STP - Spanning tree Protocol

What is STP ?

Spanning Tree Protocol
If we create redundancy in our Network, in-order to avoid single point of Failure, It leads to looping in the Network.
It helps to avoid loops in Switched Network.
Redundancy Brings Loops in the Network.

Why do we have a Loop ?

PC--1 sends an ARP request because it’s looking for the MAC address of PC--2.
An ARP request is a broadcast frame.
Switch A will forward this broadcast frame on all it links, except the link where the frame originated from.
Switch B will receive both broadcast frames.
Now what does switch B do with those broadcast frames?
It will forward it out of every link except the link where it originated from.
This means that the frame that was received on Interface 1 will be forwarded on
Interface 2.
The frame that was received on Interface 2 will be forwarded on Interface 1.

How STP works ?

Spanning Tree is Enabled by Default, All our Switches will use special frame called
as BPDU (Bridge Protocol data unit)
First of all spanning tree will elect a root bridge; this root-bridge will be the one that has the best “bridge ID”.
Bridge ID : Priority + Mac-Address.
The lower the bridge ID, the better it is.
By default the priority is a value of 32768 but you can change it if you want.
First STP checks for Priority, if Priority is same then it checks for MAC-Address.
Default STP is PVST (Per VLAN Spanning Tree )

Consider the above Example,

When you look at the 3 Switches, there is loop in the Network as SW-1 --> SW-2 --> SW-3 -->SW-1.

Lets look into the Process:

First Switches Check for Who will become Root-Bridge, (Root -Bridge is based on priority and Mac-Address)
First Switches, Checks for the Priority, By default the Priority will be the same on all switches.
If the Priority is different on each switch, the Switch with Lower Priority will become Root-Bridge.
If the Priority is same, then it chooses the Mac-Address, which is the Tie-Breaker.
Lower the Mac-Address value will be elected as the Root-Bridge.
In the Above Example, SW-1 is the Root Bridge,

Root Bridge

Priority and Mac-Address make up a Bridge ID.
The one with Lower Bridge ID becomes Root Bridge.
Lower the Priority will be Selected as a Root Bridge.
If Priority is Same, then it will check into the Mac-address.
All ports in Root Bridge will become Designated Port and it will be Forwarding state.

If you Look at the Above picture, you can see D , D is Designated port, which means the port is Active.

STP Ports

All ports in Root Bridge will become Designated Port and it will be Forwarding state.

Alternate Port will be in Blocked mode, and it will come to designated port once the current link to reach the Root Bridge is down.

Next Step is to select the Root-Port,

In the above Scenario, SW-1 is the Root-Bridge, So the Root-port will only be in SW-2 and SW-3.
SW-2 will select the interface no.2 as a Root-port, (As it is the Shortest path to reach the Root-Bridge), and SW-3 will select the interface no.1 as a Root-port.
Note : Shortest Path is Based on the cost of the interface, not by the number of routers it need to pass to reach the Root-Bridge)

Next we need see about SW-2's port no.2 and SW-3's port no.2, who's port will be up and who's port will be blocked or down.

D- Designated port , R- Root port and A - Alternate port
Now again we need check about the Priority and MAC-Address of the Switch.
When we check at SW-2 and SW-3, switch-2's Mac Address is low and Switch 2 wins the Battle and the port belongs to SW-2 will be up(Designated port), and the port of SW-3 will be in Alternate mode and it is in Blocked state. (Blocked state port won't send any traffic).

COST

STP Basic Command

STP Port States

When you connect a Cable to an interface it will be in Listening mode (15 sec )
At this stage, it will send BPDU’s , but does not learn MAC-Address and no data-transmission

Again After 15 sec it moves from Listening to Learning mode.
At this stage, it will send BPDU’s , but learns MAC-Address and no data-transmission

After 30 sec of connecting a cable it will be moving to forwarding state.
At this stage, it will send BPDU’s , but learn MAC-Address and data-transmission is done.

Note : By Default All switches run PVST (Per VLAN Spanning tree Protocol)

Each Switch will be the Root-Bridge for Each-VLAN.

That's it about STP

PGR Spot