Chapter 11 Spanning Tree Protocol

11.1.5 Multiple STP

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.

11.1.5.1MSTP 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 47 STP/RSTP Network Example

A

VLAN 1

 

VLAN 2

 

 

 

B

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.

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GS-3012/GS-3012F User’s Guide