Cisco NX-OS: Virtual Port Channel (vPC)

Three Main Types of MCEC (Multi Chassis EtherChannel)

• C3750 Cross Stack Port Channels (StackWise)
  • single control plane
• C6500 Virtual Switching System (VSS)
  • single control plane
  • via Virtual Switch Link (VSL)
• Nexus Virtaul Port Channel (vPC)
  • separate control planes
  • separate control plane protocol instances (STP/IGPs/BGP/FHRP)
  • via a Peer Link (like VSS’s VSL)

Each vPC peer has

• Peer Link
  • to sync control plane between vPC peers (CAM/ARP/IGMP)
  • uses CFSoE (Cisco Fabric Service over Ethernet)
  • used to elect a vPC Primary and vPC Secondary Role
  • normally not used for the data plane => much lower BW
• Peer Keepalive Link
  • L3 link used as heartbeat in the control plane
  • used to prevent active/active or “Split Brain” vPC roles
  • not used in the vPC data plane
  • could back to back or over routed infrastructure (vrf)
• vPC Member ports
  • from downstream neighbor the vPC peers is one switch

Note: VLANS on vPC Member ports must be added on the Peer link too

vPC Order of Operations

1. IP connectivity for Peer Keepalive
2. Enable vPC & LACP globally
3. Create vPC domain
4. Define Peer Keepalive address
5. Establish Port Channel for vPC Peer link
6. Verify vPC Consistency Parameters
7. Disable vPC Member Port (optional but recommended)
8. Configure vPC Member Ports
9. Enable vPC Member Ports

Good design for Peer Link

• redundant power
• redundant SUP
• multiple LC (Line Cards)

vPC Loop Prevention

Loops are prevented via “vPC Check”:

• Frames received on the vPC Peer link can not flood out a vPC Member Port while original vPC Peer has active vPC Members in the same vPC
• if vPC peer’s members are down, the vPC member ports => “Orphan ports” and vPC Check is disabled
• it is possible because both switches have info about status of vPC on the Peer switch over vPC member

NXOS2 will not flood frames (received over vPC Peer Link) over Po45 while NXOS1 Po45 is up.

vPC Consistency Checks

• Type 1 Global and Interface Consistency Check
  • if global mismatch - vPC failing to form
  • if interface mismatch - VLANs being suspended
• Type 2 Consistency Check
  • if mismatch - log messaged but not vPC failure, but could be data plane failures