NETGEAR GC728X User Manual page 416

Insight Managed 28-Port Gigabit Ethernet Smart Cloud Switch with 2 SFP 1G & 2 SFP+ 10G Fiber Ports
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. An MSTP bridge
transmits 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 includes 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)
Because multiple instances of spanning tree exist, an MSTP state is 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, configure an MSTP bridge with an unambiguous
assignment of VLAN IDs (VIDs) to spanning trees. For such a configuration, ensure the
following:
1. The allocation of VIDs to FIDs is unambiguous.
2. Each FID that is 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 an MSTID of 0.
VIDs might be not be allocated to an instance, but every VLAN must be allocated to one of
the other instances of spanning tree.
The portion of the active topology of the network that connects any two bridges in the same
MST region traverses only MST bridges and LANs in that region, and never bridges of any
Configuration Examples
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