Cisco WS-C2950SX-48-SI Configuration Manual page 116

Understanding How MST Works
IST Master
The IST master of an MST region is the bridge with the lowest bridge identifier and the least path cost
to the CST root. If an MST bridge is the root bridge for CST, then it is the IST master of that MST region.
If the CST root is out side the MST region, then one of the MST bridges at the boundary is selected as
the IST master. Other bridges on the boundary that belong to the same region eventually block the
boundary ports that lead to the root.
If two or more bridges at the boundary of the region have an identical path to the root, you can set a
slightly lower bridge priority to make a specific bridge IST master.
The root path cost and message age inside a region stays constant, but the IST path cost is incremented
and the IST remaining hops is decremented at each hop. Enter the show spantree mst command to
display the information about the IST master, path cost, and remaining hops for the bridge.
Edge Ports
A port that is connected to a nonbridging device (for example, a host or a router) is an edge port. A port
that connects to a hub is also an edge port, provided that the hub or any LAN that is connected by it does
not have a bridge. These ports start forwarding as soon as the link is up.
MST requires that all ports are configured for each host or router. To establish rapid connectivity after
a failure, you need to block the nonedge-designated ports of an intermediate bridge. If the port connects
to another bridge that can send back an agreement, then the port starts forwarding immediately.
Otherwise, the port requires twice the forward delay time to start forwarding again. You must explicitly
configure the ports that are connected to the hosts and routers as edge ports while using MST.
To configure a port as an edge port you enable PortFast on that port. See
Note
Spanning Tree PortFast, BPDU Guard, BPDU Filter, UplinkFast, BackboneFast, and Loop Guard."
When you enter the show spantree portfast mod/port command, if the designation for a port is
displayed as edge, that port is also a PortFast port.
To prevent a misconfiguration, PortFast turns off operationally if the port receives a BPDU. You can
display the configured and operational status of PortFast by using the show spantree mst mod/port
command.
Link Type
You can establish rapid connectivity only on point-to-point links. For correct operation of the protocol,
you must explicitly configure ports to a host or router. However, cabling in most networks meets this
requirement, and you can avoid explicit configuration by treating all full-duplex links as point-to-point
links. Enter the set spantree mst link-type command to configure point-to-point links.
Catalyst 4500 Series, Catalyst 2948G, Catalyst 2980G Switches Software Configuration Guide
7-20
Chapter 7 Configuring Spanning Tree
Chapter 8, "Configuring
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Release 8.1
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