14.
Multiple Spanning Tree Protocol (MSTP)
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If you connect two MST regions with different MST configurations, the MST regions do the
following:
•
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.
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 master port role to the port 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 an MST instance, 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.
Regional Root
The root bridge of each MSTI in a region is referred to as the MSTI’s regional root. In the
case of the IST (MSTI0), it is referred to as the CIST Regional root. Therefore, the terms “IST
Master” and “CIST Regional root” are interchangeable.
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.
MSTP 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 MSTP.
