Brocade Network OS Administrator’s Guide v4.1.1 - Manual (Page 409)

provides rapid reconvergence of edge ports, new root ports, and ports connected through point-to-point
links.
The RSTP interface states for every Layer 2 interface running RSTP are as follows:
• Learning — The interface prepares to participate in frame forwarding.
• Forwarding — The interface forwards frames.
• Discarding — The interface discards frames. Note that the 802.1D disabled, blocking, and listening
states are merged into the RSTP discarding state. Ports in the discarding state do not take part in the
active topology and do not learn MAC addresses.
The following table lists the interface state changes between STP and RSTP.
STP versus RSTP state comparison TABLE 66
STP interface state RSTP interface state Is the interface included in the
active topology?
Is the interface learning MAC
addresses?
Disabled Discarding No No
Blocking Discarding No No
Listening Discarding Yes No
Learning Learning Yes Yes
Forwarding Forwarding Yes Yes
With RSTP, the port roles for the new interface states are also different. RSTP differentiates explicitly
between the state of the port and the role it plays in the topology. RSTP uses the root port and
designated port roles defined by STP, but splits the blocked port role into backup port and alternate port
roles:
• Backup port — Provides a backup for the designated port and can only exist where two or more ports
of the switch are connected to the same LAN; the LAN where the bridge serves as a designated
switch.
• Alternate port — Serves as an alternate port for the root port providing a redundant path towards the
root bridge.
Only the root port and the designated ports are part of the active topology; the alternate and backup
ports do not participate in it.
When the network is stable, the root and the designated ports are in the forwarding state, while the
alternate and backup ports are in the discarding state. When there is a topology change, the new RSTP
port roles allow a faster transition of an alternate port into the forwarding state.
MSTP
IEEE 802.1s Multiple STP (MSTP) helps create multiple loop-free active topologies on a single physical
topology. MSTP enables multiple VLANs to be mapped to the same spanning tree instance (forwarding
path), which reduces the number of spanning tree instances needed to support a large number of
VLANs. Each MSTP instance has a spanning tree topology independent of other spanning tree
instances. With MSTP you can have multiple forwarding paths for data traffic. A failure in one instance
does not affect other instances. With MSTP, you are able to more effectively utilize the physical
resources present in the network and achieve better load balancing of VLAN traffic.
MSTP
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