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dhcp and dns clearing on windows sbs 2011

Basically i need to know how to clear the forward and reverse dns lookups and the dhcp list on our windows sbs 2011. we are having extreme network bandwidth issues. upstream of the server the speed is great, right on par with where we are paying for (50mbps with ping of 24ms )but immediately downstream of the server though it drops to about 1mbps with a ping of 1024ms. the network equipment is setup properly, they are managed switches with no bandwidth limits anywhere on the network. i have restarted the server to no change. the server is the DC and dhcp server. the only thing i can think of at this point to is to clear all the leases in the scope that aren't reserved and the dns lookups and caches. I'm just not sure how to do all this and what effects it will have on the clients on the network.
1 Solution
Restarting the server already clears the DNS lookup cache and doesn't really have anything to do with your connection bandwidth and ping.

When you say "downstream of the server", do you mean that all internet traffic passes through your SBS out to the internet? Or that your client machines use it for DNS resolution and DHCP, and all internet traffic goes out to the internet through a standard network router?
Larry Struckmeyer MVPCommented:
Agreeing that unless you have a non-standard two nic/nat config on the SBS using it as the gateway, the SBS should have nothing to do with internet speed.  DNS is to translate names to IPs and has nothing to do with sustained dl speeds once connected.  How did you test "upstream of the SBS"?  

What is the config?  Can we assume these are wired connections?  I would expect:  Internet - isp device - internal firewall - Switch - all network devices.   therefor the route to the internet should be the same for the SBS and all stations.

Unless you can connect a station directly to the firewall or the isp device and duplicate the results I would suspect failing hardware somewhere on the network.   To test the theory that it is somehow related to DHCP or DNS, give the test station a static ip with known good DNS servers such as or what ever you have confidence in.  Note this will mess up your internal speeds, but will test the external.

Suspect a bad port, bad cable, bad switch.  How is the ping speed between stations?  Between stations and the SBS?  Between stations and the gateway?
jessebruffettAuthor Commented:
I actually did connect my laptop directly to the modem and thats why i know the bandwidth right after the modem is great, and i installed chrome in the server so i could do speed tests with it and it, itself is great but everything else is not. The network goes iso->sonicwal->main switch->tertiary switches->clients, off of one of these switches is the server. There isn't a single client that is a straight home run to the main switch. bandwidth across the LAN is great, there is not a problems there, but no matter where you are, what port your using, the bandwidth remains about the same to the internet.
Olaf De CeusterCommented:
If there is no issues with the lan side, and you get good ping times between server and workstations , sounds like you issues are with the router.
Just to make sure your server is OK run the SBS BPA?
Larry Struckmeyer MVPCommented:
This is strange.. if the SBS is connected to the same switch links as other pcs and does not evidence the latency or the slower downloads there seems to be only one other possible answer.  You have gremlins in your network.  <g>.

What if you connect one of the PCs on the LAN directly to the ISP modem, then the Firewall, then a single hop switch.  If it is slow everywhere, what are the MTU settings on the system on the LAN?  Is it possible that some GPO or AV software has messed with them?  Or AV software settings in general?  If you cannot find a physical explanation, suspect anything called "internet security" installed on systems.
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