D-Link xStack DGS-3426P User Manual - Spanning Tree

xStack DGS-3400 Series Layer 2 Gigabit Ethernet Managed Switch 

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Spanning Tree 

This Switch supports three versions of the Spanning Tree Protocol; 802.1D STP, 802.1w Rapid STP and 802.1s MSTP. 802.1D 

STP will be familiar to most networking professionals. However, since 802.1w RSTP and 802.1s MSTP has been recently 

introduced to D-Link managed Ethernet switches, a brief introduction to the technology is provided below followed by a 

description of how to set up 802.1D STP, 802.1w RSTP and 802.1s MSTP. 

802.1s MSTP 

Multiple Spanning Tree Protocol, or MSTP, is a standard defined by the IEEE community that allows multiple VLANs to be 

mapped to a single spanning tree instance, which will provide multiple pathways across the network. Therefore, these MSTP 

configurations will balance the traffic load, preventing wide scale disruptions when a single spanning tree instance fails. This will 

allow for faster convergences of new topologies for the failed instance. Frames designated for these VLANs will be processed 

quickly and completely throughout interconnected bridges utilizing any of the three spanning tree protocols (STP, RSTP or 

MSTP).  

This protocol will also tag BPDU packets so receiving devices can distinguish spanning tree instances, spanning tree regions and 

the VLANs associated with them. An MSTI ID will classify these instances. MSTP will connect multiple spanning trees with a 

Common and Internal Spanning Tree (CIST). The CIST will automatically determine each MSTP region, its maximum possible 

extent and will appear as one virtual bridge that runs a single spanning tree. Consequentially, frames assigned to different VLANs 

will follow different data routes within administratively established regions on the network, continuing to allow simple and full 

processing of frames, regardless of administrative errors in defining VLANs and their respective spanning trees. 

Each switch utilizing the MSTP on a network will have a single MSTP configuration that will have the following three attributes: 

2.

A configuration name defined by an alphanumeric string of up to 32 characters (defined in the 

STP Bridge Global 

Settings 

window in the Configuration Name

field). 

3.

A configuration revision number (named here as a Revision Level and found in the 

STP Bridge Global Settings 

window) 

and; 

4.

A 4096-element table (defined here as a VID List in the 

MST Configuration Identification

 window), which will 

associate each of the possible 4096 VLANs supported by the Switch for a given instance. 

 To utilize the MSTP function on the Switch, three steps need to be taken: 

1.

The Switch must be set to the MSTP setting (found in the

 STP Bridge Global Settings

 window in the STP Version field) 

2.

The correct spanning tree priority for the MSTP instance must be entered (defined here as a Priority in the 

MST 

Configuration Identification

 window when configuring an MSTI ID

settings). 

3.

VLANs that will be shared must be added to the MSTP Instance ID (defined here as a

VID List

in the

 MST 

Configuration

Identification

 window when configuring an MSTI ID settings). 

802.1w Rapid Spanning Tree 

The Switch implements three versions of the Spanning Tree Protocol, the Multiple Spanning Tree Protocol (MSTP) as defined by 

the IEEE 802.1s, the Rapid Spanning Tree Protocol (RSTP) as defined by the IEEE 802.1w specification and a version compatible 

with the IEEE 802.1D STP. RSTP can operate with legacy equipment implementing IEEE 802.1D, however the advantages of 

using RSTP will be lost.   

The IEEE 802.1w Rapid Spanning Tree Protocol (RSTP) evolved from the 802.1D STP standard. RSTP was developed in order to 

overcome some limitations of STP that impede the function of some recent switching innovations, in particular, certain Layer 3 

functions that are increasingly handled by Ethernet switches. The basic function and much of the terminology is the same as STP. 

Most of the settings configured for STP are also used for RSTP. This section introduces some new Spanning Tree concepts and 

illustrates the main differences between the two protocols.  

Port Transition States 

An essential difference between the three protocols is in the way ports transition to a forwarding state and in the way this 

transition relates to the role of the port (forwarding or not forwarding) in the topology. MSTP and RSTP combine the transition 

states disabled, blocking and listening used in 802.1D and creates a single state Discarding. In either case, ports do not forward 

packets. In the STP port transition states disabled, blocking or listening or in the RSTP/MSTP port state discarding, there is no 

functional difference, the port is not active in the network topology. Table 7-3 below compares how the three protocols differ 

regarding the port state transition.