Chapter 5: Multiple Spanning Tree Protocol 79
MSTP functional description
Overview The MSTP algorithm and protocol provides simple and full connectivity for
frames assigned to any given VLAN throughout a bridged LAN that comprises
arbitrarily interconnected networking devices, each operating MSTP, STP, or
RSTP. MSTP allows frames assigned to different VLANs to follow separate
paths, each based on an independent multiple spanning tree instance (MSTI),
within MST regions composed of LANs and MSTP bridges. These regions and
the other bridges and LANs are connected into a single common spanning tree
(CST) (see IEEE DRAFT P802.1s/D13).
The common and
internal spanning
tree
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:
ā frames with a given VID are assigned to one and only one of the MSTIs or
the IST within the region;
ā that the assignment is consistent among all the networking devices in the
region;
ā and that the stable connectivity of each MSTI and IST at the boundary of the
region matches that of the CST.
Thus, the stable active topology of the bridged LAN with respect to frames
consistently classified as belonging to any given VLAN 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.
BPDUs All bridges, whether they use STP, RSTP, or MSTP, use BDPUs to send
configuration messages. These messages assign port roles that determine each
portās 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
unique. An MSTP bridge transmits the appropriate BPDU depending on the
received type of BPDU from a particular port.
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