Brocade Network OS Administrator’s Guide v4.1.1 - Manual (Page 411)

PVST+ is not a scalable model when there are many VLANs in the network, as it consumes a lot of
CPU power. A reasonable compromise between the two extremes of RSTP and R-PVST+ is the
Multiple Spanning Tree protocol (MSTP), which was standardized as IEEE 802.1s and later
incorporated into the IEEE 802.1Q-2003 standard. MSTP runs multiple instances of spanning tree that
are independent of VLANs. It then maps a set of VLANs to each instance.
NOTE
Network OS 4.0 and later supports PVST+ and R-PVST+ only. The PVST and R-PVST protocols are
proprietary to Cisco and are not supported.
PVST+ and R-PVST+ guidelines and restrictions
Consider the following when configuring PVST+ and R-PVST+:
• Disabling the tagging of native VLANs is required on STP/RSTP/MSTP switches in standalone mode,
otherwise PVST+/R-PVST+ does not converge and forms a loop on the native VLAN. The tagged
native VLAN data traffic is ignored. The native VLAN untagged data is forwarded.
• Disabling the tagging of native VLANs is required on edge ports in fabric cluster mode; otherwise,
PVST+/R-PVST+ does not converge and forms a loop on the native VLAN. The tagged native VLAN
data traffic is ignored. The native VLAN untagged data is forwarded.
• If a VLAN is configured with tagged ports that do not have PVST+ mode enabled on the interface and
are connected to the VDXs, and RSTP is enabled under the VLAN (PVST+), then BPDUs from the
tagged ports that are received by the VDX are dropped.
Spanning Tree Protocol and VCS mode
Network OS 4.0 and later supports any version of STP to run in VCS mode and function correctly
between interconnecting VCS nodes, or between VCS and other vendor’s switches. This feature is
called Distributed Spanning Tree Protocol (DiST).
The purpose of DiST is as follows:
• To support VCS to VCS connectivity and automatic loop detection and prevention.
• To assist deployment plans for replacing the legacy xSTP enabled switches in your network.
• To eliminate the need for standalone mode in order to be able to run xSTP.
DiST supports any of the following flavors of xSTP:
• IEEE STP
• IEEE RSTP
• IEEE MSTP
• Cisco PVST
• Cisco PVRST
DiST treats one VCS as one virtual xSTP bridge from an external view. The xSTP protocol is running
between VCS and standalone nodes. Each VCS has a unique RBridge ID and Priority. DiST can be
enabled on VCS edge ports connecting to other VCS nodes or standalone nodes. The Port Ids used for
xSTP are dynamically assigned and unique within the VCS.
It is important to note that in fabric cluster mode, it is assumed that the global xSTP configuration is the
same on all the member nodes of the VCS. Mismatched global configurations of xSTP across different
nodes in VCS are not supported. For example, if one of the nodes in a VCS is configured for RSTP and
another one is configured for STP or MSTP, this scenario is unsupported and the fabric will not form.
Each RBridge runs the spanning tree instance in a distributed manner. Each spanning tree instance
considers all the edge ports and the best information from the remote RBridges to arrive at the spanning
PVST+ and R-PVST+ guidelines and restrictions
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