Odd DHCP performance issue

[kmdcompelect]

I'm currently studying for my CCNA, so that's about my current level of Cisco knowledge.

I've noticed something odd on our small network of ~50 machines.  We have 2 Cisco Catalyst level 3 switches located in 2 buildings with a CAT 5 cable between them.  Computers are issued IP's from a windows 2008 R2 domain controller.

When I connect a machine directly to a port on the switch, it can take as long as 45 seconds for the entire DHCP process to complete. 

However, if I attach an unmanaged switch to the same port and plug the computer into the unmanaged switch, it receives an IP almost instantly. 

I've tried it in both buildings with several different switches and machines and had the same results.  It doesn't matter if the machine had recently been issued at IP or not.

Can someone explain what is going on here?  It's not a major problem or anything, more of a curiosity and a minor irritation when I need to reimage a machine over the network.  

[cadet alain · Accepted solution]

Hi,

you should enable spanning-tree Portfast on the access ports and you'll have no more 45 seconds to get an IP address.

Regards.

Alain

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[kmdcompelect]

Ok, so when a port doesn't have spaning-tree portfast enabled, it blocks new connections for a time to prevent loops.

When I plug a machine into the unmanaged switch, it works quickly because the switch has been plugged in long enough for the block to have expired. 

About right?

[cadet alain · Accepted solution]

Hi,

as your switch port where unmanaged switch is plugged into is always up/up then there is no topology change for Spanning-tree and the port is always in forwarding state. on ports where you have hosts connected directly then once they disconnect the port goes to up/down and then when they login it goes from up/down to up/up which will trigger a topolgy change for spanning-tree which will have to go from disabled to blocking then listening then learning then forwarding, with Portfast enabled you skip some stage in this process and so the port goes directly from blocking into forwarding.

Regards.

Alain

Don't forget to rate helpful posts.

[kmdcompelect]

Awesome,  thanks for the help.