Cisco 7604 Software Configuration Guide - Page 291

Chapter 20 Configuring STP and Prestandard IEEE 802.1s MST Understanding How Prestandard IEEE 802.1s MST Works OL-4266-08 Figure 20-8 Network with Interconnected SST and MST Regions MST Region r B F B B B F r SST Region b r r F F/f = Forwarding B/b = Blocking R = Root Bridge r = Root port F F F FR F F MST Region FF F r F r r b SST Region B 68285 An MST region appears as an SST or pseudobridge to STP running in the SST region. Pseudobridges operate as follows: • The same values for root identifiers and root path costs are sent in all BPDUs of all the pseudobridge ports. Pseudobridges differ from a single SST bridge as follows: - The pseudobridge BPDUs have different bridge identifiers. This difference does not affect STP operation in the neighboring SST regions because the root identifier and root cost are the same. - BPDUs sent from the pseudobridge ports may have significantly different message ages. Because the message age increases by 1 second for each hop, the difference in the message age is in the order of seconds. • Data traffic from one port of a pseudobridge (a port at the edge of a region) to another port follows a path entirely contained within the pseudobridge or MST region. • Data traffic belonging to different VLANs may follow different paths within the MST regions established by MST. • Loop prevention is achieved by either of the following: - Blocking the appropriate pseudobridge ports by allowing one forwarding port on the boundary and blocking all other ports. - Setting the CST partitions to block the ports of the SST regions. • A pseudobridge differs from a single SST bridge because the BPDUs sent from the pseudobridge's ports have different bridge identifiers. The root identifier and root cost are the same for both bridges. These guidelines apply in a topology where you configure MST switches (all in the same region) to interact with PVST+ switches: • Configure the root for all VLANs inside the MST region as shown in this example: Router# show spanning-tree mst interface gigabitethernet 1/1 GigabitEthernet1/1 of MST00 is root forwarding Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX 20-17