Cisco Systems 15310-CL, 15310-MA RSTP Features

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Cisco ONS 15310-CL and Cisco ONS 15310-MA Ethernet Card Software Feature and Configuration Guide R8.5

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Chapter 6 Configuring STP and RSTP on the ML-Series Card

RSTP Features

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 Features

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 seconds 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 6-9

• Port Roles and the Active Topology, page 6-10

• Rapid Convergence, page 6-11

• Synchronization of Port Roles, page 6-12

• Bridge Protocol Data Unit Format and Processing, page 6-13

• Topology Changes, page 6-14

The ML Series supports per-VLAN rapid spanning tree (PVRST) and a maximum of 255 rapid

spanning-tree instances.

Caution At more than 100 RSTP instances the RSTP instances may flap and may result in MAC entries flushed,

and MAC entries learned again and again. This will cause flooding in the network.. So it is recommended

to keep the RSTP instances to be less than 100, to keep system fr om being unstable.