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4 multiple stp, Figure 44 stp/rstp network example – ZyXEL Communications GS-2750 User Manual

Page 107

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Chapter 11 Spanning Tree Protocol

GS-2750 User’s Guide

107

11.1.4 Multiple STP

Multiple Spanning Tree Protocol (IEEE 802.1s) is backwards compatible with STP/RSTP and
addresses the limitations of existing spanning tree protocols (STP and RSTP) in networks to
include the following features:

• One Common and Internal Spanning Tree (CIST) that represents the entire network’s

connectivity.

• Grouping of multiple bridges (or switching devices) into regions that appear as one single

bridge on the network.

• A VLAN can be mapped to a specific Multiple Spanning Tree Instance (MSTI). MSTI

allows multiple VLANs to use the same spanning tree.

• Load-balancing is possible as traffic from different VLANs can use distinct paths in a

region.

11.1.4.1 MSTP Network Example

The following figure shows a network example where two VLANs are configured on the two
switches. If the switches are using STP or RSTP, the link for VLAN 2 will be blocked as STP
and RSTP allow only one link in the network and block the redundant link.

Figure 44 STP/RSTP Network Example

Listening

All BPDUs are received and processed.

Note: The listening state does not exist in RSTP.

Learning

All BPDUs are received and processed. Information frames are submitted to the

learning process but not forwarded.

Forwarding

All BPDUs are received and processed. All information frames are received and

forwarded.

Table 25 STP Port States

PORT STATE DESCRIPTION

A

B

VLAN 1

VLAN 2