Cisco 7609-S Software Configuration Guide - Page 255

Chapter 19 Configuring Standard-Compliant IEEE MST Understanding RSTP Figure 19-3 Detecting Unidirectional Link Failure Switch A Superior BPDU Switch B 92722 Inferior BPDU, Designated + Learning bit set Interoperability with IEEE 802.1D-1998 STP A router running MST supports a built-in protocol migration feature that enables it to interoperate with 802.1D routers. If this router receives an 802.1D configuration BPDU (a BPDU with the protocol version set to 0), it sends only 802.1D BPDUs on that port. An MST router also can detect that a port is at the boundary of a region when it receives an 802.1D BPDU, an MST BPDU (Version 3) associated with a different region, or an RSTP BPDU (Version 2). However, the router does not automatically revert to the MST mode if it no longer receives 802.1D BPDUs because it cannot detect whether the 802.1D router has been removed from the link unless the 802.1D router is the designated router. A router might also continue to assign a boundary role to a port when the router to which this router is connected has joined the region. To restart the protocol migration process (force the renegotiation with neighboring routers), use the clear spanning-tree detected-protocols privileged EXEC command. If all the 802.1D routers on the link are RSTP routers, they can process MST BPDUs as if they are RSTP BPDUs. Therefore, MST routers send either a Version 0 configuration and topology change notification (TCN) BPDUs or Version 3 MST BPDUs on a boundary port. A boundary port connects to a LAN, the designated router of which is either a single spanning tree router or a router with a different MST configuration. Understanding RSTP 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). These section describes how the RSTP works: • Port Roles and the Active Topology, page 19-10 • Rapid Convergence, page 19-11 • Synchronization of Port Roles, page 19-12 • Bridge Protocol Data Unit Format and Processing, page 19-13 OL-4266-08 Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX 19-9