Hop Count; Boundary Ports - Cisco WS-SUP32-GE-3B - Supervisor Engine 32 Software Configuration Manual

Understanding MST
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Table 17-5
Table 17-5
IEEE Standard Definition
CIST regional root
CIST internal root path cost
CIST external root path cost
MSTI regional root
MSTI internal root path cost

Hop Count

MST does not use the message-age and maximum-age information in the configuration BPDU to
compute the spanning tree topology. Instead, they use 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 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 RSTP portion of the BPDU remain the same
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 one of these STP
regions:
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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 802.1s standard. The 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
Catalyst Supervisor Engine 32 PISA Cisco IOS Software Configuration Guide, Release 12.2ZY
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The CIST regional root was called the IST master in the prestandard implementation. 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.
compares the IEEE standard and the Cisco prestandard terminology.
Prestandard and Standard Terminology
A single spanning tree region running RSTP
A single spanning tree region running PVST+ or rapid PVST+
Another MST region with a different MST configuration
Cisco Prestandard Implementation
IST master
IST master path cost
Root path cost
Instance root
Root path cost
Chapter 17 Configuring STP and MST
Cisco Standard Implementation
CIST regional root
CIST internal path cost
Root path cost
Instance root
Root path cost
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