STP and RSTP Terminology

STP (Spanning Tree Protocol) and RSTP (Rapid Spanning Tree Protocol) use many terms for their operations and functions. Learning these terms helps you understand STP and RSTP.

Root bridge

The root bridge is the switch having the lowest BID among all switches in the network. STP and RSTP run a separate instance for each VLAN. Each instance selects its root bridge. For example, if a network has ten VLANs, there will be ten root bridges, one in each VLAN.

Non-root bridge

STP and RSTP select only one switch as the root bridge in each VLAN. Apart from the elected switch, all remaining switches become non-root bridges. For example, if a network has five switches, one switch will be the root bridge, and the remaining four will be non-root bridge switches.

BPDU

A BPDU (Bridge Protocol Data Unit) is a message that STP and RSTP running switches use to communicate. Switches send a BPDU every two seconds. A BPDU contains all the information switches needed for their STP or RSTP operations.

bpdu

BID

A BID (Bridge ID) is an 8-byte value unique to each switch in each VLAN. It contains three things: bridge priority, VLAN ID, and system ID. The bridge priority is a changeable numeric value. The default priority value is 32768. The first 2-byte of the BID saves the sum of priority value and VLAN ID. For example, if the priority value is 32768 and the VLAN ID is 10, BID will save 32778 (32768 + 10) in the first 2-byte. It uses the remaining 6-byte to save the MAC address as the system ID.

Root port

After electing the root bridge, all non-root bridge switches select only one path to reach the root bridge. The port with the best path to the root the bridge is called the root port.

Port cost

Switches can have multiple types of ports. Each type supports a different bandwidth. For example, a switch can have FastEthernet and GigabitEtherent ports. A FastEtherent port supports 100 Mbps, while a GigabitEthernet port supports 1 Gbps data transfer speed. STP and RSTP assign a unique value to each port type. They use it to calculate path costs.

There are two sets of port costs. The following table lists both.

Bandwidth Old Cost Value New Cost Value
10 Gbps 1 2
1 Gbps 1 4
100 Mbps 10 19
10 Mbps 100 100

Some old series switches, like the Catalyst 1900, use the old cost value. Cisco has already discontinued these old series switches. New series switches, like the 2960, use the new cost value.

Path cost

Non-root bridge switches can have multiple paths to reach the root bridge switch. If a non-root bridge switch has more than one path to reach the root bridge, it selects only one path. It calculates the cost of each path and compares them. It chooses the path having the least cost.

The path cost calculation process starts from the root bridge. When the root bridge advertises BPDUs from all ports, it sets the path cost to zero in each BPDU. The BPDU receiving switch updates this cost by adding the port cost. For example, if a non-root bridge switch receives it on a FastEthernet port, it updates the path cost to 19 (0 + 19[received port cost]).

Designated port

A designated port connects the switch with an end device or a local segment. A switch selects only one designated port for each segment.

Non designated port

All ports apart from the root and designated ports are non-designated ports. The switch blocks non-designated ports to remove loops.

Forwarding port

A forwarding port forwards user frames. Only a root or a designated port can become a forwarding port.

Blocked port

A blocked port does not forward frames. It listens to BPDU frames from neighbor switches, but it drops any other frames received and will never transmit a frame.

Alternate port (RSTP only)

An alternate port is a port that could be used as the root port if the root port ever fails.

Backup port (RSTP only)

A backup port is a port that could be used as the designated port if the designated port fails.

Convergence

Convergence occurs when the switch finalizes the roll of all ports. A switch does not forward any frame until the convergence occurs.

Conclusion

In this tutorial, we discussed STP and RSTP terminology. These terms are essential for understanding STP and RSTP functions. These terms describe the things STP and RSTP use for their operations. RSTP is the upgraded version of STP. It uses the same terminology RSTP uses in addition to some new terms that describe RSTP-specific functions.

ComputerNetworkingNotes CCNA Study Guide STP and RSTP Terminology