Home > Switches > Port and VLAN Configuration > RSTP and its Implementation on the MS Switch

RSTP and its Implementation on the MS Switch

STP Overview

Before we begin with RSTP (Rapid Spanning Tree Protocol), we should take a moment to explain its precursor STP (Spanning Tree Protocol). STP is predominantly used to prevent layer 2 loops and broadcast storms and is also used for network redundancy. Switches within the same network need to be enabled for STP before they run the Spanning Tree Algorithm so they can accurately determine which switch should be elected the “root bridge.” This designated root bridge will be responsible for sending configuration BPDUs  (Bridge Protocol Data Units) along with other information to its directly connected switches who, in turn, forward these BPDUs to their neighboring switches. Each switch has a bridge ID priority value (BID), which is a combination of a priority value (default 32768) and the switch’s own MAC address. The switch with the lowest BID will become the Root Bridge.

Election Process

When switches are first turned on, they will send configuration BPDUs containing their BIDs, with each

switch initially believing themselves to be the root bridge. However, when a switch receives a BPDU with a superior (lower value) BID, that switch will stop originating configuration BPDUs and will instead relay these superior BPDUs to its neighboring switches.

Once a root bridge has finally been announced, a second election process begins to determine the “root port” selection process (the port on a switch that will forward frames to the root bridge). This process will follow the steps below until a root port is elected:

  1. A switch port receives superior BPDUs from another switch and identifies that switch as the root bridge

  2. The port with the lowest root path is selected as the root port if possible

  3. If the path cost is the same, the switch will select the port with the lowest sender BID as the selected root port

  4. If the sender BID is the same (usually the same switch, the port with the lowest physical port number on the sending switch  will be selected as the root bridge (as the final tie-breaker)

Election Process Diagram

Let’s follow the process for switch 2 in the diagram below

RSTP1.png

 

  1. A switch port receiving superior BPDUs – Ports fa0/1 and fa0/2 receive the same superior BPDU. Therefore, further steps are needed to determine the root port. Port Fa0/3 on switch 2 does not qualify because it’s receiving BPDUs from switch 3.

  2. The port with the lowest root path cost – We can see ports fa0/1 and fa0/2 have equal path costs, so we still have a tie.

  3. The port with the lowest sender BID – Again fa0/1 and fa0/2 have the same BID (that of switch 2) and therefore the final step will be used to determine the root port.

  4. The lowest port number of the sending switch – fa0/1 is lowest port number and is therefore elected the root port for this switch.

The 5 STP states

There are 5 STP switchport states; these are:

Disabled The result of an administrative command that will disable the port
Blocking When a device is connected, the port will first enter the blocking state
Listening The switch will listen for and send BPDUs
Learning The switch will receive a superior BPDU, will stop sending its own BPDUs and will relay the superior BPDUs
Forwarding The port is forwarding traffic

RSTP

RSTP (Rapid Spanning Tree protocol) is as its name suggests, is a faster transition to a port forwarding state. Unlike STP, which has 5 switchport states, RSTP has only 3: Discarding, Learning and Forwarding. Below is a comparison table:

STP RSTP
Disabled [NONE]
Blocking Discarding
Listening Discarding
Learning Learning
Forwarding Forwarding

Alternate and Backup Ports

In RSTP, the blocked port is split into 2 new port roles, Alternate and Backup.

Alternate

An alternate port receives BPDUs from another switch but remains in a blocked state. For example, let’s say a switch has 2 paths to the Root-bridge, it will elect 1 of the 2 ports as a root port and the other will become an Alternate port. If at any time the Root port fails, this redundant path, the Alternate port will become the new Root port.

Backup

A backup port receives BPDUs from its own switch but remains in a blocked state. For example, If a switch has 2 ports connecting to different switches, then 1 port will be elected as a root port and the other port will become the backup port.

RSTP2.png

RSTP with Meraki Switches

Per Switch Network

To manually configure all switches in a network for RSTP, navigate in Dashboard to Switch > Switch Settings, and under STP configuration you will have the option to Enable/Disable RSTP.  Enabling this feature will mean all access ports will bypass the learning state and will go immediately into a forwarding state.

 

On this switch settings page, you are also able to set the root bridge priority by selecting the desired switch and setting the priority value. The defaults value for all switches is 32768. The bridge priority increments in values of 4096 with the lowest possible value starting at 0 and the highest value being 61440. A value of 0 will mean the selected switch will become the root bridge if no other switches have that same value, which will result in the switch with the lowest MAC address being selected.

Per Port Basis

Alternatively, you may edit RSTP on a per port basis by navigating in Dashboard to Switch > Switch Ports, then click on the desired port and select edit. in the port configuration menu you have the option to Enable/Disable RSTP on the port.

In this window you can also elect to use root guard and BPDU guard.

Global RSTP Configurations

  • When RSTP is enabled globally on the switch, all ports can participate in RSTP and detect and block bridge loops. If RSTP is disabled globally on the switch, all ports will not participate in RSTP. This prevents the switch from from detecting and blocking bridge loops and is not recommended.

Per-port RSTP Configurations

  • Disabling RSTP on a switchport will prevent the port from participating in RSTP or detecting and blocking a bridge loops. This is not recommended.
  • Access ports with RSTP set to enabled run PortFast by default. PortFast allows the port to transition to a forwarding state immediately at linkup without going through the full RSTP algorithm. However, the port can still detect and block bridge loops. This is the recommended configuration for ports connected to edge devices such as workstations or servers that do not create bridge loops. 
  • Trunk ports with RSTP set to enabled run the RSTP algorithm by default. These ports will participate in the spanning tree topology and detect and block bridge loops. This is the recommended port configuration when interlinking switches. 
Last modified

Tags

Classifications

This page has no classifications.

Explore the Product

Click to Learn More

Article ID

ID: 1138

Explore Meraki

You can find out more about Cisco Meraki on our main site, including information on products, contacting sales and finding a vendor.

Explore Meraki

Contact Support

Most questions can be answered by reviewing our documentation, but if you need more help, Cisco Meraki Support is ready to work with you.

Open a Case

Ask the Community

In the Meraki Community, you can keep track of the latest announcements, find answers provided by fellow Meraki users and ask questions of your own.

Visit the Community