Interrelationship,The IEEE 802.1S standard for MSTP relates to both IEEE 802.1D and IEEE 802.1Q.

Hi Folks, 

i had one heck of a time understanding MSTP and its counter parts, you might find the below synopsis of my understanding useful :)

IEEE STP (802.1D) is the original forerunner to the different spanning tree variations/derivations that subsequently evolved, including RSTP, MSTP etc, proposed in 1990. The main purpose of the protocol was to ensure a loop free, layer 2 topology. 802.1 D standard referenced a common spanning tree that spanned the entire fabric, and included all VLANs.

IEEE standard Rapid STP (802.1W), introduced in 1998, was an evolution of STP that incorporated many improvements upon the original Standard. RSTP allowed faster convergence and explicit configuration of edge ports to allow faster transition to forwarding state. It further introduced features like port roles, link types etc.

One of the limitations of the original 802.1D standard was its use of a singular Spanning tree. If the network comprised multiple VLANs, this limitation would cause a blocked link to be blocked for all VLANs. To address this, Cisco brought forward PVST/PVST+, which allowed a “Per VLAN Spanning Tree” instance, as its name suggests. While this solution accommodate having different links in forwarding state for different VLANs in a redundant link configuration, with increasing number of VLANs, a Per VLAN instance would cause greater stress on both the resources and the CPU on the switch fabric.

Multiple Spanning Tree Protocol (802.1S), thus, was the next natural evolution in light of the aforementioned limitation. As described by IEEE, “MSTP allows frames assigned to different VLANs to follow separate paths, each based on an independent Multiple Spanning Tree Instance (MSTI), within Multiple Spanning Tree (MST) Regions composed of LANs and or MST Bridges. These Regions and the other Bridges and LANs are connected into a single Common Spanning Tree (CST).” The standard was later merged into IEEE 802.1Q, 2003.  To summarize this relation, MSTP is built on top of RSTP. MSTP allows us to group a set of VLANs into a particular instance. In an example network with 20 VLANs, MSTP can allow us to group VLANs into two or more instances, each of the instances sharing a common spanning tree providing a unique forwarding path through the switched network, independent of other instances.

MSTP is therefore, an ideal medium between a common STP approach and a Per VLAN STP approach. Frames are assigned to different VLANs to follow separate paths, each based on an independent Multiple Spanning Tree Instance (MSTI), within Multiple Spanning Tree (MST) Regions.

The original 802.1Q standard was introduced in 1998. In the context of a bridged LAN infrastructure, it defined the operation of VLANs and its relation to the 802.1 D protocol. The aim of the standard was to facilitate the sharing of the same network medium between multiple layer 2 networks, while maintaining isolation of information. It defined a Common STP instance that spans the entire bridged network, independent of the number of VLANs in operation. It also facilitates the communication between such logical segments of a bridged network, via a layer 3 device.

802.1 Q standard is frequently referenced as .1Q or VLAN Tagging. Tagging is the mechanism that allows the frames to traverse the switch fabric while retaining the ID of the logical network to which they belong. This is usually accomplished by way of inserting a VLAN tag field into the Ethernet frame.

To consider MST relation to 802.1D and 802.1Q, it is best to start by understanding that MSTP implementation assigns a special instance, called Internal Spanning tree (instance 0), as responsible for carrying all STP related information.  Thus, BPDUs are not transmitted per STP instance. All VLANs start by default in instance 0. Once further instances are enabled, this scheme changes from the classical RSTP implementation on a single instance, to MSTP implementation that resembles RSTP instances operating within regions. For networks external to the MST region, IST can denote MST as a CST virtual bridge, and can receive/transmit BPDUs to such CSTs for compatibility with 8021.D STP.



Thanks
Huzeifa Bhai