vpc peer-link
Configures an interface as a Virtual Port Channel (vPC) peer-link, which is used to synchronize state and forward traffic between vPC peer switches.
Definition: vpc peer-link is a Cisco NX-OS interface config command. Configures an interface as a Virtual Port Channel (vPC) peer-link, which is used to synchronize state and forward traffic between vPC peer switches.
Overview
The 'vpc peer-link' command is a critical configuration step in setting up a Virtual Port Channel (vPC) on Cisco Nexus switches. A vPC allows two Nexus switches to appear as a single logical device to downstream switches or servers that are dual-homed to both switches. The peer-link is the inter-switch link that carries control plane information (such as MAC address tables, ARP tables, and IGMP snooping states) as well as data traffic for orphan ports and multicast/broadcast traffic. Without a properly configured peer-link, the vPC domain cannot function.
In Cisco NX-OS, the peer-link must be a port-channel (EtherChannel) consisting of one or more physical interfaces. It is recommended to use at least two 10 Gigabit Ethernet links for redundancy and bandwidth. The peer-link is configured in interface configuration mode under the port-channel interface. Once configured, the switch will automatically use this link to exchange vPC-related information with its peer.
The peer-link is also used for the vPC keepalive mechanism, which is a separate Layer 3 link (typically a management interface or a dedicated VLAN) that monitors peer liveness. However, the keepalive is configured under the vPC domain, not on the peer-link interface.
When troubleshooting, the peer-link status is one of the first things to check. A down peer-link will cause all vPC member ports to be suspended, leading to traffic loss. Common issues include mismatched port-channel parameters (e.g., speed, duplex, allowed VLANs) or physical link failures. The 'show vpc' command provides a summary, while 'show vpc peer-link' gives detailed interface statistics.
interface <interface-type> <interface-id>
vpc peer-link
no vpc peer-linkWhen to Use This Command
- Connecting two Nexus switches in a vPC domain to form a single logical control plane for attached devices.
- Enabling inter-switch communication for vPC member ports and orphan ports.
- Providing a path for multicast and broadcast traffic between vPC peers.
- Allowing the vPC peer-link to carry BPDUs, LACP, and other control plane traffic for vPC consistency.
Parameters
| Parameter | Syntax | Description |
|---|---|---|
| interface-type | port-channel | The type of interface; must be 'port-channel' for vPC peer-link. Physical interfaces or other logical interfaces are not allowed. |
| interface-id | <channel-number> | The port-channel number (e.g., 1, 10, 100). This must match on both vPC peers for the peer-link to form. |
Command Examples
Configure vPC peer-link on port-channel 1
interface port-channel 1
vpc peer-linkThis command configures port-channel 1 as the vPC peer-link. No output is shown if successful; use 'show vpc' to verify.
Remove vPC peer-link configuration
interface port-channel 1
no vpc peer-linkRemoves the peer-link designation from the interface. The interface will become a regular port-channel.
Understanding the Output
The 'vpc peer-link' command itself does not produce output. To verify the peer-link status, use 'show vpc' or 'show vpc peer-link'. The 'show vpc' output includes a 'vPC peer-link status' field.
A healthy peer-link shows 'up' status with no errors. Problem values include 'down' (link failure), 'suspended' (incompatible configuration), or 'failed' (consistency check failure). The 'show vpc peer-link' command provides detailed information about the peer-link interface, including port-channel ID, state, and counters.
Key fields: 'Peer-link status' (up/down), 'vPC keep-alive status' (up/down), and 'Configuration consistency' (success/failure). A healthy peer-link should have both statuses as 'up' and consistency as 'success'.
Configuration Scenarios
Basic vPC Peer-Link Configuration
Two Nexus 9000 switches (N9K-1 and N9K-2) are to be configured as a vPC pair. The peer-link will use port-channel 1 with two 10GE interfaces.
Topology
N9K-1 (Eth1/1, Eth1/2) <---> N9K-2 (Eth1/1, Eth1/2)
| |
(port-channel 1) (port-channel 1)Steps
- 1.Create port-channel 1 on both switches.
- 2.Add physical interfaces to port-channel 1 on both switches.
- 3.Configure the port-channel as a trunk (allow necessary VLANs).
- 4.Enter interface configuration mode for port-channel 1 and issue 'vpc peer-link'.
- 5.Configure vPC domain and keepalive link.
! On N9K-1 interface port-channel 1 switchport mode trunk switchport trunk allowed vlan 1-100,200 vpc peer-link ! interface Ethernet1/1 channel-group 1 mode active ! interface Ethernet1/2 channel-group 1 mode active ! ! On N9K-2 (similar configuration) interface port-channel 1 switchport mode trunk switchport trunk allowed vlan 1-100,200 vpc peer-link ! interface Ethernet1/1 channel-group 1 mode active ! interface Ethernet1/2 channel-group 1 mode active
Verify: Use 'show vpc' to verify peer-link status is 'up' and consistency is 'success'. Use 'show vpc peer-link' for detailed interface counters.
Watch out: Ensure the port-channel is in 'mode active' (LACP) or 'mode on' (static) on both sides. Mismatched modes will prevent the peer-link from coming up.
Troubleshooting with This Command
When troubleshooting vPC peer-link issues on Cisco NX-OS, start with 'show vpc' to check the overall vPC status. If the peer-link is down, check the physical interfaces and port-channel status with 'show interface port-channel <id>' and 'show interface ethernet <id>'. Common causes: mismatched port-channel parameters (speed, duplex, VLAN allowed list), physical link failure, or LACP negotiation issues.
Use 'show port-channel summary' to see if the port-channel is up. If the peer-link is up but vPC is not operational, check 'show vpc consistency-parameters' to identify mismatches in global or interface parameters. The peer-link must have identical allowed VLANs on both sides.
Also verify the vPC keepalive link is working with 'show vpc keepalive'. If the peer-link flaps, check for errors on the interfaces with 'show interface counters errors'. In some cases, a software bug may cause peer-link instability; check the Cisco bug toolkit for known issues.
Always ensure both switches run the same NX-OS version for compatibility.
CCNA Exam Tips
Remember that the peer-link must be a port-channel (EtherChannel) and cannot be a single physical interface.
The peer-link is used for both control plane and data plane traffic; ensure it has sufficient bandwidth (typically at least two 10GE links).
In CCNP Data Center exams, know that the vPC peer-link must be configured on both peers with identical parameters.
Common Mistakes
Configuring vpc peer-link on a physical interface instead of a port-channel - the command will be rejected.
Forgetting to configure the peer-link on both vPC peers - the vPC domain will not form.
Using a port-channel that is also a member of a vPC - the peer-link cannot be a vPC member port.
vpc peer-link vs feature vpc
The commands 'feature vpc' and 'vpc peer-link' are closely related but serve different roles in vPC configuration. They are commonly confused because both are essential for vPC operation, yet one enables the feature globally while the other configures a specific interface as the peer-link. Understanding their distinct scopes and prerequisites is critical for correct deployment.
| Aspect | vpc peer-link | feature vpc |
|---|---|---|
| Scope | Interface-specific | Global (entire switch) |
| Configuration Mode | Interface configuration mode | Global configuration mode |
| Persistence | Saved in running-config, requires feature vpc enabled | Saved in running-config, enables vPC capability |
| Precedence | Must be configured after feature vpc and vpc domain | Must be configured before any vPC interface commands |
| Typical Use | Designates a port-channel for inter-switch synchronization | Activates the vPC feature set on the switch |
Use vpc peer-link when you need to designate a specific port-channel as the link between vPC peer switches for control and data synchronization.
Use feature vpc when you first enable vPC on a Nexus switch to allow creation of vPC domains and peer-links.
Platform Notes
On Cisco Nexus switches, the vPC peer-link must be a port-channel; this differs from Cisco IOS where a similar concept (Virtual Switch System) uses a virtual link. In NX-OS, the peer-link is configured under the interface, while in IOS VSS, the peer-link is configured under the switch virtual interface. The 'vpc peer-link' command is specific to NX-OS and is not available on IOS.
For other platforms like Arista, the equivalent is 'mlag' configuration. On Nexus 9000 and 7000 series, the peer-link supports up to 16 physical interfaces. On Nexus 3000 series, vPC is not supported.
Always check the specific Nexus model for feature support.
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