xStack
®
DGS-3400 Series Layer 2 Gigabit Ethernet Managed Switch
142
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:
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).
3.
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.
