User Manual

Configuration Examples
757
M6100, M5300, and M7100 Series Managed Switches
MSTP connects all Bridges and LANs with a single Common and Internal Spanning Tree
(CIST). The CIST supports the automatic determination of each MST region, choosing its
maximum possible extent. The connectivity calculated for the CIST provides the CST for
interconnecting these Regions, and an Internal Spanning Tree (IST) within each Region.
MSTP ensures that frames with a given VLAN ID are assigned to one and only one of the
MSTIs or the IST within the Region, that the assignment is consistent among all the
networking devices in the Region and that the stable connectivity of each MSTI and IST at
the boundary of the Region matches that of the CST. The stable active topology of the
Bridged LAN with respect to frames consistently classified as belonging to any given VLAN
thus simply and fully connects all LANs and networking devices throughout the network,
though frames belonging to different VLANs can take different paths within any Region, per
IEEE DRAFT P802.1s/D13.
All bridges, whether they use STP, RSTP or MSTP, send information in configuration
messages through Bridge Protocol Data Units (BPDUs) to assign port roles that determine
each port’s participation in a fully and simply connected active topology based on one or
more spanning trees. The information communicated is known as the spanning tree priority
vector. The BPDU structure for each of these different protocols is different. A MSTP bridge
will transmit the appropriate BPDU depending on the received type of BPDU from a particular
port.
An MST Region comprises of one or more MSTP Bridges with the same MST Configuration
Identifier, using the same MSTIs, and without any bridges attached that cannot receive and
transmit MSTP BPDUs. The MST Configuration Identifier has the following components:
1. Configuration Identifier Format Selector
2. Configuration Name
3. Configuration Revision Level
4. Configuration Digest: 16-byte signature of type HMAC-MD5 created from the MST
Configuration Table (a VLAN ID to MSTID mapping)
As there are Multiple Instances of Spanning Tree, there is a MSTP state maintained on a
per-port, per-instance basis (or on a per port per VLAN basis: as any VLAN can be in one
and only one MSTI or CIST). For example, port A can be forwarding for instance 1 while
discarding for instance 2. The port states changed since IEEE 802.1D specification.
To support multiple spanning trees, a MSTP bridge has to be configured with an
unambiguous assignment of VLAN IDs (VIDs) to spanning trees. This is achieved by:
1. Ensuring that the allocation of VIDs to FIDs is unambiguous.
2. Ensuring that each FID supported by the Bridge is allocated to exactly one Spanning Tree
Instance.
The combination of VID to FID and then FID to MSTI allocation defines a mapping of VIDs to
spanning tree instances, represented by the MST Configuration Table.
With this allocation we ensure that every VLAN is assigned to one and only one MSTI. The
CIST is also an instance of spanning tree with a MSTID of 0.
An instance might occur that has no VIDs allocated to it, but every VLAN must be allocated to
one of the other instances of spanning tree.