802.1S Mstp; 802.1W Rapid Spanning Tree; Port Transition States - D-Link DGS-3324SRi User Manual

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 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 administra tive 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) 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 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).

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 6-1 below compares how
the three protocols differ regarding the port state transition.
All three protocols calculate a stable topology in the same way. Every segment will have a single path to the root bridge.
All bridges listen for BPDU packets. However, BPDU packets are sent more frequently - with every Hello packet. BPDU
packets are sent even if a BPDU packet was not received. Therefore, each link between bridges is sensitive to the status of
the link. Ultimately this difference results in faster detection of failed links, and thus faster topology adjustment. A draw-
back of 802.1d is this absence of immediate feedback from adjacent bridges.
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