R S T P C O N F I G U R A T I O N V I A T E L N E T
Rev H
160 of 244
Bridge
Designated Port Designated Port
Root Port
Alternate Port
Disabled Port
Backup port
Bridge
Designated Port
Figure 17-1 RSTP Port Roles
The RSTP port roles are determined automatically by the following parameters:
•
Unique Bridge Identifier associated with each bridge
•
Path Cost associated with each bridge port
•
Unique Port Identifier associated with 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 manageable
Path Cost
is associated with all the ports of every bridge. 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 Root Path Cost
associated with the Root Bridge is zero.
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, 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
Port Roles
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