capacity of WS-C3750G-24WS-S25 DHCP server

[gnijs]

Hello all,

We are using the built-in DHCP server on the WS-C3750G-24WS-S25 controller to distribute IP addresses to our (wireless) guest VLANs.

Our guest VLAN has grown a lot and lately we are experiencing DHCP problems (long time to get an ip or renew for example)

Yesterday, the controller crashed with the message:

Reaper Reset: Task "DHCP Server" taking too much cpu: 39% (user 36%, system 2%) while SW Watchdog is disabled

So now i wonder:

1) how many DHCP requests / second can this controller handle ?

2) can we somehow debug or see how many requests /sec the controller needs to handle ?

Our guest WLAN has about 1000 clients, lease time 9h

Controller is WS-C3750G-24WS-S25

, running version 6.0.188.0 (left on this level, because of DHCP problems after upgrade)

regards,

Geert

[Saravanan Lakshmanan]

http://www.cisco.com/en/US/partner/products/ps6366/products_tech_note09186a0080af5d13.shtml#Internal-DHCP

The internal DHCP server was introduced initially for branch offices where an external DHCP server is not available. It is designed to support a small wireless network with less than ten APs that are on the same subnet. The internal server provides IP addresses to wireless clients, direct-connect APs, appliance-mode APs on the management interface, and DHCP requests that are relayed from APs. It is not a full-blown general purpose DHCP server. It only supports limited functionality and will not scale in a larger deployment.

However, 4000 clients could get an ip from internal pool on 7.0.220.0.

To troubleshoot dhcp on specific client:-

WLC>debug client

WLC>debug dhcp message enable

To troubleshoot dhcp only:-

WLC>debug dhcp message

WLC>debug dhcp packet

[Amjad Abdullah]

Geert,

I would recommend that you use external DHCP server. Internal DHCP is designed to help but not to fully replace external DHCP servers in enterprise.

Besides, the version you are using is old. You can upgrade to 6.0.202.0 which should handle system resources more efficently and hence mitigate your problem.

You can also upgrade to some 7.0.x versions. (avoid 7.0.98.0 though). 7.0 has more new features.

I suggest that you upgrade and if the problem is still exist then you use external DHCP.

Because of your current problem; enabling debugs on your WLC would possibly overwhelm the memory and CPU more and make the WLC more prone to crash again.

You can collect sniffer traces on the WLC port if you'd like which is better than enabling debugs.

You can restrict the debugs to be with one client only (debug mac addr ) then follow the debugs you'd like. However, debuggin only one client will not give you statistics about dhcp requests received by WLC. it only shows you dhcp process of one client.

HTH

Amjad