Cisco 3560G 48TS Software Configuration Guide - Page 344

Configuring MSTP Features Chapter 16 Configuring MSTP Specifying the Link Type to Ensure Rapid Transitions If you connect a port to another port through a point-to-point link and the local port becomes a designated port, the RSTP negotiates a rapid transition with the other port by using the proposal-agreement handshake to ensure a loop-free topology as described in the "Rapid Convergence" section on page 16-7. By default, the link type is controlled from the duplex mode of the interface: a full-duplex port is considered to have a point-to-point connection; a half-duplex port is considered to have a shared connection. If you have a half-duplex link physically connected point-to-point to a single port on a remote switch running MSTP, you can override the default setting of the link type and enable rapid transitions to the forwarding state. Beginning in privileged EXEC mode, follow these steps to override the default link-type setting. This procedure is optional. Step 1 Step 1 Command configure terminal interface interface-id Step 2 Step 3 Step 4 Step 5 spanning-tree link-type point-to-point end show spanning-tree mst interface interface-id copy running-config startup-config Purpose Enter global configuration mode. Specify an interface to configure, and enter interface configuration mode. Valid interfaces include physical ports, VLANs, and port-channel logical interfaces. The VLAN ID range is 1 to 4094. The port-channel range is 1 to 12. Specify that the link type of a port is point-to-point. Return to privileged EXEC mode. Verify your entries. (Optional) Save your entries in the configuration file. To return the port to its default setting, use the no spanning-tree link-type interface configuration command. Restarting the Protocol Migration Process A switch running MSTP supports a built-in protocol migration mechanism that enables it to interoperate with legacy 802.1D switches. If this switch receives a legacy 802.1D configuration BPDU (a BPDU with the protocol version set to 0), it sends only 802.1D BPDUs on that port. An MSTP switch also can detect that a port is at the boundary of a region when it receives a legacy BPDU, an MST BPDU (version 3) associated with a different region, or an RST BPDU (version 2). 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. A switch also might continue to assign a boundary role to a port when the switch to which it is connected has joined the region. To restart the protocol migration process (force the renegotiation with neighboring switches) on the switch, use the clear spanning-tree detected-protocols privileged EXEC command. To restart the protocol migration process on a specific interface, use the clear spanning-tree detected-protocols interface interface-id privileged EXEC command. 16-22 Catalyst 3560 Switch Software Configuration Guide 78-16156-01