Boundary Ports - Cisco Nexus 9000 Series Configuration Manual

Configuring MST Using Cisco NX-OS

Boundary Ports

A boundary port is a port that connects to a LAN, the designated bridge of which is either a bridge with a
different MST configuration (and so, a separate MST region) or a Rapid PVST+ or 802.1D STP bridge. A
designated port knows that it is on the boundary if it detects an STP bridge or receives an agreement proposal
from an MST bridge with a different configuration or a Rapid PVST+ bridge. This definition allows two ports
that are internal to a region to share a segment with a port that belongs to a different region, creating the
possibility of receiving both internal and external messages on a port.
Figure 12: MST Boundary Ports
At the boundary, the roles of MST ports do not matter; the system forces their state to be the same as the IST
port state. If the boundary flag is set for the port, the MST port-role selection process assigns a port role to
the boundary and assigns the same state as the state of the IST port. The IST port at the boundary can take up
any port role except a backup port role.
Detecting Unidirectional Link Failure: MST
Currently, this feature is not present in the IEEE MST standard, but it is included in the standard-compliant
implementation; it is based on the dispute mechanism. The software checks the consistency of the port role
and state in the received BPDUs to detect unidirectional link failures that could cause bridging loops. This
feature is based on the dispute mechanism.
Note See the Cisco Nexus 9000 Series NX-OS Interfaces Configuration Guide, for information on Unidirectional
Link Detection (UDLD).
When a designated port detects a conflict, it keeps its role, but reverts to a discarding state because disrupting
connectivity in case of inconsistency is preferable to opening a bridging loop.
This figure shows a unidirectional link failure that typically creates a bridging loop. Switch A is the root
bridge, and its BPDUs are lost on the link leading to switch B. Rapid PVST+ (802.1w) and MST BPDUs
include the role and state of the sending port. With this information, switch A can detect that switch B does
not react to the superior BPDUs that it sends and that switch B is the designated, not root port. As a result,
switch A blocks (or keeps blocking) its port, which prevents the bridging loop.
Cisco Nexus 9000 Series NX-OS Layer 2 Switching Configuration Guide, Release 7.x
Boundary Ports
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