JUNOS Enterprise Switching

Multiple Spanning Tree Protocol

RSTP solves many of STP’s limitations, but it does have one major drawback, in that it is not “VLAN-aware.” In other words, only one spanning tree is created for the entire LAN, and there is only a single path for all VLANs to use. This causes underutilization of links that are in the blocking state and results in no load balancing. MSTP (or 802.1s) provides for a new spanning tree instance per VLAN or per group of VLANs, allowing more links in the LAN to be utilized. MSTP was originally defined in IEEE 802.1s and later merged into IEEE 802.1Q-2003. That specification describes the operation of MSTP as follows:

MSTP allows frames assigned to different VLANs to follow separate paths, each based on an independent Multiple Spanning Tree Instance (MSTI), within Multiple Spanning Tree (MST) Regions composed of LANs and or MST Bridges. These Regions and the other Bridges and LANs are connected into a single Common Spanning Tree (CST).

Figure 6-30 gives an example of the goal of MSTP; here, it allows certain VLANs to utilize the link from A to D1, while others utilize the link from A to D2. This would be a common topology if Switch A were an access switch.

Figure 6-30. VLAN issue

Note

PVST+ was created to solve the multiple-STP issue on Cisco devices. However, PVST+ creates an instance per VLAN, which can have massive scaling issues as the number of VLANs grows. ...