The problem with analogies is that they often help you understand the
correct answer to a question, but seldom help you understand the "why"
behind the answer. The analogies I like to use most often are the models of
the protocols themselves. Think of each protocol as a state machine, or
rather state machines running on each router/switch in the network, and you
will seldom get the answer wrong. If you understand how that state machine
works and run it in your head, then you really }can't{ get the wrong answer.
I'd point "Curious" here:
http://standards.ieee.org/getieee802/
and just tell him/her to read the IEEE specs. It's all clear as day. It's
unbelievable how many "network engineers" don't even keep a copy of the
standards (IEEE specs, RFCs) they are supposed to be implementing in their
toolkit, and even more amazing how many of them haven't even read the
standards at least once.
Fred Reimer - CCNA
Eclipsys Corporation, 200 Ashford Center North, Atlanta, GA 30338
Phone: 404-847-5177 Cell: 770-490-3071 Pager: 888-260-2050
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-----Original Message-----
From: Fred Richards [mailto:[EMAIL PROTECTED]
Sent: Monday, September 01, 2003 10:44 AM
To: [EMAIL PROTECTED]
Subject: Re: Amazing Spanning Tree [7:74594]
I always like to think of Spanning Tree in respect to the numbers on a
clock. If the information goes around in a circle, you only need 1
blocking port to disrupt the circle. You don't need to block in two
parts of the circle. The Sybex CCNA book had an excellent example of
this (I got my CCNA June 30th, 911/1000)... they had the explaination on
one page, then you turn the page and they had 5 switches. The diagram
made all the sense in the world. Which is kind of where I got my clock
analogy. If you're disrupting the loop at the "12" spot on the clock,
you don't need to disrupt it anywhere else.
-- Fred
Curious wrote:
>Hello friends, I have an spanning tree question for you!
>
>I have a lot of switches connected between them, but I have seen
>something that I can not explain very well. Two of these switches are
>connected using two cables:
>
> Switch1 Switch2
> Port 29 ------------- Port 29
> Port 30 ------------- Port 30
>
>I expected to see one port in blocking state (spanning-tree) and the other
>in forwarding state, but suprisingly I have seen that port 30 is in
blocking
>state
>in Switch1 but it is in forwarding state in Switch 2. Let's see these
>outputs:
>
>
>
>Switch1#sh spanning-tree interface FastEthernet 0/29
>Interface Fa0/29 (port 35) in Spanning tree 1 is FORWARDING
> Port path cost 19, Port priority 128
> Designated root has priority 32768, address 0002.fd3c.18b5
> Designated bridge has priority 32768, address 0005.5e0c.57b6
> Designated port is 35, path cost 23
> Timers: message age 0, forward delay 0, hold 0
>Switch1#sh spanning-tree interface FastEthernet 0/30
>Interface Fa0/30 (port 36) in Spanning tree 1 is FORWARDING
> Port path cost 19, Port priority 128
> Designated root has priority 32768, address 0002.fd3c.18b5
> Designated bridge has priority 32768, address 0005.5e0c.57b6
> Designated port is 36, path cost 23
> Timers: message age 0, forward delay 0, hold 0
> BPDU: sent 264503, received 2
>
>
>
>Switch2#sh spanning-tree interface FAstEthernet 0/29
>Interface Fa0/29 (port 35) in Spanning tree 1 is FORWARDING
> Port path cost 19, Port priority 128
> Designated root has priority 32768, address 0002.fd3c.18b5
> Designated bridge has priority 32768, address 0005.5e0c.57b6
> Designated port is 35, path cost 23
> Timers: message age 4, forward delay 0, hold 0
> BPDU: sent 2, received 264561
>Switch2#sh spanning-tree interface FAstEthernet 0/30
>Interface Fa0/30 (port 36) in Spanning tree 1 is BLOCKING
> Port path cost 19, Port priority 128
> Designated root has priority 32768, address 0002.fd3c.18b5
> Designated bridge has priority 32768, address 0005.5e0c.57b6
> Designated port is 36, path cost 23
> Timers: message age 3, forward delay 0, hold 0
> BPDU: sent 2, received 264573
>
>
> Why a port is in blocking state and the other is in forwarding??? I
>expected to
>see both ports in blocking, but one forwarding and the other blocking
>doesn't make
>sense!!!
>
> Thanks a lot!
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