Understanding Rstp - Cisco Catalyst 3550 series Software Configuration Manual

Understanding RSTP

Understanding RSTP
The RSTP takes advantage of point-to-point wiring and provides rapid convergence of the spanning tree.
Reconfiguration of the spanning tree can occur in less than 1 second (in contrast to 50 seconds with the
default settings in the 802.1D spanning tree), which is critical for networks carrying delay-sensitive
traffic such as voice and video.
These section describes how the RSTP works:
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For configuration information, see the
Port Roles and the Active Topology
The RSTP provides rapid convergence of the spanning tree by assigning port roles and by determining
the active topology. The RSTP builds upon the IEEE 802.1D STP to select the switch with the highest
switch priority (lowest numerical priority value) as the root switch as described in the
Topology and BPDUs" section on page
ports:
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A port with the root or a designated port role is included in the active topology. A port with the alternate or
backup port role is excluded from the active topology.
In a stable topology with consistent port roles throughout the network, the RSTP ensures that every root
port and designated port immediately transition to the forwarding state while all alternate and backup
ports are always in the discarding state (equivalent to blocking in 802.1D). The port state controls the
operation of the forwarding and learning processes.
RSTP port states.
Table 17-1 Port State Comparison
Operational Status
Enabled
Enabled
Catalyst 3550 Multilayer Switch Software Configuration Guide
17-6
Port Roles and the Active Topology, page 17-6
Rapid Convergence, page 17-7
Synchronization of Port Roles, page 17-8
Bridge Protocol Data Unit Format and Processing, page 17-9
Root port—Provides the best path (lowest cost) when the switch forwards packets to the root switch.
Designated port—Connects to the designated switch, which incurs the lowest path cost when
forwarding packets from that LAN to the root switch. The port through which the designated switch
is attached to the LAN is called the designated port.
Alternate port—Offers an alternate path toward the root switch to that provided by the current root
port.
Backup port—Acts as a backup for the path provided by a designated port toward the leaves of the
spanning tree. A backup port can exist only when two ports are connected together in a loopback by
a point-to-point link or when a switch has two or more connections to a shared LAN segment.
Disabled port—Has no role within the operation of the spanning tree.
STP Port State
(802.1D)
Blocking
Listening
"Configuring MSTP Features" section on page
16-2. Then the RSTP assigns one of these port roles to individual
Table 17-1 provides a comparison of 802.1D and
RSTP Port State
Discarding
Discarding
Chapter 17 Configuring MSTP
17-11.
"Spanning-Tree
Is Port Included in the
Active Topology?
No
No
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