If the data you sent came right back to you through a secondary connection, would this help your network? In some ways it would, I suppose. It would eat up your bandwidth, it would be a security nightmare, and every interface on your network would have to keep reading it over and over continuously. It would be great to send out an e-mail and pick it up a week later with a sniffer continuing to loop around your network, wouldn’t it? Imagine how many times your friend would get that e-mail. I guess it wouldn’t be that much of good thing. So what do we do in an Ethernet network when we need to make sure we have secondary paths to a destination? Well, that is where a good understanding of Spanning Tree Protocol (STP) becomes important.

This section talks about the main purpose of STP, which is to stop network loops from occurring on your Layer 2 network (bridges or switches). STP is used to constantly monitor the switch ports and to make sure the protocol knows of all the links in your network. If more than one link exists, STP disables the secondary link until it is needed. This way the switch shuts down redundant links, putting a stop to any data loops in the network.
The STP process elects a root bridge in the network that will decide on the network topology. There can be only one root bridge in any given network. The root bridge ports are called designated ports, and these operate in what is called a forwarding state. Forwarding state ports send and receive traffic. If you have other switches in your network, as shown in Figure below, then these are non-root bridges. The switch uses a special algorithm called the spanning-tree algorithm (STA) to determine a cost to assign to each link based on the bandwidth of each hop from one switch in the network to another switch. A cost can also be assigned manually to each port. The port that has the lowest cost to the root bridge is called a root port, which sends and receives traffic.

Spanning Tree

Ports that are determined to have the lowest-cost path to the root bridge are also called designated ports and, like root bridge ports, they operate in forwarding state (noted as F in the illustration). Other ports on the bridge are considered no designated, and will not send or receive traffic. This is called blocking mode (noted as B in the illustration). STP is enabled by default on most Cisco switches with Ethernet or Fast Ethernet ports

Selecting the Root Bridge

Switches or bridges running STP exchange information with bridge protocol data units (BPDUs). BPDUs are used to send configuration messages using multicast frames, carrying the bridge ID of each device to other devices.
The bridge ID is used to determine the root bridge in the network and to determine the root port. The bridge ID is eight bytes long and includes the device’s priority value and its MAC address. The default priority on all devices running the IEEE STP version is 32768.
To determine the root bridge, the bridge’s priority and the MAC address are combined. If two switches or bridges have the same priority value, then the lower MAC address is used to determine who has the lowest ID.
For example, if two switches use the default priority of 32768, then the MAC addresses are compared. If switch A’s MAC address is 0000.0c00.1111.1111 and switch B’s MAC address is 0000.0c00.2222.2222, then switch A becomes the root bridge.

Selecting the Designated Port

To determine the port or ports that will be used to communicate with the root bridge, the path cost is determined. The STP cost is an accumulated total path cost based on the bandwidth of the links. Table below shows the typical costs associated with the different Ethernet networks.

Spanning-Tree Port States

The ports on a bridge or switch running STP can transition through four different states.
Blocking
Listening
Learning
Forwarding

Necessary Procedures

This section takes a step-by-step look at verifying the STP configuration, which can be essential to troubleshooting.

Verifying STP Information

To verify if STP is configured and running on a switch, you can use the show spantree (sh span for short) command. This will show you information for VLAN 1 only. To see information about other VLANs
Bridge Identifier has priority 32768,
address 0030.80CC.7B40
Configured hello time 2, max age 20,
forward delay 15
Current root has priority 32768, address
0030.80CC.7B40
Root port is N/A, cost of root path is 0
Topology change flag not set,
detected flag not set
Topology changes 0,
last topology change occured 0d00h00m00s ago
Times: hold 1, topology change 8960
hello 2, max age 20, forward delay 15
Timers: hello 2, topology change 35,
notification 2
Port Ethernet 0/1 of VLAN1 is Forwarding
[output cut]

The show spantree command displays the STP information for VLAN 1. Notice that the bridge ID, MAC address, and timers are displayed. The output “VLAN 1 is executing the IEEE compatible Spanning Tree Protocol” is telling you that STP is running on this VLAN.

Related posts:

1. Spanning Tree Protocol Exams Questions
2. Network Congestion Problem in Ethernet Networks
3. Virtual LANs
4. LAN segmentation using bridges and switches
5. Advantages of LAN Segmentation
6. Benefits of Network Segmentation With Routers
7. RIP Routing Protocol
8. TCP-IP Network-layer protocol
9. Virtual LANs Exams Points
10. key Frame Relay Terms and Features Exam Points