Loop Prevention - Cisco Nexus 1000V Deployment Manual

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Nexus 1000V Switch, may learn the virtual machine's MAC address dynamically. Thus, within the Cisco NX-OS

CLI, you may see the virtual machine's MAC address twice: a dynamic entry and a static entry.

Loop Prevention

Another differentiating characteristic of the Cisco Nexus 1000V is that it does not run Spanning Tree Protocol.

Although this may seem to be a significant departure from other Ethernet switches, potentially

causing catastrophic network loops, in reality the Cisco Nexus 1000V implements a simple and effective loop-

prevention strategy that does not require Spanning Tree Protocol (Figure 23).

Figure 23.

Built-in Loop Prevention Capabilities

Because the Cisco Nexus 1000V does not participate in Spanning Tree Protocol, it does not respond to Bridge

Protocol Data Unit (BPDU) packets, nor does it generate them. BPDU packets that are received by Cisco Nexus

1000V Switches are dropped.

The Cisco Nexus 1000V uses a simple technique to prevent loops. Like a physical Ethernet switch, the Cisco

Nexus 1000V Switch performs source and destination MAC address lookups to make forwarding decisions. The

VEM applies loop-prevention logic to every incoming packet on Ethernet interfaces. This logic is used to identify

potential loops. Every ingress packet on a physical Ethernet interface is inspected to help ensure that the

destination MAC address is internal to the VEM. If the source MAC address is internal to the VEM, the Cisco

Nexus 1000V Switch will drop the packet. If the destination MAC address is external, the switch will drop the

packet, preventing a loop back to the physical network.

Note:

The Cisco Nexus 1000V prevents loops between the VEMs and the first-hop access switches without the

use of Spanning Tree Protocol. However, this feature does not mean that Spanning Tree Protocol should be

disabled on any access switches. Spanning Tree Protocol is still required by access switches to prevent loops

elsewhere in the physical topology.

Spanning Tree Protocol goes through a series of states on each interface as it tries to build the network tree.

This process causes downtime on each interface when Spanning Tree Protocol needs to converge. This process

is unnecessary for ports connected to Cisco Nexus 1000V Switches. By using the PortFast feature on a switch

port, a Cisco switch can suppress the progression of Spanning Tree Protocol states and move straight to a

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