Cisco WS-C2980G-A Software Guide - Page 112

Understanding How MST Works Chapter 7 Configuring Spanning Tree • Do not connect switches with access links because access links may partition a VLAN. • Any MST configuration involving a large number of either existing or new logical VLAN ports should be carried out during the maintenance window. This action should be taken because the complete MST database gets re-initialized for any incremental changes (such as adding new VLANs to instances or moving VLANs across instances). Rapid Spanning Tree Protocol RSTP significantly reduces the time it takes you to reconfigure the active topology of the network when changes to the physical topology or its configurations parameters occur. RSTP selects one switch as the root of a spanning-tree-connected active topology and assigns port roles to individual ports of the switch, depending on whether that port is part of the active topology. RSTP provides rapid connectivity following the failure of a switch, switch port, or a LAN. A new root port and the designated port on the other side of the bridge transition to forwarding through an explicit handshake between them. RSTP allows switch port configuration so the ports can transition to forwarding directly when the switch reinitializes. RSTP, specified in 802.1w, supersedes STP specified in 802.1D while retaining compatibility with STP. RSTP provides the structure on which the MST operates. You configure RSTP when you configure the MST feature. For more information, see the "Configuring MST" section on page 7-46. RSTP provides backward compatibility with 802.1D bridges, as follows: • RSTP selectively sends 802.1D-configured BPDUs and Topology Change Notification (TCN) BPDUs on a per-port basis. • When a port initializes, the Migration Delay timer starts and RSTP BPDUs are transmitted. While the Migration Delay timer is active, the bridge processes all BPDUs that are received on that port. RSTP BPDUs are not visible on the port. Only version 3 BPDUs are visible on the port. • If the bridge receives an 802.1D BPDU after a port's Migration Delay timer expires, the bridge assumes it is connected to an 802.1D bridge and starts using only 802.1D BPDUs. • When RSTP uses 802.1D BPDUs on a port and receives an RSTP BPDU after the migration delay expires, RSTP restarts the Migration Delay timer and begins using RSTP BPDUs on that port. RSTP Port Roles RSTP uses the following definitions for port roles: • Root-A forwarding port elected for the spanning tree topology. • Designated-A forwarding port elected for every switched LAN segment. • Alternate-An alternate path to the root bridge to that provided by the current root port. • Backup-A backup for the path that is provided by a designated port toward the leaves of the spanning tree. Backup ports can exist only where two ports are connected together in a loopback by a point-to-point link or bridge with two or more connections to a shared LAN segment. • Disabled-A port that has no role within the operation of spanning tree. Port roles are assigned as follows: • A root port or designated port role includes the port in the active topology. • An alternate port or backup port role excludes the port from the active topology. 7-16 Catalyst 4500 Series, Catalyst 2948G, Catalyst 2980G Switches Software Configuration Guide-Release 8.1 78-15486-01