STP (Spanning Tree Protocol)

STP (Spanning Tree Protocol) is used to prevent any network for looping of frames by keeping some ports or interfaces of device in forward state and some ports or interfaces in blocked mode.

When two or more than two switches are connected with each other to create redundant links then it may cause loop occurrence. STP is layer 2 protocol and it enabled on switches by default. STP is used to stop loop occurrence.

STP - IEEE 802.1D (Open Standard)
RSTP - IEEE 802.1W
MST (Multiple Spanning Tree) - IEEE 802.1S
PVST (Per VLAN Spanning Tree) - CISCO Proprietary
PVST+ - CISCO Proprietary
RPVST - CISCO Proprietary
CST (Common Spanning Tree) - All VLANs will participate in single number of instance
IST (Inter Spanning Tree) - Different - Different VLANs Different - Different number of instance

Problems which occur if STP is not used

Storm of Broadcast
High Process Utilization
Instability of MAC Table
Transmission of Multiple Frames

Tasks performed by STP

Root Bridge Elect
Designated Port Elect
Root Port Elect

Root Bridge Elect

A switch having lowest best bridge ID which is the combination of switch priority and MAC addresses. This ID is consist of 8 bytes. 2 bytes for priority and 6 bytes for MAC address.

By default priority of switches is 32768.

Priority can be changed and set between 0 - 65535.

Designated Port Elect

Ports which advertise BPDUs having lowest cost are know as designated ports. BPDUs are sent by designated ports towards Non Root Bridge.

Root Port Elect

Port that receives the BPDUs of lowest cost is know as root port. It is the shortest path to the root bridge.

COST - Cost is an integer value which is used to perform Designated Port (DP) and Root Port (RP) elections.

Ethernet Standards	Cost
10 Mbps	100
100 Mbps	19
1 Gbps	4
10 Gbps	2

BPDU (Bridge Protocol Data Unit)

Each switch send hello packet to other switch in every 2 sec and that hello packet is known as hello BPDU.

There are two types of BPDU

Configuration BPDU - Sent from Root to Non-Root
TCN (Topology Change Notification) BPDU - Sent from Non-Root to Root

Contents of Configuration BPDU

Protocol ID is always 0
Version is always 0
Type of Message
Flag
ID of Root Bridge
Cost of Root
ID of Sender Bridge
Priority of Sender Port
Maximum Age
Age of Message
Hello
Forward Delay

Content of TCN BPDU

ID of Protocol
Version
Type of Message

Root Bridge sends configuration BPDU after every 2 seconds.
Cost of BPDU send by root bridge is always 0.

Root Bridge Requirements

Low Bridge Priority
Low MAC Address

(NOTE: COST OF BPDU GENERATED BY ROOT BRIDGE IS ALWAYS 0)

DP and RP Requirements

Lower Bridge ID which is only for DP and between Root and Non Root
Low Cost for DP and RP
Lower Sender ID between Root to Non Root
Lower Sender Port Priority between Non Root to Non Root
Lower Sender Port ID

Configurations of STP

As we have learnt above that STP is enabled by default on all the switches, so here we will see that how the cost and priority of any interface can be changed or how to make any switch as root bridge.

If we want to make switch as root bridge for one vlan

Switch(config)# spanning-tree vlan 1 priority 0

If we want to make switch as root bridge for all vlans

Switch(config)# spanning-tree vlan 1 - 4094 priority 0

To remove switch from root bridge

Switch(config)# no spanning -tree vlan 1 - 4094 priority 0

If one switch goes down then other switch becomes root bridge

Switch(config)# spanning-tree vlan 1 - 5 root primary

Switch(config)# spanning-tree vlan 6 - 10 root secondary

On other switch issue the reverse commands

Switch(config)# spanning-tree vlan 1 - 5 root secondary

Switch(config)# spanning-tree vlan 6 - 10 root primary

To change the cost of any interface

Switch(config)# interface fa 0/1

Switch(config-if)# spanning-tree vlan 1 cost 20

Switch(config-if)# exit

To change the priority of any port

Switch(config)# interface fa 0/1

Switch(config-if)# spanning-tree vlan 1 port priority 100

Switch(config-if)# exit

To configure port fast

Switch(config)# interface fa 0/1

Switch(config-if)# spanning-tree portfast

We can also configure range to configure port fast

Switch(config)# interface range fa 0/1 - 24

Switch(config-if-range)# spanning-tree portfast

Switch(config-if-range)# exit

To stop or disable spanning tree protocol

Switch(config)# no spanning-tree vlan 1

Set timer for Hello Packet

Switch(config)# spanning-tree vlan 1 hello time <select form 1 to 10 sec>

Change Maximum age

Switch(config)# spanning-tree vlan 1 max-age <select from 6 to 40 sec>

Change forward delay time

Switch(config)# spanning-tree vlan 1 forward-time <select form 4 to 30 sec>

Basic Networking

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