Cisco Systems 3750E Hop Count, Ieee 802.1s Terminology, Cisco Prestandard Cisco Standard, 19-5

Chapter 19 Configuring MSTP

Understanding MSTP

MSTP switches use Version 3 RSTP BPDUs or IEEE 802.1D STP BPDUs to communicate with legacy IEEE 802.1D switches. MSTP switches use MSTP BPDUs to communicate with MSTP switches.

IEEE 802.1s Terminology

Some MST naming conventions used in Cisco’s prestandard implementation have been changed to identify some internal or regional parameters. These parameters are significant only within an MST region, as opposed to external parameters that are relevant to the whole network. Because the CIST is the only spanning-tree instance that spans the whole network, only the CIST parameters require the external rather than the internal or regional qualifiers.

•The CIST root is the root switch for the unique instance that spans the whole network, the CIST.

•The CIST external root path cost is the cost to the CIST root. This cost is left unchanged within an MST region. Remember that an MST region looks like a single switch for the CIST. The CIST external root path cost is the root path cost calculated between these virtual switches and switches that do not belong to any region.

•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 switch 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.

Table 19-1 on page 19-5compares the IEEE standard and the Cisco prestandard terminology.

Table 19-1	Prestandard and Standard Terminology
IEEE Standard		Cisco Prestandard	Cisco Standard
CIST regional root		IST master	CIST regional root
CIST internal root path cost		IST master path cost	CIST internal path cost
CIST external root path cost		Root path cost	Root path cost
MSTI regional root		Instance root	Instance root
MSTI internal root path cost		Root path cost	Root path cost

Hop Count

The IST and MST instances do 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-hopsglobal 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 switch 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.

Catalyst 3750-E and 3560-E Switch Software Configuration Guide

OL-9775-02 19-5