Cisco Nexus 1000V Manuale d'uso - Pagina 13

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Deployment Guide

MAC Address Learning

This distributed forwarding model within a centrally managed switch is demonstrated by the way the Cisco Nexus

1000V Series handles MAC address learning. A MAC address can be learned multiple times within a single Cisco

Nexus 1000V Series Switch in either of two ways: statically or dynamically. Static entries are automatically generated

for virtual machines running on the VEM; these entries do not time out. For devices not running on the VEM, the VEM

can learn a MAC address dynamically, through the physical NICs in the server.

Each VEM maintains a separate MAC address table. Thus, a single Cisco Nexus 1000V Series Switch may learn a

given MAC address multiple times: as often as once per VEM. For example, one VEM may be hosting a virtual

machine, and the virtual machine's MAC address will be statically learned on the VEM. A second VEM, in the same

Cisco Nexus 1000V Series 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 Series is that it does not run Spanning Tree Protocol.

Although this might seem to be a significant departure from other Ethernet switches, potentially causing catastrophic

network loops, in reality the Cisco Nexus 1000V Series implements a simple and effective loop-prevention strategy

that does not require Spanning Tree Protocol.

Because the Cisco Nexus 1000V Series 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 Series Switches are dropped.

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

Nexus 1000V Series 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 destination MAC address is external, the Cisco Nexus 1000V Series will drop

the packet preventing a loop back to the physical network.

Note:

The Cisco Nexus 1000V Series 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 the access switches to prevent loops

elsewhere in the physical topology.

VEM-VSM Communication

Like the VSM, each VEM has a control and packet interface. These interfaces are unmanaged and not directly

configurable by the end user. The VEM uses the opaque data provided by VMware vCenter Server to configure the

control and packet interfaces with the correct VLANs. The VEM then applies the correct uplink port profile to the

control and packet interfaces to establish communication with the VSM.

After the VSM recognizes the VEM, a new module will be virtually inserted into the Cisco Nexus 1000V Series

Switch's virtual chassis. The VSM CLI will notify the network administrator that a new module has powered on, much

as with a physical chassis.

The module assignment is sequential, meaning that the VEM will be assigned the lowest available module number

between 3 and 66. When a VEM comes online for the first time, the VSM assigns the module number and tracks that

module using the unique user ID (UUID) of the VMware ESX server, helping ensure that if the VMware ESX host

loses connectivity or is powered down for any reason, the VEM will retain its module number when the host comes

back online.

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