Cisco 3560G 48TS Software Configuration Guide - Page 328

Understanding RSTP Chapter 16 Configuring MSTP However, the switch does not automatically revert to the MSTP mode if it no longer receives 802.1D BPDUs because it cannot detect whether the legacy switch has been removed from the link unless the legacy switch is the designated switch. Also, a switch might continue to assign a boundary role to a port when the switch to which this switch is connected has joined the region. To restart the protocol migration process (force the renegotiation with neighboring switches), use the clear spanning-tree detected-protocols privileged EXEC command. If all the legacy switches on the link are RSTP switches, they can process MSTP BPDUs as if they are RSTP BPDUs. Therefore, MSTP switches send either a version 0 configuration and TCN BPDUs or version 3 MSTP BPDUs on a boundary port. A boundary port connects to a LAN, the designated switch of which is either a single spanning-tree switch or a switch with a different MST configuration. Understanding RSTP The RSTP takes advantage of point-to-point wiring and provides rapid convergence of the spanning tree. Reconfiguration of the spanning tree can occur in less than 1 second (in contrast to 50 seconds with the default settings in the 802.1D spanning tree), which is critical for networks carrying delay-sensitive traffic such as voice and video. These section describes how the RSTP works: • Port Roles and the Active Topology, page 16-6 • Rapid Convergence, page 16-7 • Synchronization of Port Roles, page 16-8 • Bridge Protocol Data Unit Format and Processing, page 16-9 For configuration information, see the "Configuring MSTP Features" section on page 16-11. Port Roles and the Active Topology The RSTP provides rapid convergence of the spanning tree by assigning port roles and by learning the active topology. The RSTP builds upon the IEEE 802.1D STP to select the switch with the highest switch priority (lowest numerical priority value) as the root switch as described in the "Spanning-Tree Topology and BPDUs" section on page 15-3. Then the RSTP assigns one of these port roles to individual ports: • Root port-Provides the best path (lowest cost) when the switch forwards packets to the root switch. • Designated port-Connects to the designated switch, which incurs the lowest path cost when forwarding packets from that LAN to the root switch. The port through which the designated switch is attached to the LAN is called the designated port. • Alternate port-Offers an alternate path toward the root switch to that provided by the current root port. • Backup port-Acts as a backup for the path provided by a designated port toward the leaves of the spanning tree. A backup port can exist only when two ports are connected together in a loopback by a point-to-point link or when a switch has two or more connections to a shared LAN segment. • Disabled port-Has no role within the operation of the spanning tree. A port with the root or a designated port role is included in the active topology. A port with the alternate or backup port role is excluded from the active topology. 16-6 Catalyst 3560 Switch Software Configuration Guide 78-16156-01