HP 6120XG HP ProCurve Series 6120 Blade Switches Advanced Traffic Management G - Page 114

Multiple Instance Spanning-Tree Operation 802.1s Multiple Spanning Tree Protocol (MSTP) Between regions there is a single, active spanning-tree topology. How Separate Instances Affect MSTP Operation. Assigning different groups of VLANs to different instances ensures that those VLAN groups use independent forwarding paths. For example, in figure 4-3 each instance has a different forwarding path. Path through IST Instance to Other Regions Region "X" Blocks redundant link for MSTI "B". Switch 1 IST Root VLAN Memberships: • IST Instance: VLANs 1, 2 • MSTI "A": 4, 5 • MSTI "B": 7, 9 Blocks redundant link for MSTI "A". Switch 2 MSTI "A" Root VLAN Memberships: • IST Instance: VLANs 1, 2 • MSTI "A": 4, 5 • MSTI "B": 7, 9 Blocks redundant link for IST instance. Switch 3 MSTI "B" Root VLAN Memberships: • IST Instance: VLANs 1, 2 • MSTI "A": 4, 5 • MSTI "B": 7, 9 Figure 4-3. Active Topologies Built by Three Independent MST Instances While allowing only one active path through a given instance, MSTP retains any redundant physical paths in the instance to serve as backups (blocked) paths in case the existing active path fails. Thus, if an active path in an instance fails, MSTP automatically activates (unblocks) an available backup to serve as the new active path through the instance for as long as the original active path is down. Note also that a given port may simultaneously operate in different states (forwarding or blocking) for different spanning-tree instances within the same region. This depends on the VLAN memberships to which the port is assigned. For example, if a port belongs to VLAN 1 in the IST instance of a region and also belongs to VLAN 4 in MSTI "x" in the same region, the port may apply different states to traffic for these two different instances. 4-10