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Workstations that connect to a switch cannot get a Dynamic Host Configuration Protocol (DHCP) address

Core issue

Spanning tree delays may cause a workstation to report a No DHCP Servers Available message.


A workstation may report a No DHCP Servers Available message when booting up because of Spanning Tree Protocol (STP) loop detection delays. This issue has become more common as boot-up times for workstation OSs have decreased.

Turning off spanning tree on the entire switch (globally) is usually not recommended.  Spanning tree serves many useful purposes, including the detection of physical loops and preventing them from bringing down the entire network. Issue the portfast command on end ports only to reduce the detection delay.

In order to reduce the delay, issue the set spantree portfast command on the switch port. For example, to enable portfast on port 3/2, the command would be set spantree portfast 3/2 enable. This reduces the STP delay by about 30 seconds. By default, STP spends approximately 15 seconds listening and 15 seconds learning.

This delay is not necessary when a single end station is directly connected  to a switch port. Because a loop is not possible in that situation, there  is no reason for spanning tree to spend 30 seconds checking for loops on that port.

The portfast feature should never be used on switch ports that connect to  other switches, hubs, or routers. These connections may cause physical loops  and it is very important that spanning-tree go through the full initialization  procedure. A spanning-tree loop can bring the network down. If portfast is  turned on for a port that is part of a physical loop, it can cause a window  of time where packets could be continuously forwarded (and even multiply) in such a way that the network cannot recover.


For more information regarding startup delay issues and configuration, refer to Using Portfast and Other Commands to Fix Workstation Startup Connectivity Delays.

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‎06-22-2009 04:09 PM
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