On 1/14/2013 6:36 PM, Edward Ned Harvey (lopser) wrote:
From: [email protected] [mailto:discuss-
[email protected]] On Behalf Of Morse, Richard E.MGH
Hi! Thanks to everyone who responded with suggestions! Apparently, it
works if I turn *off* spanning-tree on the child switch, leaving the parent
switch configured with RSTP. I'm not sure why, not what deleterious side
effects I've just introduced, sadly. But at least everything seems to be
working so far.
Is "portfast" the same thing as "edge port"? If so, I may go over again and
look more closely at those settings.
Just to make sure you know what to be afraid of, definitely DON'T do this with
STP disabled:
Take an ethernet cable. Click both ends into the same switch. If you do this,
you will almost instantaneously bring down the whole network, while all forms
of traffic are propagated endlessly in a circle.
This error is kind-of rare (but certainly not unheard of) as long as you have just one
switch in the closet, with fairly untangled cables. But this problem becomes much more
common when you have network closets connected to network closets, and people scatter
little 5-port switches throughout the office, and then "bring your child to work
day" comes around, and one of the negligent parents allows their kid to play with an
ethernet cable.
With spanning tree protocol enabled, the switch waits 30 seconds before allowing any
traffic to pass. It's detecting loops and disables that port if a loop is detected.
When you disable stp, you're saying, "don't do that annoying 30 second wait thing.
I accept the risk. I want the ethernet to come on immediately when a cable is
connected."
One effective way to prevent the 'misc switch in the office' but still
allow for dhcp and friends to work is to combine portfast (or edge-port)
with the 'bpduguard' directive. I use this in many places and make it a
default setting for end stations. That way you can get the advantages of
having the port forwarding enabled quickly, but if somebody plugs in a
device that generates spanning tree bridge protocol data units (BPDUs),
like a misc switch, the port will be disabled.
(PS - spanning tree does not give immunity from network loop problems. I
have the purple heart, as I'm sure many others do here)
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