strange Connectivity Issue

The server may be trying to do a reverse DNS lookup on your IP address whenever you try to access it. This is relatively common. If there isn't a reverse DNS entry for your IP, DNS can take a long while to timeout.

New Member

Re: Strange Connectivity Issue

This is one of the first things that my ISP checked and it seems that my reverse DNS is ok.

Thanks for your opinion.

VIP Purple

Re: Strange Connectivity Issue

Since your ISP is blocking ICMP traffic you can´t do a full traceroute, but I am thinking your email server might take a different path from your router than the other servers. You could use a traceroute tool like PingPlotter, available at http://www.pingplotter.com/ to maybe find out where the delay occurs.

Regards,

Georg

New Member

Re: Strange Connectivity Issue

My ISP does not block the ICMP traffic. The email server is probably behind a firewall that blocks the ICMP traffic. Also, a traceroute to the email server shows connectivity with the email server at less than 100ms for each hop (except for the email server itself that has ICMP blocked). I'll try the PingPlotter. Thanks for answering.

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