Boundary Ports - Cisco Catalyst 4500 Series Configuration Manual

Chapter 20 Configuring STP and MST
To form an MST region, bridges can be either of the following:
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If you connect two MST regions with different MST configurations, the MST regions do the following:
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Boundary Ports

A boundary port is a port that connects to a LAN, the designated bridge of which is either an SST bridge
or a bridge with a different MST configuration. A designated port knows that it is on the boundary if it
detects an STP bridge or receives an agreement message from an RST or MST bridge with a different
configuration.
At the boundary, the role of MST ports do not matter; their state is forced to be the same as the IST port
state. If the boundary flag is set for the port, the MSTP Port Role selection mechanism assigns a port
role to the boundary and the same state as that of the IST port. The IST port at the boundary can take up
any port role except a backup port role.
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 outside 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 a region have an identical path to the root, you can set a slightly
lower bridge priority to make a specific bridge the IST master.
The root path cost and message age inside a region stay constant, but the IST path cost is incremented
and the IST remaining hops are decremented at each hop. Enter the show spanning-tree 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 switch) is an edge port. A port
that connects to a hub is also an edge port if the hub or any LAN that is connected to it does not have a
bridge. An edge port can start forwarding as soon as the link is up.
MST requires that you configure each port connected to a host. To establish rapid connectivity after a
failure, you need to block the non-edge 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 needs twice the forward delay time to start forwarding again. You must explicitly
configure the ports that are connected to the hosts and switches as edge ports while using MST.
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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.
Load balance across redundant paths in the network. If two MST regions are redundantly connected,
all traffic flows on a single connection with the MST regions in a network.
Provide an RSTP handshake to enable rapid connectivity between regions. However, the
handshaking is not as fast as between two bridges. To prevent loops, all the bridges inside the region
must agree upon the connections to other regions. This situation introduces a delay. We do not
recommend partitioning the network into a large number of regions.
Software Configuration Guide—Release IOS XE 3.3.0SG and IOS 15.1(1)SG
About MST
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