MST differs from the other spanning tree implementation in that it combines
some, but not necessarily all, VLANs into logical spanning tree instances. This
raises the problem of determining what VLAN is to be associated with what
instance. More precisely, tagging BPDUs so that receiving devices can identify
the instances and the VLANs to which they apply.
The issue is irrelevant
in the case of the 802.1Q standard, where all instances are mapped to a unique
and common instance Common Spanning Tree (CST). In the PVST+ implementation,
different VLANs carry the BPDUs for their respective instance (one BPDU per
VLAN) based on the VLAN tagging information.
To provide this logical
assignment of VLANS to spanning trees, each switch running MST in the network
has a single MST configuration that consists of three attributes:
- An alphanumeric configuration name (32 bytes)
- A configuration revision number (two bytes)
- A 4096-element table that associates each of the potential 4096 VLANs
supported on the chassis to a given instance
To be part of a common MST region, a group of switches must share the
same configuration attributes. It is up to the network administrator to
properly propagate the configuration throughout the region.
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NOTE:
If two switches differ on one or more configuration attributes, they
are part of different regions.
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To ensure a 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 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, it
extracts the digest (a numerical value derived from the VLAN-to-instance
mapping table through a mathematical function) and compares it with its own
computed digest. If the digests differ, the mapping must be different, so 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.
In the figure, the port on B1 is at the boundary of region A, whereas the ports
on B2 and B3 are internal to region B.