BCMSN Exam Cram™ 2 (Exam Cram 642-811)

Multiple Spanning Tree

Multiple Spanning Tree (MST) is an enhancement to IEEE's RSTP. MST is similar to Cisco's PVST. You'll recall from the last chapter that PVST has a separate instance of STP for each VLAN, and is supported on trunk connections to Cisco devices. For each STP instance, there is a separate set of BPDUs, root switches, and STP configuration information. One problem of PVST is that it adds a lot of overhead to your switching equipment. An example of PVST is shown in the top-left corner of Figure 5.3. In this example, there are 100 VLANs. Half of the VLANs (1–50) are forwarding on the left trunk from switch 1 to switch 2 and the other half (51–100) are forwarding on the right trunk from switch 1 to switch 3. With PVST, you have ...