Chapter 4: Feature Description
Ethernet Features
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Common Internal Spanning Tree (CIST) – A collection of the ISTs in each MST Region, and the
CST that interconnects the MST regions and individual spanning trees. MSTP connects all
bridges and LANs with a single CIST.
MSTP specifies:
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An MST Configuration Identifier that enables each bridge to advertise its configuration for
allocating frames with given VIDs to any of a number of MSTIs.
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A priority vector that consists of a bridge identifier and path cost information for the CIST.
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An MSTI priority vector for any given MSTI within each MST Region.
Each bridge selects a CIST priority vector for each port based on the priority vectors and MST
Configuration Identifiers received from the other bridges and on an incremental path cost
associated with each receiving port. The resulting priority vectors are such that in a stable network:
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One bridge is selected to be the CIST Root.
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A minimum cost path to the CIST Root is selected for each bridge.
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The CIST Regional Root is identified as the one root per MST Region whose minimum cost
path to the root is not through another bridge using the same MST Configuration Identifier.
Based on priority vector comparisons and calculations performed by each bridge for each MSTI,
one bridge is independently selected for each MSTI to be the MSTI Regional Root, and a minimum
cost path is defined from each bridge or LAN in each MST Region to the MSTI Regional Root.
The following events trigger MSTP re-convergence:
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Addition or removal of a bridge or port.
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A change in the operational state of a port or group (LAG or protection).
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A change in the service to instance mapping.
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A change in the maximum number of MSTIs.
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A change in an MSTI bridge priority, port priority, or port cost.
Note
All except the last of these triggers can cause the entire MSTP to re-converge. The last
trigger only affects the modified MSTI.
MSTP Interoperability
MSTP in PTP 820S units is interoperable with:
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Third-party bridges running MSTP.
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Third-party bridges running RSTP
OAM
PTP 820S provides complete Service Operations Administration and Maintenance (SOAM)
functionality at multiple layers, including:
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