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Cisco Catalyst Blade Switch 3130 for Dell Software Configuration Guide
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Chapter18 Configuring MSTP
Understanding MSTP
The message-age and maximum-age information in the RSTP portion of the BPDU rema in the sam e
throughout the region, and the same values are propagated by the region designated ports at the
boundary.
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 rapid 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 stan dard. 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. I f 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 respecti ve
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 designa ted
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 bel ong ing 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 de fined
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 o n 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 swit che s.
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 st andard, as
well as some of the desirable prestandard functionality that is not yet incorporated into the published
standard.