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I have a very strange problem. DHCP requests (to my router, going through my PowerConnect switch) take 35 - 40 seconds.

Here is my setup:

Router (pfSense) --> PowerConnect 2824 switch --> LAN

DHCP server is disabled on the PowerConnect 2824 but is enabled on my router. However, any requests on my LAN take 30 - 40 seconds to receive a DHCP response.

I've tried a different switch (Netgear Prosafe) and it works perfectly and I receive an instant DHCP response so I know the problem is not with my router.

Can anybody please help me out?

SofaKng
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2 Answers2

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It's probably got nothing to do with DHCP. It's probably spanning tree. The switch is simply unwilling to enable the port until it confirms that the port will not cause a disastrous loop in the network. The solution, assuming you know that the port will never, ever form a loop is to enable portfast (or 'fast link'). See this link:

http://www.mcbsys.com/techblog/2010/02/gigabit-switch-spanning-tree-causes-slow-logon/

David Schwartz
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    Dell calls this fast link. And I do believe they default to having STP enabled and fast link off on all ports. With some devices this can cause more than just a delay leaving the device unable to negotiate a network connection. This tends to only happen with embedded devices which are less forgiving of odd networking behaviors. – 3dinfluence Aug 28 '11 at 16:27
  • Thanks so much! That was the problem! I'm going to have to read up on spanning tree to understand what exactly it's doing... – SofaKng Aug 28 '11 at 19:43
  • I never thought spanning tree would cause such a big lag. I currently have the problem that several workstations don´t install AD GPO policies.... because on bootup they don´t have a DHCP IP address yet. – cljk May 29 '13 at 08:02
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It's spanning tree. It's enabled by default on Dell switches. You can turn on portfast like this:

interface range ethernet e1-24
spanning-tree portfast

Jason Berg
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