Spanning Tree Protocol is a hot exam topic for Cisco, CompTIA, Juniper, and other vendors. You will be grilled about it in technical job interviews and, of course have to fix STP issues in the real world of IT.

What is Spanning Tree Protocol? STP Mini-Course

The Spanning Tree Protocol (STP), defined by IEEE 802.1D, is a loop-prevention protocol that allows switches to communicate with each other in order to discover physical loops in a network. If a loop is found, the STP specifies an algorithm that switches can use to create a loop-free logical topology. This algorithm creates a tree structure of loop-free leaves and branches that spans across the Layer 2 topology.

Loops occur most often as a result of multiple connections between switches, which provides redundancy, as shown below in the figure.

Layer 2 Loop Scenario

Referring to the figure above, if none of the switches run STP, the following process takes place: Host A sends a frame to the broadcast MAC address (FF-FF-FF-FF-FF-FF) and the frame arrives at both Switch 1 and Switch 2. When Switch 1 receives the frame on its Fa0/1 interface, it will flood the frame to the Fa0/2 port, where the frame will reach Host B and the Switch 2 Fa0/2 interface. Switch 2 will then flood the frame to its Fa0/1 port and Switch 1 will receive the same frame it transmitted. By following the same set of rules, Switch 1 will re-transmit the frame to its Fa0/2 interface, resulting in a broadcast loop. A broadcast loop can also occur in the opposite direction (the frame received by Switch 2 Fa0/1 will be flooded to the Fa0/2 interface, which will be received by Switch 1).

Bridging loops are more dangerous than routing loops because, as mentioned before, a Layer 3 packet contains a special field called TTL (Time to Live) that decrements as it passes through Layer 3 devices. In a routing loop, the TTL field will reach 0 and the packet will be discarded. A Layer 2 frame that is looping will stop only when a switch interface is shut down. The negative effects of Layer 2 loops grow as the network complexity (i.e., the number of switches) grows, because as the frame is flooded out to multiple switch ports, the total number of frames multiplies at an exponential rate.

Broadcast storms also have a major negative impact on the network hosts, because the broadcasts must be processed by the CPU in all devices on the segment. In the figure, both Host A and Host B will try to process all the frames they receive. This will eventually deplete their resources unless the frames are removed from the network.

Spanning Tree Protocol (STP) Calculations

STP calculations are based on the following two concepts:

• Bridge ID
• Path Cost

A Bridge ID (BID) is an 8-byte field composed of two subfields: the high-order Bridge Priority (2 bytes) and the low-order MAC address (6 bytes). The MAC address is expressed in hexadecimal format, while the Bridge Priority is a 2-byte decimal value with values from 0 to 65535 and a default value of 32768.

Switches use the concept of cost to evaluate how close they are to other switches. The original 802.1D standard defined a cost of 1000 Mbps divided by the bandwidth of the link in Mbps. For example, a 10 Mbps link was assigned a cost of 100, and a FastEthernet link had a cost of 10. Lower STP costs are better. However, as higher bandwidth connections have gained popularity, a new problem has emerged, namely, that cost is stored as an integer value only. The option of using a cost of 1 for all links greater than 1 Gbps would narrow the accuracy of the STP cost calculations, so it is considered invalid. As a solution to this problem, the IEEE decided to modify the cost values on a non-linear scale, as illustrated below:

 

These values were carefully chosen to allow the old and new schemes to interoperate for the link speeds in common use today.

STP Four-step Process

To create a loop-free logical topology, STP uses a four-step decision process as follows:

1. Lowest Root BID
2. Lowest Path Cost to Root Bridge
3. Lowest Sender BID
4. Lowest Port ID

Switches exchange STP information using special frames called Bridge Protocol Data Units (BPDUs). Switches evaluate all the BPDUs received on a port and store the best BPDU seen on every port. Every BPDU received on a port is checked against the four-step sequence to see whether it is more attractive than the existing BPDU saved for that port.

When a switch first becomes active, all of its ports send BPDUs every 2 seconds. If a port hears a BPDU from another switch that is more attractive than the BPDU it has been sending, the port stops sending BPDUs. If the more attractive BPDU stops arriving for a period of 20 seconds (by default), the local port will resume sending its own BPDUs.

The two types of BPDUs are as follows:

• Configuration BPDUs, which are sent by the Root Bridge and flow across active paths
• Topology Change Notification (TCN) BPDUs, which are sent to announce a topology change

The initial STP convergence process is accomplished in the following three steps:

1. Root Bridge election
2. Root Ports election
3. Designated Ports election

When a network is powered on, all the switches announce their own BPDUs. After they analyze the received BPDUs, a single Root Bridge is elected. All switches except the Root Bridge calculate a set of Root Ports and Designated Ports to build a loop-free topology. After the network converges, BPDUs flow from the Root Bridge to every segment in the network. Additional changes in the network are handled using TCN BPDUs.

The first step in the convergence process is electing a Root Bridge. The switches do this by analyzing the received BPDUs and looking for the switch with the lowest BID, as shown below the below figure:

STP Convergence

Referring to the figure above, Switch 1 has the lowest BID of 32768.AA.AA.AA.AA.AA.AA and will be elected as the Root Bridge because it has the lowest MAC address, considering they all have the same Bridge Priority (i.e., the default of 32768).

The switches learn about Switch 1’s election as the Root Bridge by exchanging BPDUs at a default interval of 2 seconds. BPDUs contain a series of fields, among which include the following:

• Root BID – identifies the Root Bridge
• Root Path Cost – information about the distance to the Root Bridge
• Sender BID – identifies the bridge that sent the specific BPDU
• Port ID – identifies the port on the sending bridge that placed the BPDU on the link

Only the Root BID and Sender BID fields are considered in the Root Bridge election process. When a switch first boots, it places its BID in both the Root BID and the Sender BID fields. For example, Switch 1 boots first and starts sending BPDUs announcing itself as the Root Bridge every 2 seconds. After some time, Switch 3 boots and announces itself as the Root Bridge. When Switch 2 receives these BPDUs, it discards them because its own BID has a lower value. As soon as Switch 3 receives a BPDU generated by Switch 2, it starts sending BPDUs that list Switch 2 as the Root BID (instead of itself) and Switch 3 as the Sender BID.

The two switches now agree that Switch 2 is the Root Bridge. Switch 1 boots a few minutes later, and it initially assumes that it is the Root Bridge and starts advertising this fact in the BPDUs it generates. As soon as these BPDUs arrive at Switch 2 and Switch 3, these two switches give up the Root Bridge position in favor of Switch 1. All three switches are now sending BPDUs that announce Switch 1 as the Root Bridge.

The next step is electing the Root Ports. A Root Port on a switch is the port that is closest to the Root Bridge. Every switch except the Root Bridge must elect one Root Port. As mentioned before, switches use the concept of cost to determine how close they are from other switches. The Root Path Cost is the cumulative cost of all links to the Root Bridge.

When Switch 1 sends BPDUs, they contain a Root Path Cost of 0. As Switch 2 receives them, it adds the path cost of its interface Fa0/1 (a value of 19 for a FastEthernet link) to the Root Path Cost value. Switch 2 sends the new Root Path Cost calculated value of 19 in its BPDUs generated on the Fa0/2 interface. When Switch 3 receives the BPDUs from Switch 2, it increases the Root Path Cost by adding 19, the cost of its Fa0/2 interface, for a total of 38. At the same time, Switch 3 also receives BPDUs directly from the Root Bridge on Fa0/1. This enters Switch 3 with a value of 0, and Switch 3 increases the cost to 19 because Fa0/1 is a FastEthernet interface.

At this point, Switch 3 must select a single Root Port based on the two different BPDUs it received, one with a Root Path Cost of 38 from Switch 2 and the other with a Root Path Cost of 19 from Switch 1. The lowest cost wins; thus, Fa0/1 becomes the Root Port and Switch 3 begins advertising this Root Path Cost of 19 to downstream switches. Switch 2 goes through the same set of calculations and elects its Fa0/1 interface as the Root Port. This Root Port selection process on Switch 3 is based on the lowest Root Path Costs it receives in the BPDUs, as illustrated below:

 

The next step in the STP convergence process is electing Designated Ports. Each segment in a Layer 2 topology has one Designated Port. This port sends and receives traffic to and from that segment and the Root Bridge. Only one port handles traffic for each link, guaranteeing a loop-free topology. The bridge that contains the Designated Port for a certain segment is considered the Designated Switch on that segment.

Analyzing the link between Switch 1 and Switch 2, Switch 1 Fa0/1 has a Root Path Cost of 0 (being the Root Bridge) and Switch 2 Fa0/1 has a Root Path Cost of 19. Switch 1 Fa0/1 becomes the Designated Port for that link because of its lower Root Path Cost. A similar election takes place for the link between Switch 1 and Switch 3. Switch 1 Fa0/2 has a Root Path Cost of 0 and Switch 3 Fa0/1 has a Root Path Cost of 19, so Switch 1 Fa0/2 becomes the Designated Port.

When considering the link between Switch 2 and Switch 3, both Switch 2 Fa0/2 and Switch 3 Fa0/2 ports have a Root Path Cost of 19, resulting in a tie. To break the tie and declare a winner, STP uses the four-step decision process described below:

1. Lowest Root BID: All three bridges are in agreement that Switch 1 is the Root Bridge; advance to the next step.
2. Lowest Root Path Cost: Both Switch 2 and Switch 3 have a cost of 19; advance to the next step.
3. Lowest Sender BID: Switch 2’s BID (32768.BB.BB.BB.BB.BB.BB) is lower than Switch 3’s BID (32768.CC.CC.CC.CC.CC.CC), so Switch 2 Fa0/2 becomes the Designated Port and Switch 3 Fa0/2  is considered a non-Designated Port; end of the decision process.
4. Lowest Port ID: N/A.

Five Spanning Tree Protocol States

In a loop-free topology, Root and Designated Ports forward traffic and non-Designated Ports block traffic. The five STP states are listed below:

 

1. After initialization, the port starts in the Blocking state, where it listens for BPDUs. The port will transit into the Listening state after the booting process, when it thinks it is the Root Bridge or after not receiving BPDUs for a certain period of time.
2. In the Listening state, no user data passes through the port; it is just sending and receiving BPDUs in order to determine the Layer 2 topology. This is the phase in which the election of the Root Bridge, Root Ports, and Designated Ports occur.
3. Ports that remain Designated or Root Ports after 15 seconds progress to the Learning state, and during another 15-second period, the bridge builds its MAC address table but does not forward user data.
4. After the 15-second period, the port enters the Forwarding state, in which it sends and receives data frames.
5. The Disabled state means the port is administratively shut down.

STP Timers

The STP process is controlled by the three timers listed below:

 

RSTP in Brief

A modern variation of the STP is the Rapid STP (RSTP), as defined by IEEE 802.1W. The main advantage of RSTP is its ability to achieve fast convergence (i.e., neighbor switches can communicate between each other and determine the state of the links in less time). RSTP ports have the following roles:

• Root
• Designated
• Alternate
• Backup
• Disabled

RSTP port states are also different, as the Blocking, Learning, and Disabled states converge into a Discarding state. Although some important differences exist between RSTP and STP, they are compatible and can work together in any network.

We cover STP, RSTP and more in our Cisco CCNA and Cisco CCNP online courses. These include labs and practice tests. You can also read the Cisco papers on STP.