spanning tree question

Posted on 2009-07-01
Last Modified: 2012-05-07
On a normal Dell switch, if we disable Spanning tree, does it still take 30-50 seconds for the access ports to come in working condition ? Secondly, if a PC has been booted up for 1 min, the STP convergence time starts after that or before that ?

Basically, the DHCP requests are really taking long (30 seconds) ..... DHCP server is on the LAN

Secondly, if ive a Dell switch (STP disabled) connected to a Cisco 877 router (that has STP enabled), then will the Cisco switch give its spanning tree info to Dell switch and the dell switch would somehow become active with Spanning tree ? I dont think so but just double checking ! I know this second bit is a stupid question to ask but am just clarifying the doubts of one of the guys

Question by:nabeel92
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Expert Comment

by:Ken Boone
ID: 24759818
I'm not a Dell expert, but if spanning-tree is not enabled then the port should not participate in spanning-tree therefore there would be no need to put the port in a non-forwarding state while spanning tree goes through the first two phases.  STP convergence time starts when there is activity on the port.  

STP is really important when connecting switches together, but not important when connecting clients to the switch.  So it is rather silly to go through the full spanning-tree process for a PC, since the PC cannot put the layer 2 network in a loop as it is just an end-station.  So most switches have a method whereby you can declare on a port by port basis that spanning tree can skip the first two phases and go immediately to a forwarding state on client ports.  On Cisco this is called spanning-tree portfast, on HP I tihnk it is called FAST spanning-tree.   I am sure there is a similar setting on your Dell switch.  This is what you want to configure on all ports where clients are connected.  This is the recommended way to .  Leave STP on but put client ports into the fast phase.

Secondly the switch ports on the 877 running STP will send out BPDU so the switches will be in agreement on the STP topoology.  One of the switches will be the root and the tree will be built from there.  But if the Dell switch has STP disabled it will not participate in STP.

Hope that helps.

Author Comment

ID: 24759856
Ok, thanks for that explanation ... .

Basically, whats happening is (and am not sure if thats related to portfast) is that clients get their I.P address from a DHCP server (linux based DHCP server) and as long as STP is running, it takes ages (50 seconds) for the clients to fetch the TFTP config file from the DHCP server ... when we disabled spanning tree, the process speeds up and clients get their I.P in normal time (few seconds) .... that's what got me thinking that there is some conflict between spanning tree and DHCP ....
LVL 25

Accepted Solution

Ken Boone earned 500 total points
ID: 24759873
Yes you are right on.  That is what is causing your problem.  You need to find the setting which will be a per port setting that will put spanning-tree into a mode where it will bypass the first two phases which takes approx 30 secs to get through.
LVL 25

Expert Comment

by:Ken Boone
ID: 24759878
A quick google shows that dell uses the same syntax as cisco on the command line.  Issue spanning-tree portfast on all client ports and that will take care of it for you.

Author Comment

ID: 24760384
Check this link ... That's right

Ok I've emailed the I.T support guy 3 solutions
1. Disable STP completely
2. Enable STP (with portfast option)
3. Use RSTP instead (still with portfast)

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