STP uses Spanning-Tree Algorithm (SPA) to the create topology database of network. To the loops, SPA places some are interfaces in the forwarding state and other interfaces in the blocking state. How does STP decides in the which state port will be placed? A couple of the criteria exist:
1.
all the switches in a network select a root switch. the working interfaces on root switch are placed
in the forwarding state.
2. all other the switches, called no root switches, determine best path to the
get to root switch. The port used to the reach root switch (root port) placed
in the forwarding state.
3. On the shared Ethernet segments, the switch with best path to the reach root
switch is placed in the forwarding state. That switch are called the designated switch and it’s port is known
as designated
port.
4. all other interfaces are placed in the blocking state and will be not
forward frames.
The
example will be help you understand the concept are:
Let
say that the SW1 is selected as root switch. All ports on the SW1 are placed
into the forwarding state. SW2 and SW3 choose ports with lowest cost to the
reach root switch to the root ports. These ports are also placed in the
forwarding state. On shared Ethernet segment between SW2 & SW3, port Fa0/1
on SW2 has lowest cost to the reach root switch. This port is placed in the forwarding
states. To the prevent loops, port Fa0/1 on the SW3 is placed in the blocking
state.
BPDU (Bridge Protocol Data Unit)
BPDUs messages used by the switches to the share STP information with
the each other in the order to the select a root switch and detect loops. The
most common messages are Hello BPDUs which are include the following information:
·
Root switch ID
·
sender switch ID
·
sender root cost
·
Hello, Max Age, & forward delay timer
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