Cisco IE-3000-8TC Software Configuration Guide - Page 395

Chapter 19 Configuring MSTP Understanding MSTP 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 19-5 compares the IEEE standard and the Cisco prestandard terminology. Table 19-1 Prestandard and Standard Terminology IEEE Standard CIST regional root CIST internal root path cost CIST external root path cost MSTI regional root MSTI internal root path cost Cisco Prestandard IST master IST master path cost Root path cost Instance root Root path cost Cisco Standard CIST regional root CIST internal path cost Root path cost Instance root 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-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 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. 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. OL-13018-03 Cisco IE 3000 Switch Software Configuration Guide 19-5