D-Link DES-3552 [56/260] Spanning tree

DES-3550 Fast Ethernet Layer 2 Switch

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 either of the three spanning tree protocols

(STP, RSTP or MSTP).

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

and the VLANs asscociated with them. These instances will be classified by an MSTI ID. 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:

1. 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).

2. A configuration revision number (named here as a Revision Level and found in the STP Bridge Global Settings

window) and;

3. A 4096-element table (defined here as a VID List in the MST Configuration Table window) that 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 Table 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 Table window when configuring an MSTI ID settings).