RSTP, an enhancement to STP, implements fast convergence of the network topology. There is a defect for both RSTP and STP: All VLANs on a LAN use one spanning tree, and thus VLAN-based load balancing cannot be performed. Once a link is blocked, it will no longer transmit traffic, wasting bandwidth and causing the failure in forwarding certain VLAN packets.
Figure 1 STP/RSTP defect
On the network shown in Figure 1, STP or RSTP is enabled. The broken line shows the spanning tree. S6 is the root switching device. The links between S1 and S4 and between S2 and S5 are blocked. VLAN packets are transmitted by using the corresponding links marked with "VLAN2" or "VLAN3."
Host A and Host B belong to VLAN 2 but they cannot communicate with each other because the link between S2 and S5 is blocked and the link between S3 and S6 denies packets from VLAN 2.
To fix the defect of STP and RSTP, the IEEE released 802.1s in 2002, defining the Multiple Spanning Tree Protocol (MSTP). MSTP implements fast convergence and provides multiple paths to load balance VLAN traffic.
MSTP divides a switching network into multiple regions, each of which has multiple spanning trees that are independent of each other. Each spanning tree is called a Multiple Spanning Tree Instance (MSTI) and each region is called a Multiple Spanning Tree (MST) region.
An instance is a collection of VLANs. Binding multiple VLANs to an instance saves communication costs and reduces resource usage. The topology of each MSTI is calculated independent of one another, and traffic can be balanced among MSTIs. Multiple VLANs that have the same topology can be mapped to one instance. The forwarding status of the VLANs for a port is determined by the port status in the MSTI.
Figure 2 Multiple spanning trees in an MST region
As shown in Figure 2, MSTP maps VLANs to MSTIs in the VLAN mapping table. Each VLAN can be mapped to only one MSTI. This means that traffic of a VLAN can be transmitted in only one MSTI. An MSTI, however, can correspond to multiple VLANs.
Two spanning trees are calculated:
MSTI 1 uses S4 as the root switching device to forward packets of VLAN 2.
MSTI 2 uses S6 as the root switching device to forward packets of VLAN 3.
In this manner, devices within the same VLAN can communicate with each other; packets of different VLANs are load balanced along different paths.
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