Руководство По Проектированию для Cisco Cisco Nexus 5010 Switch

Design Guide

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secondary device (operational primary) will not change, to avoid unnecessary disruptions. This behavior is achieved

with a sticky-bit method, whereby the sticky information is not saved in the startup configuration, thus making the

device that is up and running win over the reloaded device. Hence, the vPC primary becomes the vPC operational

secondary.

If the peer link is disconnected but the vPC peers are still connected through the vPC peer keepalive link, the vPC

operational roles stay unchanged.

If both the peer link and peer-keepalive link are disconnected, both vPC peers become operational primary, but upon

reconnection of the peer-keepalive link and the peer link, the vPC secondary device (operational primary) keeps the

primary role, and the vPC primary becomes the operational secondary device.

Spanning Tree

vPC modifies the way in which spanning tree works on the switch to help ensure that the vPC peers in a vPC domain

appear as a single spanning-tree entity on vPC ports. Also, vPC helps ensure that devices can connect to a vPC

domain in a non-vPC fashion with classic spanning-tree topology. vPC is designed to support hybrid topologies.

Depending on the Cisco NX-OS Software release, this can be achieved in slightly different ways.

In all Cisco NX-OS releases, the peer link is always forwarding because of the need to maintain the MAC address

tables and Internet Group Management Protocol (IGMP) entries synchronized.

vPC by default ensures that only the primary switch forwards BPDUs on vPCs. This modification is strictly limited to

vPC member ports. As a result, the BPDUs that may be received by the secondary vPC peer on a vPC port are

forwarded to the primary vPC peer through the peer link for processing.

Note: Non-vPC ports operate like regular spanning-tree ports. The special behavior of the primary vPC member
applies uniquely to ports that are part of a vPC.

Starting from Cisco NX-OS Releases 4.2(6) and 5.0(2), vPC allows the user to choose the peer-switch option. This

option optimizes the behavior of spanning tree with vPC as follows:

●

The vPC primary and secondary are both root devices and both originate BPDUs

●

The BPDUs originated by both the vPC primary and the vPC secondary have the same designated bridge ID

on vPC ports

●

The BPDUs originated by the vPC primary and secondary on non-vPC ports maintain the local bridge ID

instead of the vPC bridge ID and advertise the Bridge ID of the vPC system as the root

The peer-switch option has the following advantages:

●

It reduces the traffic loss upon restoration of the peer link after a failure.

●

It reduces the disruption associated with a dual-active failure (whereby both vPC members become primary).

Both devices keep sending BPDUs with the same bridge ID information on vPC member ports, which prevents

errdisable from potentially disabling the PortChannel for an attached device.

●

It reduces the potential loss of BPDUs if the primary and secondary roles change.

Cisco Discovery Protocol

From the perspective of the Cisco Discovery Protocol, the presence of vPC does not hide the fact that the two Cisco

Nexus Switches are two distinct devices, as illustrated by the following output:

tc-nexus5k01# show cdp neigh

Capability Codes: R - Router, T - Trans-Bridge, B - Source-Route-Bridge

S - Switch, H - Host, I - IGMP, r - Repeater,

V - VoIP-Phone, D - Remotely-Managed-Device,