Specifications

8-44 Rapier Switch Software Reference
Software Release 2.7.3
C613-03098-00 REV A
Compatibility with Previous Spanning Tree
Protocols
MSTP provides for compatibility with older spanning tree protocols in several
ways. In addition to the MST region described in the previous section, the
protocol provides for single spanning tree systems by employing a common
and internal spanning tree (CIST) protocol. The CIST applies a common and
internal spanning tree protocol to the whole of the bridged network and is a
direct equivalent the internal spanning tree (IST) protocol of earlier versions.
In common with legacy spanning tree systems, the CIST protocol first
determines its root bridge from all the bridges on the network. This is the
bridge that contains the lowest bridge identifier. The protocol then selects a
regional root bridge for each MSTR. This is the bridge that provides the best
path to the CIST root. After the MSTR root bridges have been chosen, they then
act on the region’s behalf in such a way that the region appears to the CST as a
virtual bridge. So in addition to having multiple MSTIs, each region must
operate as a bridge in a CST.
MSTI Port Priority 15 Bits 5 through 8 are used to convey the value of the port
identifier priority for this MSTI. Bits 1
through 4 are transmitted as 0, and ignored on receipt.
MSTI Remaining
Hops
16 Value of remaining hops for this MSTI.
Table 8-14: MSTI configuration messages