The Virtual Port Channel (vPC) technology, integral to Cisco’s Nexus series of switches, introduces many advantages, notably the capability to eliminate the blocking state for a port-channel in an Ethernet network. One of its salient features is the peer-switch.
What is the Peer-Switch Feature?
The peer-switch command allows two vPC peer devices to function as if they are a single spanning tree root, simplifying the STP topology and resulting in faster convergence times.
Key Components
1. Single Virtual Switch Emulation: When peer-switch is enabled, vPC peers emulate a single virtual switch for Spanning Tree Protocol purposes.
2. Consistent Bridge IDs and Priorities: In a typical environment without peer-switch, each Nexus device has its own bridge ID. This could lead to scenarios where different switches become the root for different VLANs. With peer-switch, both vPC peers adopt a common bridge ID and priority for all vPC VLANs, projecting themselves as a singular STP root to the entire network.
3. Benefits of the Peer-Switch Feature:
• No Disruption from STP Re-convergence: The bridge ID remains consistent, even if one of the peers is unavailable. Thus, STP doesn’t need to reconverge, eliminating transient disruptions.
• Avoidance of Root Port Flaps: Devices dual-homed to the vPC peers experience no root port change, which could otherwise disrupt traffic.
• Stable MAC Address Table: The MAC address table remains intact and doesn’t undergo flushing, avoiding potential disturbances.
4. Compatibility with Multi-chassis EtherChannel (MEC): The peer-switch command augments the efficiency of multi-chassis EtherChannel. Devices dual-homed using an EtherChannel to both vPC peers perceive the connection as a single EtherChannel to a unified logical switch.
Essential Configurations
For the peer-switch feature to function correctly, certain configurations need to be consistent across both vPC peer devices:
• STP Configuration: The Spanning Tree Protocol configuration must be identical on both vPC peers. This includes settings such as the STP priority. Inconsistencies can lead to unpredictable behavior.
Example:
On both vPC peers:
spanning-tree vlan 10-100 priority 4096
Conclusion
The vPC peer-switch feature is essential for network architects looking to simplify their STP topologies and hasten convergence times. Network reliability and efficiency are enhanced by ensuring consistency in STP settings across vPC peers.