Rapid Spanning Tree Protocol; Multiple Spanning Tree Protocol - Nortel 4500 Series Overview-System Configuration

134 About the Nortel Ethernet Routing Switch 4500 Series
The Spanning Tree Groups (STG) forms a loop-free topology that includes
one or more VLANs. The Nortel Ethernet Routing Switch 4500 Series
supports a maximum of eight STGs running simultaneously.
The Nortel Ethernet Routing Switch 4500 Series supports a maximum of
256 VLANs. Therefore each STG may have 32 VLANs.

Rapid Spanning Tree Protocol

The standard spanning tree implementation in 4500 Series switches is
based on IEEE 802.1d, which is slow to respond to a topology change in the
network (for example, a dysfunctional link in a network).
The Rapid Spanning Tree Protocol (RSTP or IEEE 802.1w) reduces
the recovery time after a network breakdown. RSTP also maintains a
backward compatibility with the IEEE 802.1d. In certain configurations, the
recovery time of RSTP is less than 1 second. You can maintain backward
compatibility by configuring a port to be in STP-compatible mode. A port
that operates in the STP-compatible mode transmits and receives only STP
BPDUs and drops any RSTP BPDUs.
RSTP also reduces the amount of flooding in the network by enhancing the
way Topology Change Notification (TCN) packet is generated.

Multiple Spanning Tree Protocol

With Multiple Spanning Tree Protocol (MSTP or IEEE 802.1s), you can
configure multiple instances of RSTP on the same switch. Each RSTP
instance can include one or more VLANs. The operation of the MSTP is
similar to the current Nortel proprietary STP.
The 4500 Series switch can use RSTP and MSTP to achieve the following:
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Copyright © 2007, Nortel Networks
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Reduce converging time from 30 seconds to less than 1 second when
topology changes in the network (that is, the port goes up or down).
Eliminate unnecessary flushing of the MAC database and flooding of
traffic to the network with a new Topology Change mechanism.
Provide backward compatibility with other switches that run legacy
802.1d STP or Nortel MSTG (STP group 1 only).
Provide simultaneous support for eight instances of RSTP under MSTP
mode. Instance 0 or CIST is the default group, which includes default
VLAN 1. Instances 1 to 7 are called MSTIs 1-7.
You can configure the switch to run Nortel STPG (IEEE 802.1d), RSTP
(IEEE 802.1w), or MSTP (IEEE 802.1s).
Nortel Ethernet Routing Switch 4500 Series
Overview — System Configuration
NN47205-500 02.01 Standard
5.0 23 February 2007
Nortel Networks Confidential