Nexus VPC Configuration: How to Set Up Peer Link for Seamless Connectivity

When it comes to building resilient and high-performing data center networks, Cisco Nexus switches often lead the conversation. But to fully unlock their potential, one configuration stands out, Nexus vPC configuration.

It’s not just about load balancing or link redundancy. It’s about maintaining stable L2 adjacency between switches, minimizing downtime, and keeping your critical applications flowing, even when something breaks.

This guide breaks down the process of configuring a vPC peer link from scratch, the real-world failure scenarios you need to prep for, and the best practices Cisco doesn’t always spell out clearly.

Let’s get right into it.

What Is vPC and Why It’s Worth Your Attention?

Virtual Port Channel (vPC) allows two Nexus switches to logically appear as a single switch to downstream devices. This means devices can use multi-chassis EtherChannel without worrying about STP blocking half the traffic.

You avoid loops. You get full active-active bandwidth. And if configured properly, failover becomes seamless.

vPCs are foundational in Nexus 9300 vPC configurations, especially in environments where link redundancy and high availability aren’t optional—they’re expected.

You’ll often hear terms like peer link, keepalive, orphan port, and consistency check. We’ll walk through all of those and how to get this setup running correctly.

Terminology You Should Know Before Configuring vPC

Step-by-Step Nexus vPC Configuration

Let’s walk through the correct order and commands for setting up vPC.

This applies across multiple platforms, including Nexus 9000 vPC best practices and even older 5Ks, with minor adjustments.

1. Enable vPC and Define the Domain

conf t
feature vpc
vpc domain 10
  

Both switches must have the same domain ID. This number should be unique across the network.

2. Set Up the Keepalive Link

Use the out-of-band management interface (like mgmt0) for this.

interface mgmt0
  ip address 10.10.10.1/24
  no shutdown

vpc domain 10
  peer-keepalive destination 10.10.10.2 vrf management
  

Keepalive should never go over the peer link. That creates a single point of failure. Use a dedicated interface in a VRF instead.

This is where many guides fall short. Misconfiguring this leads to dual-active issues, known as split-brain.

3. Create and Configure the Peer Link

Choose two physical interfaces (like eth1/1 and eth1/2) and bundle them.

interface port-channel10
  switchport
  switchport mode trunk
  spanning-tree port type network
  vpc peer-link

interface ethernet1/1
  channel-group 10 mode active

interface ethernet1/2
  channel-group 10 mode active
  

Use at least 2 x 10G or 40G interfaces. And run them across different modules or line cards to avoid shared failure domains.

4. Create vPC Member Port-Channels

Example: Connecting to a downstream switch or server.

interface port-channel20
  switchport
  switchport mode trunk
  spanning-tree port type edge trunk
  vpc 20

interface ethernet1/3
  channel-group 20 mode active

interface ethernet1/4
  channel-group 20 mode active
  

Repeat this on both switches. And yes, configs must be identical—LACP mode, VLANs, STP settings, everything.

This is where vPC configuration guide entries in Cisco documentation get unnecessarily vague. Matching the configs isn’t just good practice—it’s mandatory. Mismatched port-channel configs will fail vPC consistency checks.

What Happens If Peer Link Fails?

This is the question that matters most. If the peer link goes down but the keepalive link stays active, the secondary switch disables all its vPC member ports. This avoids loops.

If both peer link and keepalive fail? Now you’ve got split brain.

Here’s what to expect:

• Both switches think they’re primary
  
• Duplicate MAC addresses flood the network
  
• Orphan ports may go rogue
  
• STP may not save you
  

To prepare, use auto-recovery reload-delay commands and even external monitoring scripts if needed.

Troubleshooting Table

This table isn’t for show. It’s the stuff network engineers use in production.

Handling Orphan Ports and Special Cases

Orphan ports—devices only connected to one switch—are often overlooked.

They’re harmless during normal operation, but if you lose the link to the other peer, traffic behavior changes fast.

Best practices:

• Avoid them if possible
  
• If necessary, clearly document where they exist
  
• Consider backup Layer 3 paths

This is especially important when dealing with Nexus vPC with different code. If one switch runs a slightly older NX-OS version, behavior around orphan ports, consistency checks, or STP reactions can vary.

Make sure you validate compatibility and behavior before mixing OS versions.

Can You Have More Than Two vPC Peers?

No. vPC supports exactly two switches per domain.

If you need more horizontal scaling or multi-site support, look at VXLAN EVPN instead. vPC is designed for dual-switch redundancy, not a full fabric topology.

Best Practices for Nexus vPC Configuration

If you’re setting up a vPC Nexus environment and miss even one of these, you’ll likely end up with ports stuck in down state or unpredictable MAC behavior.

Resolving The Peer Link Flap Issue

One of the most frustrating issues is intermittent peer link failure. You may see:

• MAC flapping on connected switches
  
• ARP instability
  
• Dropped sessions for connected apps

What’s the fix?

• Monitor the link continuously with NMS
  
• Use ip arp synchronize and mac address-table synchronize commands to prevent loss during failover
  
• Check optics and replace transceivers if errors rise

Many engineers don’t realize that ARP sync is off by default. That one missed command leads to real-world downtime.

vPC vs Other Technologies

If you’re building a network today, you should know how vPC stacks up.

For most mid-sized DCs, vPC gets the job done without needing a full BGP EVPN fabric. But if you’re expanding, keep an eye on VXLAN.

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