Руководство По Проектированию для Cisco Cisco Nexus 5010 Switch

 

 

Design Guide 

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Figure 8.    LACP Behavior with Various Wiring Configurations 

 

System ID in a vPC System 

Spanning tree and LACP use the switch MAC address for, respectively, the bridge ID field in the spanning-tree BPDU 

and as part of LACP LAGID. In a single chassis, they use the systemwide MAC address for this purpose. For 

systems that use vPCs, use of the systemwide MAC address would not work because the vPC peers needs to 

appear as a single entity as shown in example C in Figure 8. To meet this requirement, vPC offers both an automatic 

configuration and a manual configuration of the system ID for the vPC peers. 

The automatic solution implemented by vPC consists of generation of a system ID composed of a priority and a MAC 

address, with the MAC derived from a reserved pool of MAC addresses combined with the domain ID specified in the 

vPC configuration. The domain ID is encoded in the last octet and the trailing 2 bits of the previous octet of the MAC 

address. 

By configuring domain IDs to be different on adjacent vPCs complexes (and to be identical on each vPC peer 

complex), you will help ensure the uniqueness of the system ID for LACP negotiation purposes. You also help ensure 

that the spanning-tree BPDUs use a MAC address that is representative of the vPC complex. 

You can override the automatic generation of the system ID by using the command-line interface (CLI) and 

configuring the system ID on both vPC peers manually, as follows: 

(config-vpc-domain)#system-mac <mac> 

Primary and Secondary vPC Roles 

In a vPC system, one vPC switch is defined as primary and one is defined as secondary, based on defined priorities. 

The lower number has higher priority, so it wins. Also, these roles are nonpreemptive, so a device may be 

operationally primary, but secondary from a configuration perspective. 

To understand the operational role of a vPC member, you need to consider the status of the peer-keepalive link and 

the peer link. 

When the two vPC systems are joined to form a vPC domain, the priority decides which device is the vPC primary 

and which is the vPC secondary. If the primary device were to reload, when the system comes back online and 

connectivity to the vPC secondary device (now the operational primary) is restored, the operational role of the