Unable to access certain sites after router upgrade?

I have inherited a site: Win2K11 SBS server with a fairly standard looking setup. They have an ISP provided Zyxel D1000 dsl modem/router.
I am attempting to install a Sonicwall TZ205: I have put the d1000 in bridge mode (changed its i.p. address), configured the sonicwall with the original i.p. address of the d1000 and opened the ports for exchange, owa etc.

The sbs2011 box is doing dhcp & dns on the server. There is no proxy server on the network.

Everything works except for web browsing, which is erratic. Some sites cannot be accessed at all, but they can be pinged and do appear to resolve but time out, this is not limited to any one tld.

For example microsoft.com resolves to http://www.microsoft.com/en-us/ but the site never loads, it either times out or cointinuously tries to load.

This behaviour is confined to the domain joined machines on site! My own laptop, whether connected to wireless or wired to either the sonicwall or to the switch has no problem accessing any sites!

Please Help

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That sounds like DNS problem.
Try to set on some of problematic hosts google public DNS as primary DNS
If problems are resolved... your DNS server is problematic.
RantCanSr. Systems AdministratorCommented:
Agreed with Predrag. Try setting the DNS on the SonicWall to the DNS from the ISP, and set the IP of the SonicWall as downstream DNS for your domain machines.
Doctor-BobAuthor Commented:
Thanks to you both, I had already tried your suggestions with no luck. My own non-domain joined laptop when connected to the switch or sonicwall gets the sbs box as primary dns and web browsing works fine.

Also, If I take the dsl modem router out of bridge mode and remove the sonicwall everything works fine, in all cases the sbs box is doing dhcp and dns.

Its very confusing...
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Doctor-BobAuthor Commented:
DNS also appears to be functioning as internet URI's are correctly resolving, i.e. ping microsoft.com returns a response from the microsoft i.p. address......
Try to clear server DNS cache
run as admin either of these
Dnscmd /ClearCache

and on hosts try to clear local dns cache (at least one of them to check are issues resolved)
Ipconfig /FlushDNS
Doctor-BobAuthor Commented:
Thanks Predrag, but again I have already tried these multiple times :(
Do you have VPN there?
One of situation where this happens is when MTU is too big...
Just for fun, can you change MTU on SonicWall to, let's say, 1480 (even if you don't have VPN try to do so), or even smaller 1450?

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Doctor-BobAuthor Commented:
And that, ladies and gentlemen is why I keep coming back.

Well done  Predrag Jovic: I followed the Kb to discover the correct MTU is 1460, Nice catch sir.....**tips hat
Thank you.
This was a tough cookie.
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