DES-1228/ME Metro Ethernet Managed Switch CLI Reference Guide
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11
M
ULTIPLE
S
PANNING
T
REE
P
ROTOCOL
(MSTP)
C
OMMANDS
This Switch supports three versions of the Spanning Tree Protocol; 802.1D STP, 802.1w Rapid STP and 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 associated with them. These instances will be classified by an instance_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:
A configuration name defined by an alphanumeric string of up to 32 characters (defined in the config stp mst_config_id command
as name <string>).
A configuration revision number (named here as a revision_level) and;
A 4094 element table (defined here as a vid_range) which will associate each of the possible 4094 VLANs supported by the
Switch for a given instance.
To utilize the MSTP function on the Switch, three steps need to be taken:
The Switch must be set to the MSTP setting (config stp version)
The correct spanning tree priority for the MSTP instance must be entered (config stp priority).
VLANs that will be shared must be added to the MSTP Instance ID (config stp instance_id).
