Brocade FastIron Ethernet Switch Platform and Layer 2 Switching Configuration Guide - Manual (Page 178)

STP/RSTP
Configuring STP on MCT VLANs at MCT cluster devices is not recommended. By default, the
spanning tree is disabled in the MCT VLANs. If the network topology may create Layer 2 loops
through external connections, STP may be enabled on switches outside the MCT cluster to prevent
the Layer 2 loop. The MCT cluster devices then performs a pass-through forwarding of STP BPDUs
received through its ports in the MCT VLAN.
• In rare cases in which the network topology consists of Layer 2 loops outside the MCT cluster that
require STP/RSTP to be enabled on MCT VLANs in the cluster, the CCEPs are always in the
spanning tree disabled state. Refer to
MCT configuration examples using STP on page 208 to view
deployment scenarios where STP is used in an MCT configuration to prevent Layer 2 loops.
• The STP/RSTP algorithms have been modified so that the ICL never enters blocking state. The ICL
guard mechanism ensures that if the ICL is about to go into a blocking state, the port on which the
superior BPDUs are being received is moved to blocking state, and the ICL guard timer starts
running on it. This timer runs as long as superior BPDUs are received on this interface. As long as
this timer runs on an interface, the superior BPDUs are dropped.
• The new BLK_BY_ICL STP state indicates that superior BPDUs were received on this interface,
which could have led to blocking of the ICL interface, with the result that the CL port guard
mechanism has been triggered on this port.
• In an 802.1s MSTP deployment, Brocade recommends disabling spanning tree on MCT cluster
devices at the global level. MSTP cannot be configured on individual cluster devices.
• An MCT cluster can support up to 32 spanning tree instances.
BPDU forwarding
If the network deploys single STP or IEEE 802.1s (MSTP), the MCT cluster devices must be
configured using the bpdu-flood-enable command to flood the single STP/MSTP BPDUs in the
SSTP/MSTP domain (that is, to forward to all of the ports in the cluster switch, irrespective of VLAN.)
Syntax: [no] bpdu-flood-enable
When bpdu-flood-enable is configured, only the ICL should connect the two MCT cluster devices.
(The keep-alive VLAN link should not connect them.) If there is an additional link, the flooded BPDU
will cause a loop and high CPU utilization.
NOTE
The bpdu-flood-enable command is not supported on the Brocade ICX 7750.
Protocol-based VLANs
Protocol and subnet VLANs can be configured on MCT VLANS; however, the ICL and CCEPs cannot
be configured as dynamic members of protocol-based VLANs (and vice versa). The ICL and CCEP
can either be excluded from protocol-based VLANs, or they can be configured as static members.
CEPs can be configured as dynamic or static members of protocol-based VLANs, or they can be
excluded from them.
In a cluster, both cluster devices should have exactly the same protocol VLAN membership
configuration for the ICL and CCEPs. The ICL and CCEPs should be configured with the same type of
protocol/VLAN membership, although the CLI does not impose any restriction.
STP/RSTP
178 FastIron Ethernet Switch Platform and Layer 2 Switching Configuration Guide
53-1003086-04