Allied Telesis SwitchBlade x3100 Series Manual page 403

Multiple Spanning Tree Protocol (MSTP) Introduction
4.5.8.3 Bridge ID
Bridge IDs are used in root bridge elections. The root bridge is the switch in the extended LAN with the
numerically lowest bridge ID value. This is guaranteed to identify a single bridge due to the unique MAC address
component. The user is allowed to change the bridge priority component to override the arbitrary root selec-
tion that will result from only comparing MAC addresses when the default bridge priorities are in use.
Bridge IDs are also used in designated bridge elections. Normally the switch with the lowest root path cost is
the designated bridge for a physical LAN. If more than one switch has the same lowest root path cost, then the
designated bridge is the switch with the numerically lowest bridge ID value.
The default bridge priority value is 32768. A bridge priority can be configured as a value from zero to 65535, in
accordance with IEEE Std 802.1D, 1998 Edition. For MSTP, however, the priority component of the bridge ID is
reduced to support MSTP operations, to allow for the unique identification of each MSTI in a bridge as part of a
"system ID" that represents a (12-bit) numerical extension to the MAC address. This avoids the potential need
to allocate up to 4094 additional MAC addresses per bridge to uniquely identify each MSTI. The reallocation of
(bits in) the bridge ID contents was done in a manner that supports backwards compatibility with IEEE Std.
802.1D, 1998 Edition.
As a result, the bridge priority component has been modified to be a (4-bit) value between 0-65535 that can
only be provisioned in increments of 4096. This was done to allow for direct comparison with values from ear-
lier versions of STP.
For Bridges that are running MSTP, there will be MSTI definitions to support the different VLANS defined for
the bridge. Each of the MSTIs will have its own Bridge Identifier with the composition described above, except
that each will include the Bridge MAC address as a component of the Bridge ID. Each will have a priority com-
ponent, as described above, which can be independently provisioned from the other spanning tree instances
defined for the same bridge.
The final component is an identifier called the "system ID extension" that is used to uniquely identify each of the
MSTIs for a bridge. The CIST for each bridge will use the system ID extension value of zero. Any other MSTI
defined for the bridge will utilize a value called the MSTID that identifies the MSTI. The MSTID parameter is
described in a later section.
4.5.8.4 Port ID
Port IDs are used in root port elections. Normally, the port with the lowest root path cost is the root port for
the switch. If more than one port ties for the lowest root path cost, then the root port is the port with the low-
est numerical port ID.
The default port priority value is 128. The IEEE Std 802.1D, 1998 Edition includes priority values on a per-port
basis from zero to 255. For the Allied Telesis SBx3112, the storage space (number of bits) allocated to the pri-
ority component of the port ID is reduced to support bridges with larger numbers of ports, since this only left
room for port numbers from 1-255. To maintain compatibility for comparison with previous versions of STP, the
port priority is a value between 0-240 that can only be provisioned in increments of 16.
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Software Reference for SwitchBlade x3100 Series Switches (Layer Two Switching)