SPANNING TREE PROTOCOL (STP)

In an enterprise environment, redundancy is one of the key factors in a well-designed network. It enables a network to become more fault-tolerant, and it can increase the throughput of traffic by balancing the load between devices. The simplest way to achieve redundancy is by adding the same device, or, for the sake of this topic, an additional switch in the topology. However, in a layer-2 network, adding switches to the topology introduces problems such as broadcast storms, frame loops, and unstable entries in the MAC address table. This issue is caused by the way Ethernet frames work. To address this problem, the Spanning Tree Protocol (STP) was created and enabled on switches.

STP is a layer-2 protocol that enables switches to become aware of other switches connected within the same network. It uses Bridge Protocol Data Units (BPDU) frames to communicate with other switches, determining which interfaces an Ethernet frame should be forwarded through, thereby avoiding the mentioned problems that may occur in a layer-2 network.

STP Fundamentals

STP Iteration

Protocol	Proprietary	Spanning-Tree Calculation
STP	IEEE 802.1D	All VLANs
PVST+	Cisco	Per VLAN
RSTP	IEEE 802.1W	All VLANs
Rapid PVST+	Cisco	Per VLANs
MSTP	IEEE 802.1S	Per Instance

STP Key Terminology

KeyTerms

Root Bridge

• a switch that has been elected as a root.

Bridge Protocol Data Unit (BPDU)

• a PDU that is used for network switches to process STP.

BPDU Types:

1. Configuration BPDU
2. Topology Change Notification (TCN) BPDU

Root Path Cost

• a metric used for measuring the path to the root bridge.

System Priority

• the lowest value is elected as root (value between 0-61440, increments of 4096).

System ID Extension

• a 12-bit value indicates the VLAN that the BPDU correlates to.

Root Bridge ID

• details of the switch that has been elected as a root.

Local Bridge ID

• details of the switch that is currently being managed.

STP Timers

Timers

Hello Timer

• time interval that BPDU is being advertised (default value is 2 seconds).

Max Age

• maximum time that a bridge saves the BPDU information.
• 802.1D (STP) = default value is 10 × Hello Timer = 20 seconds
• 802.1W (RSTP) = default value of 6 seconds

Forward Delay

• amount of time that a port stays in a listening and learning state (default value is 15 seconds per state).

STP Convergence Time (Approx.)

• 802.1D (STP)
  • around 50 seconds = 30s (Forward Delay) + 20s (Time that the switch needs to realize that the link is either up/down)
• 802.1W (RSTP)
  • from milliseconds to under 10 seconds.

Root Bridge Election Process

A part of the process of building an STP topology is the election of which switch will assume the role of the Root Bridge. The elected switch serves as a central reference point for all other switches in calculating the path cost and selecting the best path to the frame’s destination. This election is crucial in building a loop-free network by knowing which port role (root port, designated port, blocking port, etc.) will be configured on each of the connected interfaces in a switch. 

Election Criteria

The criteria given below are an ordered list of how a Root Bridge is selected in a STP topology.

1. Lowest Bridge Priority.
2. Lowest MAC Address.

Bridge ID Structure

A Bridge ID is a unique value that identifies each switch in an STP topology. It is used to determine which switch will be elected as a Root Bridge.

Figure 1: Bridge ID Structure

• Bridge ID
  • Bridge Priority
    • System Priority
    • VLAN ID
  • MAC Address

STP vs RSTP

Port States

The figure below illustrates the states that a switch port transitions through before reaching the forwarding state. Figure 2: STP and RSTP Port States

Port Roles

The figure below displays the roles assigned to each switch port in STP or RSTP. Figure 3: STP and RSTP Port Roles

Port Roles	Definition
Root Port (RP)	A port on a switch that has the closest path (Lowest Cost) to the Root Bridge.
Designated Port (DP)	A port that can send the best BPDU frame on its segment.
Blocking Port	A port that does not send an BPDU frame.
Alternate Port	A blocking port that serves as a backup of Root Port.
Backup Port	A blocking port that serves as a backup of Designated Port.

STP Configuration

This section provides several CLI commands on how to configure and modify an STP topology.

Checking the STP Topology

show spanning-tree

Change STP Mode (STP or RSTP)

configure terminal
	spanning-tree mode [pvst | rapid-pvst]

Note: On Cisco devices, PVST and Rapid-PVST are the default settings when configuring STP and RSTP.

Bridge Priority Modification

configure terminal
	spanning-tree vlan [vlan-id] priority [0 - 61440]

Note: Bridge priority must be in increments of 4096.

STP Timer Modification

configure terminal
	spanning-tree vlan [vlan-id] [forward-time | hello-time | max-age] [seconds]

Port Modification

Portfast

• Enable on all access ports

    configure terminal
        spanning-tree portfast default
  

• Enable on a specific port

    configure terminal
        interface [interface-id]
            spanning-tree portfast
  

BPDU Guard

• Enable on all portfast ports

    configure terminal
        spanning-tree portfast bpduguard default
  

• Enable on a specific port

    configure terminal
        interface [interface-id]
            spanning-tree bpduguard enable
  

BPDU Filter

• Enable on all portfast ports

    configure terminal
        spanning-tree portfast bpdufilter default
  

• Enable on a specific port

    configure terminal
        interface [interface-id]
            spanning-tree bpdufilter enable