Cisco IE 4000, Software Configuration Manual

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Configuring MSTP
Information About Configuring MSTP
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 a legacy STP switch on the segment, messages are always considered external.
The other change from the prestandard implementation is that the CIST regional root switch ID field is now inserted where
an RSTP or legacy IEEE 802.1Q switch has the sender switch 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 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.
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 2 on page 6 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 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.