MSTP

CST

The original IEEE 802.1q standard defines much more than simply trunking. This standard defines a Common Spanning Tree (CST) that only assumes one spanning tree instance for the entire bridged network, regardless of the number of VLANs.
In a network running the CST, these statements are true:
. No load balancing is possible; one Uplink needs to block for all VLANs.
. The CPU is spared; only one instance needs to be computed.

MST Case
Several VLANs can be mapped to a reduced number of spanning tree instances because most networks do not need more than a few logical topologies.
. The desired load balancing scheme can still be achieved.
. The CPU is spared because only limited instances are computed.

MST Region
The main enhancement introduced by MST is that several VLANs can be mapped to a single spanning tree instance. This raises the problem of how to determine which VLAN is to be associated with which instance. More precisely, how to tag BPDUs so that the receiving devices can identify the instances and the VLANs to which each device applies.

The IEEE 802.1s committee adopted a much easier and simpler approach that introduced MST regions. Think of a region as the equivalent of Border Gateway Protocol (BGP) Autonomous Systems, which is a group of switches placed under a common administration.

MST Configuration and MST Region

Each switch running MST in the network has a single MST configuration that consists of these three attributes:

1. An alphanumeric configuration name (32 bytes)
2. A configuration revision number (two bytes)
3. A 4096-element table that associates each of the potential 4096 VLANs supported on the chassis to a given instance

In order to be part of a common MST region, a group of switches must share the same configuration attributes.
Note: If for any reason two switches differ on one or more configuration attribute, the switches are part of different regions.

Region Boundary

In order to ensure consistent VLAN-to-instance mapping, it is necessary for the protocol to be able to exactly identify the boundaries of the regions. For that purpose, the characteristics of the region are included in the BPDUs. The exact VLANs-to-instance mapping is not propagated in the BPDU, because the switches only need to know whether they are in the same region as a neighbor. Therefore, only a digest of the VLANs-to-instance mapping table is sent, along with the revision number and the name. Once a switch receives a BPDU, the switch extracts the digest (a numerical value derived from the VLAN-to-instance mapping table through a mathematical function) and compares this digest with its own computed digest. If the digests differ, the port on which the BPDU was received is at the boundary of a region.

In generic terms, a port is at the boundary of a region if the designated bridge on its segment is in a different region or if it receives legacy 802.1d BPDUs.

MST Instances

According to the IEEE 802.1s specification, an MST bridge must be able to handle at least these two instances:
. One Internal Spanning Tree (IST)
. One or more Multiple Spanning Tree Instance(s) (MSTIs)

The terminology continues to evolve, as 802.1s is actually in a pre-standard phase. It is likely these names will change in the final release of 802.1s.

IST Instances

In order to clearly understand the role of the IST instance, remember that MST originates from the IEEE. Therefore, MST must be able to interact with 802.1q-based networks, because 802.1q is another IEEE standard. For 802.1q, a bridged network only implements a single spanning tree (CST). The IST instance is simply an RSTP instance that extends the CST inside the MST region.

The IST instance receives and sends BPDUs to the CST. The IST can represent the entire MST region as a CST virtual bridge to the outside world.

These are two functionally equivalent diagrams. Notice the location of the different blocked ports. In a typically bridged network, you expect to see a blocked port between Switches M and B. Instead of blocking on D, you expect to have the second loop broken by a blocked port somewhere in the middle of the MST region. However, due to the IST, the entire region appears as one virtual bridge that runs a single spanning tree (CST). This makes it possible to understand that the virtual bridge blocks an alternate port on B. Also, that virtual bridge is on the C to D segment and leads Switch D to block its port.



MSTIs

The MSTIs are simple RSTP instances that only exist inside a region. These instances run the RSTP automatically by default, without any extra configuration work. Unlike the IST, MSTIs never interact with the outside of the region. Remember that MST only runs one spanning tree outside of the region, so except for the IST instance, regular instances inside of the region have no outside counterpart. Additionally, MSTIs do not send BPDUs outside a region, only the IST does.

MSTIs do not send independent individual BPDUs. Inside the MST region, bridges exchange MST BPDUs that can be seen as normal RSTP BPDUs for the IST while containing additional information for each MSTI. Each switch only sends one BPDU, but each includes one MRecord per MSTI present on the ports.

Note: The first information field carried by an MST BPDU contains data about the IST. This implies that the IST (instance 0) is always present everywhere inside an MST region. However, the network administrator does not have to map VLANs onto instance 0, and therefore this is not a source of concern.


Unlike regular converged spanning tree topology, both ends of a link can send and receive BPDUs simultaneously. This is because, each bridge can be designated for one or more instances and needs to transmit BPDUs. As soon as a single MST instance is designated on a port, a BPDU that contains the information for all instances (IST+ MSTIs) is to be sent. The diagram shown here demonstrates MST BDPUs sent inside and outside of an MST region.

The MRecord contains enough information (mostly root bridge and sender bridge priority parameters) for the corresponding instance to calculate its final topology. The MRecord does not need any timer-related parameters such as hello time, forward delay, and max age that are typically found in a regular IEEE 802.1d or 802.1q CST BPDU. The only instance in the MST region to use these parameters is the IST; the hello time determines how frequently BPDUs are sent, and the forward delay parameter is mainly used when rapid transition is not possible (remember that rapid transitions do not occur on shared links). As MSTIs depend on the IST to transmit their information, MSTIs do not need those timers.