ZyXEL ES-3148 Guida Utente

Chapter 11 Spanning Tree Protocol

ES-3148 User’s Guide

Multiple Spanning Tree Protocol (IEEE 802.1s) is backward 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. 

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 46   STP/RSTP Network Example 

With MSTP, VLANs 1 and 2 are mapped to different spanning trees in the network. Thus 
traffic from the two VLANs travel on different paths. The following figure shows the network 
example using MSTP.