13.
Rapid Spanning Tree Protocol (RSTP)
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The RSTP port roles are determined automatically by the following parameters:
•
a unique Bridge Identifier associated to each bridge;
•
a Path Cost associated to each bridge port;
•
a unique Port Identifier associated to each bridge port.
Selection of the Root Bridge and Root Port
RSTP automatically selects the bridge that has the best
Bridge Identifier
as the
Root Bridge
.
Each bridge has a unique Bridge Identifier that is derived from the Bridge Address and from a
manageable priority component (described in
IEEE Std 802.1w-2001, Part 3: Media Access
Control (MAC) Bridges, Amendment 2: Rapid Reconfiguration, Section 9.2.5:
Encoding
of Bridge Identifiers
)
.
The unique Bridge Identifiers are compared numerically, assigning the
highest priority to the lowest identifier value (the best Bridge Identifier).
A Root
Path Cost
is associated with every Bridge, by summing up the path costs for each
Bridge Port receiving frames on the least cost path from the Root Bridge to that Bridge. The
path cost associated with the Root Bridge this is zero. The Path Cost associated with all other
ports may be manageable.
For each bridge except for the Root Bridge, RSTP automatically assigns the role of
Root Port
to the Bridge Port that receives frames on the least cost path from the Root Bridge. If two or
more ports on a bridge have the same least Path Cost sum from the Root, then RSTP selects
the port that has the best
Port Identifier
as the Root Port.
The
Port Identifier
comprises two parts. One part is fixed and unique for each Port on a
Bridge. The other part is a manageable priority component (as described in
IEEE Std
802.1w-2001, Part 3: Media Access Control (MAC) Bridges, Amendment 2: Rapid
Reconfiguration, Section
9.2.7
:
Encoding of Port Identifiers
). The unique Port Identifiers
are compared numerically, assigning the highest priority to the lowest identifier value (the
best Port Identifier).
Selection of the Designated Bridge and Designated
Port
RSTP associates a Root
Path Cost
to every LAN in the Bridged Local Area Network. This is
the Root Path Cost of the lowest cost Bridge with a Bridge Port connected to that LAN. RSTP
selects this Bridge as the
Designated Bridge
for that LAN. If two or more Bridges have the
same Root Path Cost, then the Bridge with the best priority (least numerical value) is selected
as the Designated Bridge. The Bridge Port on the Designated Bridge that is connected to the
LAN is assigned the role of
Designated Port
for that LAN. If the Designated Bridge has two
or more ports connected to the LAN, then the Bridge Port with the best priority Port Identifier
(least numerical value) is selected as the Designated Port.
In a Bridged LAN with a stable physical topology (i.e., the information communicated by the
RST Algorithm is consistent throughout the network), each LAN has one single Designated
Port, and each Bridge except for the Root Bridge has a Root Port connected to a LAN. Any
operational Bridge Port that is not assigned a role of Root Port or Designated Port is either of
the following:
