Hop Count; Boundary Ports - Cisco Nexus 7000 Series Configuration Manual

Configuring MST Using Cisco NX-OS
• The CIST external root path cost is the cost to the CIST root. This cost is left unchanged within an MST
• If the CIST root is in the region, the CIST regional root is the CIST root. Otherwise, the CIST regional
• The CIST internal root path cost is the cost to the CIST regional root in a region. This cost is only relevant

Hop Count

MST does not use the message-age and maximum-age information in the configuration BPDU to compute
the STP topology inside the MST region. Instead, the protocol uses the path cost to the root and a hop-count
mechanism similar to the IP time-to-live (TTL) mechanism.
The hop count achieves the same result as the message-age information (triggers a reconfiguration). The root
bridge of the instance always sends a BPDU (or M-record) with a cost of 0 and the hop count set to the
maximum value. When a device receives this BPDU, it decrements the received remaining hop count by one
and propagates this value as the remaining hop count in the BPDUs that it generates. When the count reaches
zero, the device discards the BPDU and ages the information held for the port.
The message-age and maximum-age information in the 802.1w portion of the BPDU remain the same throughout
the region (only on the IST), and the same values are propagated by the region-designated ports at the boundary.
You configure a maximum aging time as the number of seconds that a device waits without receiving spanning
tree configuration messages before attempting a reconfiguration.

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 14: MST Boundary Ports
region. An MST region looks like a single device to the CIST. The CIST external root path cost is the
root path cost calculated between these virtual devices and devices that do not belong to any region.
root is the closest device to the CIST root in the region. The CIST regional root acts as a root bridge for
the IST.
to the IST, instance 0.
Cisco Nexus 7000 Series NX-OS Layer 2 Switching Configuration Guide, Release 5.x
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