Define Rapid Spanning Tree Protocol on SFE/SGE Managed Switches

Objective

Rapid Spanning Tree Protocol (RSTP) is an advanced version of standard Spanning Tree Protocol (STP). The advantage of RSTP is that it converges faster after a topology change. It is also backwards compatible with STP. In general, standard STP takes around 30 to 50 seconds to respond when there is a topology change while RSTP responds within a few milliseconds.

The objective of this article is to guide you on how to define RSTP settings.

Applicable Devices

• SFE/SGE Managed Switches

Software Version

• v3.0.2.0

RSTP Interface Settings

Note: Before you enable RSTP on the port you have to enable STP on that particular port. For more information go through the article titled Edit Spanning Tree Protocol Interface Settings on SGE/SFE Managed Switches.

Step 1. Log in to the web configuration utility and choose Bridging > Spanning Tree (STP) > RSTP. The RSTP page opens:

The Interface Settings page contains the following fields:

• Copy from Entry Number — This field allows you to copy the interface settings of one port to another port. In this field you have to enter the port from which the settings are to be copied.

• To Entry Number(s) — In this field you have to enter the port to which the settings are to be copied.

Step 2. Click the Port radio button if you want the STP settings to be applied on a particular port whereas click the LAG radio button if you want STP settings to be applied on a bundle of ports.

Step 3. Click the Edit button. The Edit Rapid Spanning Tree window opens:

It contains the following information:

• Role — Shows what is the role assigned to a port by the algorithm. The following are the possible port states:

– Root — This is the port which receives the best Bridge Protocol Data Units (BPDU) and this is the port very close to the root bridge in terms of path cost.

– Designated — A port is assigned the designated role if it can send best BPDU on the segment to which it is connected such as an Ethernet segment. For a given path there can be only a single root towards the root bridge because if there are more than one then a bridging loop will occur which is not desired by the spanning tree.

– Alternate — A port is assigned the alternate role if it has an alternate path to the root bridge. These ports are in a blocking state. A blocked port is defined as not being the designated or root port.

– Backup — A port is assigned the backup role if it has a backup path to the root bridge. This is similar to Alternate role except that here the backup port and the designated port belong to the same switch.

• Mode — Shows whether RSTP is enabled or STP is enabled on the port.

• Fast Link Operational Status — Shows whether Fast Link is enabled or disabled on the port or LAG. There is a benefit of using PortFast on the network. Whenever a link becomes active and goes into forwarding state in Spanning Tree Protocol, the switch needs to send a special packet called the Topology Change Notification (TCN) which causes the MAC addresses to age out after 15 seconds instead of the usual 300 seconds even though the topology does not change significantly.

Note: For Fast Link Operational Status to be active you have to enable Port Fast in the STP settings . For more information go through the article titled Edit Spanning Tree Protocol Interface Settings on SGE/SFE Managed Switches.

• Port State — Shows the current STP state of the port. The following are the possible port states:

– Disabled — It means that STP is disabled on this port but the port will search for MAC addresses while forwarding the traffic at the same time.

– Blocking — It means that the port is currently blocked. A port is blocked when it is known to cause a switching loop. In this state it cannot send or receive data. But this port will go into forwarding state if the other active ports fail.

– Listening — It means that the port is in listening mode. In this state the switch processes BPDUs and will wait for information that would cause it to return to the blocking state.

– Learning — It means that the port is in learning mode. Even though it does not forward frames it learns about source addresses from the received frames and adds them to the filtering database.

– Forwarding — It means that the port is in forwarding mode. In this state it receives and sends the data.  STP continues to monitor the incoming BPDUs which may indicate that the port should return to blocking state to avoid formation of loops.

• Point to Point Operational Status — Shows the Point to Point Operating State.

Step 4. (Optional) From the Interface drop-down list choose the particular port or LAG for which you want to add to the Spanning Tree Protocol Interface settings.

Step 5. From the Point to Point Admin Status drop-down list choose Enable if you want full duplex port links, choose Disable if you want half duplex port links or choose Auto if you want the device to determine the state automatically.

Step 6. Check the Active Protocol Migration Test check box if you want to enable the Protocol Migration Test. This test will identify the STP mode of an unknown interface which is connected to a known interface.

Step 7. Click Apply. Rapid Spanning Tree Protocol settings are defined.

Caution: This only saves your configuration to the running configuration file. This means any changes made will be lost if the device is rebooted. If you wish to save these changes even after a system reboot, you need to copy the running configuration file to the startup configuration file. See Copy Configuration File on SFE/SGE Series Managed Switches for more information on how to do this.