Specifications

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Cisco MWR 2941 Mobile Wireless Edge Router Release 3.5 Software Configuration Guide, Cisco IOS Release 15.1(3)MR
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Chapter 10 Configuring MSTP
Understanding MSTP
Boundary Ports
In the Cisco prestandard implementation, a boundary port connects an MST region to a single
spanning-tree region running RSTP, to a single spanning-tree region running PVST+ or to another MST
region with a different MST configuration. A boundary port also connects to a LAN, the designated
switch of which is either a single spanning-tree switch or a switch with a different MST configuration.
There is no definition of a boundary port in the IEEE 802.1s standard. The IEEE 802.1Q-2002 standard
identifies two kinds of messages that a port can receive: internal (coming from the same region) and
external. When a message is external, it is received only by the CIST. If the CIST role is root or alternate,
or if 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. This means a port cannot receive a mix of internal and external messages.
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 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 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 the example in Figure 10-1, 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 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 (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.