STP and RSTP Terminology

STP (Spanning Tree Protocol) and RSTP (Rapid Spanning Tree Protocol) rely on specific terminology for their operations and functions. This tutorial explains the meaning of each key term in detail. Understanding this terminology lets you manage STP and RSTP more effectively.

Root bridge

The root bridge is the switch with the lowest BID 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 will be the root bridge, and the remaining four will be non-root bridges.

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 need for their STP or RSTP operations.

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 bytes of the BID save the sum of the priority value and the 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 bytes. It uses the remaining 6 bytes to save the MAC address as the system ID.

Root port

After electing the root bridge, all non-root bridge switches select a single path to reach it. The port with the best path to the root of 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 Gigabit Ethernet ports. A Fast Ethernet port supports 100 Mbps, while a Gigabit Ethernet port supports 1 Gbps. 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.

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 may access the root bridge switch through several paths. If multiple options exist, a non-root bridge switch selects only one. It computes each path's cost and picks the path with the lowest cost. Path cost calculation starts at the root bridge. The root bridge sets the path cost to zero in all transmitted BPDUs. Receiving switches increment this cost by adding the cost of each port. For example, if received on a FastEthernet port, the path cost becomes 19 (0 + 19).

Designated port

A designated port connects the switch to an endpoint or a network segment. Only one designated port is selected per segment.

Non-designated port

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

Forwarding port

A forwarding port transmits user frames. Only root ports or designated ports function as forwarding ports.

Blocked port

A blocked port does not forward frames. It listens for BPDU frames from neighboring switches but drops all other frames and will never transmit them.

Alternate port (RSTP only)

An alternate port can serve as the root port if the root port fails.

Backup port (RSTP only)

A backup port can act as the designated port if the current designated port fails.

Convergence

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

Conclusion

This tutorial explained the terminology for STP and RSTP. These terms are essential for understanding the functions of STP and RSTP. These terms describe the things STP and RSTP use for their operations. RSTP is the upgraded version of STP. It uses the same terminology as RSTP, along with some new terms that describe RSTP-specific functions.