Brocade Network OS Administrator’s Guide v4.1.1 - Manual (Page 410)

NOTE
In MSTP mode, RSTP is automatically enabled to provide rapid convergence.
Multiple switches must be configured consistently with the same MSTP configuration to participate in
multiple spanning tree instances. A group of interconnected switches that have the same MSTP
configuration is called an MSTP region.
NOTE
Brocade supports 32 MSTP instances and one MSTP region.
MSTP introduces a hierarchical way of managing switch domains using regions. Switches that share
common MSTP configuration attributes belong to a region. The MSTP configuration determines the
MSTP region where each switch resides. The common MSTP configuration attributes are as follows:
• Alphanumeric configuration name (32 bytes)
• Configuration revision number (2 bytes)
• 4096-element table that maps each of the VLANs to an MSTP instance
Region boundaries are determined based on the above attributes. A multiple spanning tree instance is
an RSTP instance that operates inside an MSTP region and determines the active topology for the set
of VLANs mapping to that instance. Every region has a common internal spanning tree (CIST) that
forms a single spanning tree instance that includes all the switches in the region. The difference
between the CIST instance and the MSTP instance is that the CIST instance operates across the
MSTP region and forms a loop-free topology across regions, while the MSTP instance operates only
within a region. The CIST instance can operate using RSTP if all the switches across the regions
support RSTP. However, if any of the switches operate using 802.1D STP, the CIST instance reverts
to 802.1D. Each region is viewed logically as a single STP/RSTP bridge to other regions.
PVST+ and Rapid PVST+
A network topology of bridges typically contains redundant connections to provide alternate paths in
case of link failures. However, because there is no concept of TTL in Ethernet frames, this could result
in permanent circulation of frames if there are loops in the network. To prevent loops, a spanning tree
connecting all the bridges is formed in real time. The redundant ports are put in a
blocking (nonforwarding) state. They are enabled when required.
In order to build a spanning tree for the bridge topology, the bridges must exchange control frames
(BPDUs - Bridge Protocol Data Units). The protocols define the semantics of the BPDUs and the
required state machine. The first Spanning Tree Protocol (STP) became part of the IEEE 802.1d
standard.
Because the convergence time of STP is 50 seconds in the case of link failures, this delay soon
became increasingly unacceptable. Keeping the main skeleton of STP the same, the state machine
was changed to speed up the convergence time as part of the Rapid Spanning Tree protocol (RSTP).
RSTP became part of the IEEE 802.1w standard.
Both STP and RSTP build a single logical topology. A typical network has multiple VLANs. A single
logical topology does not efficiently utilize the availability of redundant paths for multiple VLANs. If a
port is set to "blocked/discarding" for one VLAN (under STP/RSTP), it is the same for all other VLANs
too.
Per-VLAN Spanning Tree Plus (PVST+) protocol runs a spanning tree instance for each VLAN in the
network. The version of PVST+ that uses the RSTP state machine is called Rapid-PVST Plus (R-
PVST+). R-PVST+ has one instance of spanning tree for each VLAN on the switch.
PVST+ and Rapid PVST+
410 Network OS Administrator’s Guide
53-1003225-04