Cisco WS-SUP32-GE-3B - Supervisor Engine 32 Software Configuration Manual page 248

Understanding MST
Spanning Tree Interoperation Between Legacy and Standard-Compliant Switches
Because automatic detection of prestandard switches can fail, you can use an interface configuration
command to identify prestandard ports. A region cannot be formed between a standard and a prestandard
switch, 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 switch receives a prestandard BPDU on a port that has not been configured for prestandard BPDU
transmission.
Figure 17-11
is the standard-compliant switch and B is a prestandard switch, 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 switch 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 17-11
We recommend that you minimize the interaction between standard and prestandard MST
Note
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 17-12
the root bridge, and its BPDUs are lost on the link leading to switch B. RSTP and MST BPDUs include
the role and state of the sending port. With this information, switch A can detect that switch B does not
react to the superior BPDUs it sends and that switch B is the designated, not root bridge. As a result,
switch A blocks (or keeps blocking) its port, thus preventing the bridging loop.
Catalyst Supervisor Engine 32 PISA Cisco IOS Software Configuration Guide, Release 12.2ZY
17-24
illustrates a standard-compliant switch connected to a prestandard switch. Assume that A
Standard-Compliant and Prestandard Switch Interoperation
Segment X
Switch A
Segment Y
illustrates a unidirectional link failure that typically creates a bridging loop. Switch A is
Chapter 17
MST
Region
Switch B
Configuring STP and MST
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