A few comments. First, not being able to obtain a DHCP lease upon initial
boot isn't "a problem related to Cisco's Spanning Tree Protocol"
implementation. Cisco implements the IEEE 802.1D STP algorithm that
specifies
when a port becomes active, it must go through the blocking, listening, and
learning phases before it can be switched to forwarding mode.
By default, Spanning Tree Protocol to transition from the blocking phase to
the forwarding phase is 50 seconds. A port is to remain in the blocking
phase
for 20 seconds. It then transitions to the listening phase that lasts 15
seconds. Once the listening phase has been completed, the port transitions
to
the learning phase, which is 15 seconds in length. It's become commonplace
for many newer PCs and operating systems to send DHCP requests well in
advance
of 50 seconds of system boot - which creates the problem of not being able to
initially obtain a DHCP lease.
If a PC is not configured to bridge frames between LAN segments, the switch
port to which the PC is connected can safely begin forwarding frames
immediately.
-- Leigh Anne Chisholm
> -----Original Message-----
> From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]]On Behalf Of
> John Gotti
> Sent: April 30, 2001 8:44 AM
> To: [EMAIL PROTECTED]
> Subject: Spanning Tree Protocol [7:2564]
>
>
> Hey all...we are having a problem where workstations sporatically will not
> be able to obtain an IP address from our DHCP server. After about 4
> minutes,
> you can perform a manual renew from WINIPCFG and you get your IP address.
> This has baffled me for quite some time and I have recently been told it is
> our Cisco 2924 Switch to blame. The story I was told is below. I
> welcome any
> comments for or against this opinion. Thank you for your time.
>
>
> "It appears the problem is connected to the spanning tree algorithm used
> by the CISCO switches. By default, ports on the switch block as they are
> initialised; during this phase the port is in its spanning tree algorithm
> learning and listening state - it is not forwarding. This is specifically
> aimed at ports that will be used to connect to other switches/routers in a
> stack. After a default time (4 mins?) they switch to the standard
> forwarding
> mode and everything seems normal, the problem is that you have missed all
> the important DHCP broadcast and acknowledgment from client to DHCP server
> during this period.
>
> You can change this default state by changing the PORT-FAST setting on
> each port. The port is then immediately in the FORWARDING mode as it is
> initialised. By default this setting is DISABLED, I have ENABLED all
> ports except the ports doing the linking to other switches"
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