HP Cisco Nexus 5000 Cisco Nexus 5000 Series Switch CLI Software Configuration - Page 162

Information About MST Chapter 9 Configuring MST Send feedback to [email protected] • Interoperability with Rapid PVST+: Understanding PVST Simulation, page 9-9 MST Overview Note You must enable MST; Rapid PVST+ is the default spanning tree mode. MST maps multiple VLANs into a spanning tree instance, with each instance having a spanning tree topology independent of other spanning tree instances. This architecture provides multiple forwarding paths for data traffic, enables load balancing, and reduces the number of STP instances required to support a large number of VLANs. MST improves the fault tolerance of the network because a failure in one instance (forwarding path) does not affect other instances (forwarding paths). MST provides rapid convergence through explicit handshaking as each MST instance uses the IEEE 802.1w standard, which eliminates the 802.1D forwarding delay and quickly transitions root bridge ports and designated ports to the forwarding state. (See Chapter 8, "Configuring Rapid PVST+" for complete information on the explicit handshake agreement.) MAC address reduction is always enabled while you are using MST. (See Chapter 8, "Configuring Rapid PVST+" for complete information on MAC address reduction.) You cannot disable this feature. MST improves spanning tree operation and maintains backward compatibility with these STP versions: • Original 802.1D spanning tree • Rapid per-VLAN spanning tree (Rapid PVST+) Note • IEEE 802.1w defined the Rapid Spanning Tree Protocol (RSTP) and was incorporated into IEEE 802.1D. • IEEE 802.1s defined MST and was incorporated into IEEE 802.1Q. MST Regions To allow switches to participate in MST instances, you must consistently configure the switches with the same MST configuration information (see "MST Configuration Information" section on page 9-3). A collection of interconnected switches that have the same MST configuration is an MST region. An MST region is a linked group of MST bridges with the same MST configuration. The MST configuration controls the MST region to which each switch belongs. The configuration includes the name of the region, the revision number, and the MST VLAN-to-instance assignment map. A region can have one or multiple members with the same MST configuration. Each member must be capable of processing 802.1w bridge protocol data units (BPDUs). There is no limit to the number of MST regions in a network. Each region can support up to 65 MST instances (MSTIs). Instances are identified by any number in the range from 1 to 4094. The system reserves Instance 0 for a special instance, which is the IST. You can assign a VLAN to only one MST instance at a time. (See "IST, CIST, and CST" section on page 9-4 for more information on the IST.) The MST region appears as a single bridge to adjacent MST regions and to other Rapid PVST+ regions and 802.1D spanning tree protocols. Cisco Nexus 5000 Series Switch CLI Software Configuration Guide 9-2 OL-16597-01