Cannot access certain websites...

Ok, I have read and read and read and researched and not found a damn thing!!!  I am sure it is something simple but anyways, here's the problem.

I cannot seem to get to yahoo.com, romeoville.org and a few other selected sites.  I just chalked it up to the sites bouncing, but have dropped that theory since I can remote to my home computer and get on the sites with no problem.  I can go into my firewall here at work and ping each site without a problem, I can go into the INTERNET router and ping the sites with no problems.  I can ping the sites from the Web Filter as well with no problems.  The problem lies within trying to ping/traceroute from our internal core router.  I get a "Destination unreachable" back from the router.  Nothing has changed in either router to make this happen.  I have checked, double checked and triple checked DNS and Name-Servers.  I have cleared the ARP Cache and the DNS Cache on our routers and DNS servers.  I have cleared Temp Internet files, Cookies and even gone as far and building a machine from scratch to test this theory.  Somewhere between the firewall and the outside internet router, something is lost in translation......here is an example of the network.....

INTERNET ROUTER (outside of firewall)  xxx.xxx.xxx.xxx
Sonicwall Pro Firewall               Wan port xxx.xxx.xxx.xxx    Lan port 192.168.1.3
Network Core Router (LAN Port of firewall)  192.168.1.1

This network has been in place and functioning for quite sometime and now these certain sites aren't accessible.  I have also run repeated tests for spyware/adware/malware/viruses and checked the hosts files for redirects.  Please let me know what information I can provide to someone to help me resolve this issue.  I have contacted the ISP as well and they report no trouble, which is proven through the traceroutes and pings from the internet router.  I can only assume that it's a routing issue with the core router or the firewall.

Thanks.....
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Timothy KashinInfrastructure ManagerAsked:
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celsmkCommented:
Hi,

Looks like you have got MTU size problems. Some of your routers are not handling well path MTU and is dropping larger packets.

Please look at these threads:

A problem similar to yours:
http://www.experts-exchange.com/Networking/Q_20621043.html?query=Path+MTU%2C+web&clearTAFilter=true

A way to diagnose fragmentation sizes in your network path:
http://www.experts-exchange.com/Networking/Q_20025708.html?query=Path+MTU%2C+web&clearTAFilter=true


Hope this helps.

Regards,

Celsmk
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Timothy KashinInfrastructure ManagerAuthor Commented:
Actually, I found what the issue was.  It was a pain to nail down, but here goes....
n our network are the following components/appliances...
Core Router
Internet Router
Sonicwall Firewall Pro
St. Bernard Webfilter
Messagescreen Spam appliance.

The problem is the person prior to my employment here setup the network and had the core router (192.168.1.1) as the gateway.  Eash other appliance pointed this way as well.  This seemed to be working fine, but, in all reality the firewall (192.168.1.3) should have been the default route for internet traffic.  Once I found this discrepancy, everything starting working beautifully.

Thanks for the suggestions though.......

Tim
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Timothy KashinInfrastructure ManagerAuthor Commented:
Mods, please award Celsmk 100pts and an A for effort on this one.  I did make some MTU adjustments as well after I found the root cause and it made a difference as well.
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