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Chapter 17
Configuring MSTP
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Interoperation Between Legacy and Standard 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 by using the CIST. Only the capability of load balancing over different
instances is lost in that particular case. The CLI displays different flags depending on the port
configuration when a 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 17-2
configured to be in the same region. A is the root switch for the CIST, and thus 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 thus 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 17-2
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 this Cisco IOS release.
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 discarding state because
disrupting connectivity in case of inconsistency is preferable to opening a bridging loop.
OL-23389-01
The boundary port is not the root port of the CIST regional root—The MSTI ports follow the state
and role of the CIST port. The standard provides less information, and it might be difficult to
understand why an MSTI port can be alternately blocking when it receives no BPDUs (MRecords).
In this case, although the boundary role no longer exists, the show commands identify a port as
boundary in the type column of the output.
illustrates this scenario. Assume that A is a standard switch and B a prestandard switch, both
Standard and Prestandard Switch Interoperation
Segment X
Switch A
Segment Y
MST
Region
Switch B
Catalyst 2928 Switch Software Configuration Guide
Understanding MSTP
17-7
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Troubleshooting
