Standard-Compliant Mst Implementation - Cisco 7604 Configuration Manual
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Chapter 19
Configuring Standard-Compliant IEEE MST
the external BPDU is a topology change, it could have an impact on the MST instances. When a message
is internal, the CIST part is received by the CIST, and each MST instance receives its respective
M-record. The Cisco prestandard implementation treats a port that receives an external message as a
boundary port, which means a port cannot receive a mix of internal and external messages.
An MST region includes both routers and LANs. A segment belongs to the region of its designated port.
Therefore, a port in a different region from the designated port for a segment is a boundary port. This
definition allows two ports internal to a region to share a segment with a port belonging to a different
region, creating the possibility of receiving both internal and external messages on a port.
The primary change from the Cisco prestandard implementation is that a designated port is not defined
as boundary unless it is running in an STP-compatible mode.
Note
If there is an 802.1D STP router on the segment, messages are always considered external.
The other change from the prestandard implementation is that the CIST regional root bridge ID field is
now inserted where an RSTP or legacy 802.1s router has the sender switch ID. The whole region
performs like a single virtual router by sending a consistent sender switch ID to neighboring routers. In
this example, router C would receive a BPDU with the same consistent sender switch ID of root, whether
or not A or B is designated for the segment.
Standard-Compliant MST Implementation
The standard-compliant MST implementation includes features required to meet the standard, as well as
some of the desirable prestandard functionality that is not yet incorporated into the published standard.
These sections describe the standard-compliant MST implementation:
•
•
•
Changes in Port-Role Naming
The boundary role was deleted from the final MST standard, but this boundary concept is maintained in
the standard-compliant implementation. However, an MST instance (MSTI) port at a boundary of the
region might not follow the state of the corresponding CIST port. The following two situations currently
exist:
•
•
OL-4266-08
Changes in Port-Role Naming, page 19-7
Spanning Tree Interoperation Between Legacy and Standard-Compliant Routers, page 19-8
Detecting Unidirectional Link Failure, page 19-8
The boundary port is the root port of the CIST regional root—When the CIST instance port is
proposed and is synchronized, it can send back an agreement and move to the forwarding state only
after all the corresponding MSTI ports are synchronized (and thus forwarding). The MSTI ports now
have a special master role.
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 (M-records).
In this situation, although the boundary role no longer exists, when you enter the show commands,
they identify a port as boundary in the type column of the output.
Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
Understanding MST
19-7
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