Cisco Systems 15327 manual RSTP

7-9
Ethernet Card Software Feature and Configuration Guide, R7.2
January 2009
Chapter 7 Configuring STP and RSTP
Accelerated Aging to Retain Connectivity
Accelerated Aging to Retain Connectivity
The default for aging dynamic addresses is 5 minutes, which is the default setting of the bridge
bridge-group-number aging-time global configuration command. However, a spanning-tree
reconfiguration can cause many station locations to change. Beca use these stations could be unreachable
for 5 minutes or more during a reconfiguration, the address-aging time is accelerat ed so that station
addresses can be dropped from the address table and then relea rned.
Because each VLAN is a separate spanning-tree instance, the switch accelerates aging on a per-VLAN
basis. A spanning-tree reconfiguration on one VLAN can cause the dynamic addresses learned on that
VLAN to be subject to accelerated aging. Dynamic add resses on other VLANs can be unaffected and
remain subject to the aging interval entered for the switch.
RSTP
RSTP provides rapid convergence of the spanning tree. It improves the fault tolerance o f the network
because a failure in one instance (forwarding path) does not affect other instances (forwarding paths).
The most common initial deployment of RSTP is in the backbone and distribution layers of a Layer 2
switched network; this deployment provides the highly available network required in a service -provider
environment.
RSTP improves the operation of the spanning tree while maintaining backward compatibility with
equipment that is based on the (original) IEEE 802.1D spanning tree.
RSTP takes advantage of point-to-point wiring and provides rapid convergence of the spanni ng tree.
Reconfiguration of the spanning tree can occur in less than 2 second (in contrast to 50 secon ds with the
default settings in the IEEE 802.1D spanning tree), which is critical for networks carrying
delay-sensitive traffic such as voice and video.
These sections describe how RSTP works:
• Supported RSTP Instances, page 7-9
• Port Roles and the Active Topology, page 7-9
• Rapid Convergence, page 7-10
• Synchronization of Port Roles, page 7-12
• Bridge Protocol Data Unit Format and Processing, page 7-13
• Topology Changes, page 7-14
Supported RSTP Instances
The ML Series supports per-VLAN rapid spanning tree (PVRST) and a maximum of 255 rapid
spanning-tree instances.
Port Roles and the Active Topology
The RSTP provides rapid convergence of the spanning tree by assigning port roles and by de termining
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 “Election of the Root
Switch” section on page 7-3. Then the RSTP assigns one of these port roles to individual ports: