Disabled
Blocked
Listening 
Learning 
Forwarding

this are the 5 steps which take 50 sec to become amber to green

now come to its detail
WHAT IS STP
A loop avoidance mechanism called STP has been developed to
alleviate the 3 issues .

There are two varieties of Spanning Tree Protcol, STP
(802.1d) and RSTP ('Rapid' 802.1w) both work in a similar way.

A Spanning Tree Algorithm examines the switched network,
and, through a series of decisions (discussed below) places
each port in the loop into either Forwarding or Blocking
state, therefore breaking the loop.

Types of port in STP:

   1. Designated Port (DP) Closest to Root i.e. All Ports on
the Root Switch.
   2. Route Port (RP) One Per Switch.
   3. Blocking Port.

The process the switches go through to decide which port
they place in blocking or forwarding state is governed by
the position of a 'Root Switch' (also known as 'Root Bridge'):

    * To decide which switch is the root switch, each switch
begins by claiming to be the Root Switch they all send out
an STP messages called BPDU's (Bridge Protocol Data Unit). A
fight-off happens to determine the final root based on a
unique BID (Bridge Identifier) contained within the BPDU,
the BID is made up of two components; a priority value and
the switch MAC address. By default each switch begins off
with a priority of 32,768. The switch with the lowest BID
wins the honor of being the Root Bridge.
    * All ports on the root switch are placed into
forwarding state. These ports are known as 'Designated
Ports' (DP). You can NOT have a Root Port or Blocking Port
on the root switch.

    * All the remaining non-root switches determine which
port is closest to the root switch and places the port with
the least cost (based on hops and/or bandwidth) into
forwarding state. These are known as 'Root Ports' (RP).
    * There can only ever be one RP per segment, and so if
there are more than one paths with the same cost the
decision is made based on the BID of the neighboring
switches, the port connecting to the switch with the lowest
BID becomes a RP.
    * All ports are placed in blocking state and the loops
stopped.


Once the switches have all assigned ports then the switched
network is said to be "Converged".

As discussed above the BID is made up of:

    * A Priority Value
    * MAC Address

The Priority Value of the BID can be manually changed by an
administrator, this is useful of you want to force a switch
in the middle of a network to be a Root Switch.

note: If a newer switch is plugged into a working network of
a different manufacturer it may disrupt the network if
manual BID priorities are not set. A Cisco Switch (MAC
Vendor ID 00000C) will win over a 3COM switch (MAC Vendor ID
001AFC) because Cisco equipment has a lower MAC address.


You could switch STP off on all the switches, but this is
not advisable, as all it would take is someone to plug a
cable in the wrong port to cause a loop.

note: If two links between two switches, the decision of
which port to block can no longer be based on the BID
because they would be the same for both links, the selection
is therefore made on the lowest port number.


During the STP selection process ports can be in one of the
following statuses:

    * Disabled
    * Blocked
    * Listening *
    * Learning *
    * Forwarding

* Ports in Listening & Learning state can take upto 30
seconds, during which no traffic will pass through the port.
This may disrupt services such as DHCP and Network
Neighborhood. Switching on 'portfast' by using the
"spanning-tree portfast" command will skip the Listening and
Learning stage. Do not set portfast on inter switch ports or
branch ports, however you could set portfast on a leaf port
(ports connected to hosts).


I HAVE WRITEN THAT WHAT I HAVE KNOW AND WHAT I HAVE IN MY
MIND IT MIGHT BE CONFUSING BUT LETS HPE IT WILL GIVE YOU
CONFIDENCE

Hi Jitu... Above mention's MAC vendors ID for CISCO & 3COM 
are fixed... ya for example u took..!!!! dats the point got 
confused..!!!!