Cisco 7609-S Software Configuration Guide - Page 252

Understanding MST Chapter 19 Configuring Standard-Compliant IEEE MST • 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 router 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. Table 19-1 compares the IEEE standard and the Cisco prestandard terminology. Table 19-1 Prestandard and Standard Terminology IEEE Standard Definition CIST regional root CIST internal root path cost CIST external root path cost MSTI regional root MSTI internal root path cost Cisco Prestandard Implementation IST master IST master path cost Root path cost Instance root Root path cost Cisco Standard Implementation CIST regional root CIST internal path cost Root path cost Instance root 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 router 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 router 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: • 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 A boundary port also connects to a LAN, the designated router of which is either a single spanning tree router or a router 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 19-6 Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX OL-4266-08