14.
Multiple Spanning Tree Protocol (MSTP)
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Link Type
Rapid connectivity is established only on point-to-point links. If you connect a port to another
port through a point-to-point link and the local port becomes a designated port, the RSTP
negotiates a rapid transition with the other port by using the proposal-agreement handshake to
ensure a loop-free topology. By default, the link type is automatically determined by the
duplex state of the port. If you have a half-duplex link physically connected point-to-point to
a single port on a remote switch running RSTP, you can override the default setting of the link
type and enable rapid transitions to the forwarding state.
Message Age and Hop Count
IST and MST instances do not use the message age and maximum age timer settings in the
BPDU. IST and MST use a separate hop count mechanism that is very similar to the IP time-
to live (TTL) mechanism. You can configure each MST Bridge with a maximum hop count.
The root bridge of the instance sends a BPDU (or M-record) with the remaining hop count
that is equal to the maximum hop count. When a bridge receives a BPDU (or M-record), it
decrements the received remaining hop count by one. The bridge discards the BPDU (M-
record) and ages out the information held for the port if the count reaches zero after
decrementing. The nonroot bridges propagate the decremented count as the remaining hop
count in the BPDUs (M-records) they generate.
The message age and maximum age timer settings in the RST portion of the BPDU remain
the same throughout the region, and the region’s designated ports at the boundary propagate
the same values.
Port Priority
The MSTP uses the port priority when selecting an interface to put into the forwarding state if
a loop occurs. To interfaces that you want selected first, you can assign higher priority values,
and to interfaces that you want selected last you can assign lower priority values. A higher
priority value corresponds to a lower numerical value and a lower priority value corresponds
to a higher numerical value. If all interfaces have the same priority value, the MSTP puts the
interface with the lowest interface number in the forwarding state and blocks the other
interfaces.
Path Cost
The MSTP uses path cost when selecting an interface to put in the forwarding state if a loop
occurs. The MSTP path cost default value is derived from the link speed of an interface. You
can assign lower cost values that you want selected first and higher cost values to interfaces
that you want selected last. If all interfaces have the same cost value, the MSTP puts the
interface with the lowest interface number in the forwarding state and blocks the other
interfaces.
Interoperability with 802.1D STP
A switch running both MSTP and RSTP supports a built-in protocol migration mechanism
that enables it to interoperate with legacy 802.1D switches. If this switch receives a legacy
802.1D configuration BPDU (a BPDU with the protocol version set to 0), it sends only
802.1D BPDUs on that port. An MST switch can also detect that a port is at the boundary of a
region when it receives a legacy BPDU, an MST BPDU (version 3) associated with a
different region, or an RST BPDU (version 2). However, the switch does not automatically
revert to the MSTP mode if it no longer receives 802.1D BPDUs because it cannot determine
whether the legacy switch has been removed from the link unless the legacy switch is the
