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Cisco IE 2000 Switch Software Configuration Guide
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Chapter 21 Configuring MSTP
Information About Configuring MSTP
An MST region includes both switches and LANs. A segment belongs to the region of its designated
port. Therefore, a port in a different region than the designated port for a segment is a boundary port.
This definition allows two ports internal to a region to share a segment wi th 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 designat ed port is not defined
as boundary, unless it is running in an STP-compatible mode.
Note If there is a legacy STP switch on the segment, messages are always considered external.
The other change from the prestandard implementati on is that the CIST regional root switch ID field is
now inserted where an RSTP or legacy IEEE 802.1Q switch has the sender sw itch ID. The whole region
performs like a single virtual switch by sending a consistent sender switch ID to neighboring switches.
In this example, switch 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.
IEEE 802.1s Implementation
The Cisco implementation of the IEEE MST standard 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.

Port Role Naming Change

The boundary role is no longer in the final MST standard, but this boundary concept is maintained in
Cisco’s implementation. However, an MST instance port at a boundary of the region might not follow
the state of the corresponding CIST port. Two cases exist now:
• The boundary port is the root port of the CIST regional root —When the CIST instance port is
proposed and is in sync, it can send back an agreement and move to the forwarding state only after
all the corresponding MSTI ports are in sync (and forwarding) . The MSTI ports now have a special
master role.
• The boundary port is not the root port of the CIST regional roo t—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.

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 b etween 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 21-2 illustrates this scenario. Assume that A is a standard switch and B a prestandard switch, both
configured to be in the same region. A is the root switch for the CIST, and 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 be comes