Cisco 7609-S Software Configuration Guide - Page 254

Understanding MST Chapter 19 Configuring Standard-Compliant IEEE MST Spanning Tree Interoperation Between Legacy and Standard-Compliant Routers Because automatic detection of prestandard routers can fail, you can use an interface configuration command to identify prestandard ports. A region cannot be formed between a standard and a prestandard router, but they can interoperate before using the CIST. Only the capability of load balancing over different instances is lost in this specific situation. The CLI displays different flags depending on the port configuration when the port receives prestandard BPDUs. A syslog message also appears the first time a router receives a prestandard BPDU on a port that has not been configured for prestandard BPDU transmission. Figure 19-2 illustrates a standard-compliant router connected to a prestandard router. Assume that A is the standard-compliant router and B is a prestandard router, both configured to be in the same region. A is the root bridge for the CIST, and so B has a root port (BX) on segment X and an alternate port (BY) on segment Y. If segment Y flaps, and the port on BY becomes the alternate before sending out a single prestandard BPDU, AY cannot detect that a prestandard router is connected to Y and continues to send standard BPDUs. The port BY is fixed in a boundary, and no load balancing is possible between A and B. The same problem exists on segment X, but B might transmit topology changes. Figure 19-2 Standard-Compliant and Prestandard Router Interoperation Segment X MST Region Switch A Switch B 92721 Segment Y Note We recommend that you minimize the interaction between standard and prestandard MST implementations. Detecting Unidirectional Link Failure This feature is not yet present in the IEEE MST standard, but it is included in the standard-compliant implemtation. The software checks the consistency of the port role and state in the received BPDUs to detect unidirectional link failures that could cause bridging loops. When a designated port detects a conflict, it keeps its role, but reverts to a discarding state because disrupting connectivity in case of inconsistency is preferable to opening a bridging loop. Figure 19-3 illustrates a unidirectional link failure that typically creates a bridging loop. Router A is the root bridge, and its BPDUs are lost on the link leading to router B. RSTP and MST BPDUs include the role and state of the sending port. With this information, router A can detect that router B does not react to the superior BPDUs it sends and that router B is the designated, not root bridge. As a result, router A blocks (or keeps blocking) its port, thus preventing the bridging loop. 19-8 Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX OL-4266-08