14.
Multiple Spanning Tree Protocol (MSTP)
MN700004 Rev 01
144
The message-age and maximum-age information in the RSTP portion of the BPDU remain
the same throughout the region, and the region’s designated ports at the boundary propagate
the same values.
MST-to-SST Interoperability
A virtual bridged LAN may contain interconnected regions of SST and MST bridges.
To enable running STP in the SST region, an MST region appears as a single SST or
pseudobridge, which operates as follows:
•
Although the values for root identifiers and root path costs match for all BPDUs in all
pseudobridges, a pseudobridge differs from a single SST bridge as follows:
The pseudobridge BPDUs have different bridge identifiers. This difference does not
affect STP operation in the neighboring SST regions because the root identifier and
root cost are the same.
BPDUs sent from the pseudobridge ports may have significantly different message
ages. Because the message age increases by one second for each hop, the difference in
the message age is measured in seconds.
•
Data traffic from one port of a pseudobridge (a port at the edge of a region) to another port
follows a path entirely contained within the pseudobridge or MST region. Data traffic
belonging to different VLANs might follow different paths within the MST regions
established by MST.
•
The system prevents looping by doing either of the following:
Blocking the appropriate pseudobridge ports by allowing one forwarding port on the
boundary and blocking all other ports
Setting the CST partitions to block the ports of the SST regions.
MST Instances
BiNOS supports up to 16 instances. Each spanning tree instance is identified by an instance
ID that ranges from 0 to 15. Instance 0 is mandatory and is always present. Instances 1
through 15 are optional.
MST Region Overview
Interconnected bridges that have the same MST configuration are referred to as an MST
region. There is no limit on the number of MST regions in the network.
To form an MST region, bridges can be either of the following:
•
An MST bridge that is the only member of the MST region.
•
An MST bridge interconnected by a LAN. A LAN’s designated bridge has the same MST
configuration as an MST bridge. All the bridges on the LAN can process MST BPDUs.
