Hop Count; Boundary Ports - Cisco nexus 5000 series Cli Configuration Manual

Chapter 1 Configuring MST
S e n d f e e d b a c k t o n x 5 0 0 0 - d o c f e e d b a c k @ c i s c o . c o m
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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.
By using the spanning-tree mst max-hops global configuration command, you can configure the
maximum hops inside the region and apply it to the IST and all MST instances in that region.
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 switch 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 switch 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 switch 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 (see
Figure
Figure 1-3
B2 designated => B1 boundary,
OL-16597-01
If the CIST root is in the region, the CIST regional root is the CIST root. Otherwise, the CIST
regional root is the closest switch to the CIST root in the region. The CIST regional root acts as a
root bridge for the IST.
The CIST internal root path cost is the cost to the CIST regional root in a region. This cost is only
relevant to the IST, instance 0.
1-3).
MST Boundary Ports
MST region A
Bridge B1
MST region B
Bridge B2
B2 & B3 internal
Cisco Nexus 5000 Series Switch CLI Software Configuration Guide
Information About MST
Bridge B3
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