vPC Feature for Consistent STP Bridge ID

B.

In a vPC environment, two Cisco Nexus switches are configured as a vPC pair to provide a loop-free topology and to achieve redundancy. The vPC pair presents itself as a single logical switch to downstream devices such as non-vPC switches, servers, and storage devices. However, in some cases, the downstream non-vPC switches may not receive the same STP bridge ID from the upstream vPC switch peers.

This issue occurs because each vPC switch in a vPC pair has a unique MAC address, and when the vPC peer link is brought up, both vPC switches use the same virtual MAC address for the vPC domain. However, non-vPC switches use the MAC address of the directly connected switch as the STP bridge ID, resulting in different bridge IDs for the upstream vPC switch peers.

To ensure that vPC and non-vPC switches receive the same STP bridge ID from the upstream vPC switch peers, the "peer-switch" feature must be enabled on the vPC domain. The "peer-switch" feature makes both vPC switches in a vPC pair appear as a single switch to the downstream non-vPC switches. It causes both vPC switches to use the same MAC address for the vPC domain, which in turn ensures that both vPC switches present the same STP bridge ID to the downstream non-vPC switches.

Option A, "vpc local role-priority 4000," is used to set the local vPC priority for the vPC switch. Option B, "peer-switch," is the correct answer to this question. Option C, "system-mac 0123.4567.89ab," is used to manually set the system MAC address for the vPC domain. Option D, "peer-gateway," is used to enable vPC on the default gateway to allow for Layer 3 traffic to be load-balanced across both vPC switches.