Question about network connections dropping

joemagueta · ‎04-20-2011

Greetings everyone.

I'm having some issues with a connection for approximately 100 people and phones 50/50 dropping off inconsitently on various days but always at around 2pm. I've setup a syslog server and configured it to be very noisy in debug mode but all I seem to get are entries about DHCP server conflicts which seem to happen after the router has been rebooted so it makes sense to me that there would be DHCP devices with addresses the router didn't know about. Our provider has a portal that allows us to see the amount of traffic used as well as the time of day it's used and there isn't anything weird around 2 pm. The Syslog server also captures debug info from the switches and it all seems to be very normal ports going up and down as people connect and disconnect. Just wondering if any of you more experienced network administrators have any ideas on what it might be and how I might go about checking.

Thank you.

Joe

Roman Rodichev · ‎04-20-2011

Maybe configure DHCP snooping to allow DHCP replies only from your authorized DHCP server:

http://www.cisco.com/en/US/docs/switches/lan/catalyst4500/12.1/13ew/configuration/guide/dhcp.html

joemagueta · ‎04-21-2011

This actually seems kind of interesting, I might do this just for general security purposes.

Thank you.

Joe

garapoglou · ‎04-20-2011

Hi,

Do you have a DCHP server on your network? I mean a DHCP server other than the router.

It would be better when DHCP requests are handled by a dedicated server. Routers can handle DHCP but I wouldn't suggest it for large networks.

On a large network routers have a lot of work to do. There's no need to deal with it too.

Best regards,

Giorgos

joemagueta · ‎04-21-2011

There isn't another DHCP server onsite just because there are approximately 30 VLANs that each need DHCP and the router seems like the most convenient place. Also, they have another building with more users and the same router (1841) without issues. However, if there's an opportunity to install another DHCP server I'll give it a try.

Thanks

Joe

garapoglou · ‎04-21-2011

I would still use a stand-alone DHCP server and leave the routers do their real job.

30 VLANs full of clients that send and receive DHCP information, is an extra job for a router. It may not be so hard to deal, but problems can always occur.

In order to forward DHCP requests among VLANs to a server you can specify IP helper-addresses for each VLAN, since DHCP requests are broadcasts that are normally blocked by VLANs. The broadcast will be converted to a unicast packet and it will be forwarded to the DHCP server.

If you need help, feel free to ask me.

Regards,

Giorgos

joemagueta · ‎04-21-2011

Okay, thanks Giorgos.

Joe

garapoglou · ‎04-22-2011

You are welcome, Joe.

Good luck!

Giorgos

joemagueta · ‎04-23-2011

The organization has many computers but there are many people who are temporary that jack in and out throughout the day. The Syslog also showed VLANs going down and back up so the implication is a problem with STP. I've noticed the switches use classic STP do people think this might be causing the problem? Is there some way to verify if STP is the problem?

Joe