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After configuring FabricPath, the following appears in the syslog:
%STP-2-L2GW_BACKBONE_BLOCK: L2 Gateway Backbone port inconsistency blocking port port-channel100 on VLAN0010.
What is the cause of this message?
A. STP must be disabled; message indicates that STP is enabled.
B. STP is misconfigured; root guard must be disabled.
C. An inferior BPDU is received on a Cisco FabricPath edge port.
D. A superior BPDU is received on a Cisco FabricPath edge port.
Correct Answer: D
Explanation/Reference:
Explanation:
The devices that are in both the FabricPath domain and CE domain are considered to be edge devices or gateway devices. Edge ports have a FabricPath root guard-like function enabled implicitly. If a superior BPDU is received on an edge port, the port is placed in the Layer 2 Gateway inconsistent state until the condition is cleared.
%STP-2-L2GW_BACKBONE_BLOCK: L2 Gateway Backbone port inconsistency blocking port port-channel100 on VLAN0010.
As a best practice, you should configure all edge devices with the lowest STP priority of all devices in the STP domain to which they are attached. By setting all of the edge devices to be the root bridge, the entire FabricPath domain looks like one virtual bridge to the CE domain. The same recommendation applies to a virtual port channel+ (vPC+) domain; you must configure each device (primary and secondary) as the root. You configure all FabricPath edge devices by manually setting the bridge priority lower than any STP bridge or by entering these commands.
Sw7-vpc(config)# spanning-tree vlan root primary
sw7-vpc(config)# spanning-tree vlan 1-50 root primary
To have a loop-free topology for the CE/FabricPath hybrid network, the FabricPath network automatically displays as a single bridge to all connected CE devices. The STP domains do not cross into the FabricPath network. If multiple STP domains are defined, BPDUs and topology change notifications (TCNs) are localized to the domain. If a connected STP domain is multihomed to the FabricPath domain, a TCN must be able to reach to all devices in the STP domain through the FabricPath domain. As a result, the TCN is sent to the FabricPath domain through the IS-IS protocol data unit (PDU) by default.
Reference: http://www.cisco.com/en/US/docs/switches/datacenter/nexus5000/sw/mkt_ops_guides/513_n1_1/n5k_ops_fabricpath.html (see Fabricpath versus classical Ethernet networks, see the paragraph just below figure 1-2)