A virtual port channel (vPC) allows links that are physically connected to two different Cisco Nexus devices or Cisco Nexus Fabric Extenders to appear as a single port channel by a third device (see the following figure). The third device can be a switch, server, or any other networking device. You can configure vPCs in topologies that include Cisco Nexus devices connected to Cisco Nexus Fabric Extenders. A vPC can provide multipathing, which allows you to create redundancy by enabling multiple parallel paths between nodes and load balancing traffic where alternative paths exist.

Figure 1. vPC Architecture

You configure the EtherChannels by using one of the following:

• No protocol
• Link Aggregation Control Protocol (LACP)

When you configure the EtherChannels in a vPC—including the vPC peer link channel—each switch can have up to 16 active links in a single EtherChannel. When you configure a vPC on a Fabric Extender, only one port is allowed in an EtherChannel.

Note	You must enable the vPC feature before you can configure or run the vPC functionality.

To enable the vPC functionality, you must create a peer-keepalive link and a peer-link under the vPC domain for the two vPC peer switches to provide the vPC functionality.

To create a vPC peer link you configure an EtherChannel on one Cisco Nexus device by using two or more Ethernet ports. On the other switch, you configure another EtherChannel again using two or more Ethernet ports. Connecting these two EtherChannels together creates a vPC peer link.

Note	We recommend that you configure the vPC peer-link EtherChannels as trunks.

The vPC domain includes both vPC peer devices, the vPC peer-keepalive link, the vPC peer link, and all of the EtherChannels in the vPC domain connected to the downstream device. You can have only one vPC domain ID on each vPC peer device.

Note	Always attach all vPC devices using EtherChannels to both vPC peer devices.

A vPC provides the following benefits:

• Allows a single device to use an EtherChannel across two upstream devices
• Eliminates Spanning Tree Protocol (STP) blocked ports
• Provides a loop-free topology
• Uses all available uplink bandwidth
• Provides fast convergence if either the link or a switch fails
• Provides link-level resiliency
• Assures high availability

The terminology used in vPCs is as follows:

• vPC—combined EtherChannel between the vPC peer devices and the downstream device.

• vPC peer device—One of a pair of devices that are connected with the special EtherChannel known as the vPC peer link.

• vPC peer link—link used to synchronize states between the vPC peer devices.

• vPC member port—Interfaces that belong to the vPCs.

• Host vPC port—Fabric Extender host interfaces that belong to a vPC.

• vPC domain—domain that includes both vPC peer devices, the vPC peer-keepalive link, and all of the port channels in the vPC connected to the downstream devices. It is also associated to the configuration mode that you must use to assign vPC global parameters. The vPC domain ID must be the same on both switches.

• vPC peer-keepalive link—The peer-keepalive link monitors the vitality of a vPC peer Cisco Nexus device. The peer-keepalive link sends configurable, periodic keepalive messages between vPC peer devices.

  No data or synchronization traffic moves over the vPC peer-keepalive link; the only traffic on this link is a message that indicates that the originating switch is operating and running vPCs.

The terminology used for the Cisco Nexus Fabric Extender is as follows:

• Fabric interface—A 10-Gigabit Ethernet uplink port designated for connection from the Fabric Extender to its parent switch. A fabric interface cannot be used for any other purpose. It must be directly connected to the parent switch.

• EtherChannel fabric interface—An EtherChannel uplink connection from the Fabric Extender to its parent switch. This connection consists of fabric interfaces bundled into a single logical channel.

• Host interface—An Ethernet interface for server or host connectivity. These ports are 1-Gigabit Ethernet interfaces or 10-Gigabit Ethernet interfaces, depending on the fabric extender model.

• EtherChannel host interface—An EtherChannel downlink connection from the Fabric Extender host interface to a server port.

  Note	An EtherChannel host interface consists of only one host interface and can be configured either as a Link Aggregation Control Protocol (LACP) or non-LACP EtherChannel.

You can connect a server with dual or quad or more network adapters that are configured in a vPC to a pair of Cisco Nexus Fabric Extenders which are connected to the Cisco Nexus devices as depicted. Depending on the FEX model, you may be able to connect one or more network adapter interfaces to each fabric extender. As an example, the following figure refers to a topology built with the Cisco Nexus 2148T fabric extender, where a server has one link only to each fabric extender. A topology with Cisco Nexus 2248TP or with Cisco Nexus 2232PP fabric extender could consist of more links from the server to a single fabric extender.

. The topology that is shown in the following figure provides the vPC functionality to dual homed servers with 1-Gigabit Ethernet uplink interfaces.

Figure 2. Single Homed Fabric Extender vPC Topology

The Cisco Nexus device can support up to 12 configured single homed Fabric Extenders (576 ports) with this topology however only 480 576 dual homed host servers can be configured in a vPCs with this configuration.

Note	The Cisco Nexus 2148T fabric extender does not support EtherChannels on its host interfaces. Therefore a maximum of two links can be configured in an EtherChannel from the server where each link is connected to a separate Fabric Extender.

You can connect the Cisco Nexus Fabric Extender to two upstream Cisco Nexus devices and downstream to a number of single homed servers. The topology shown in the following figure provides the vPC functionality to singly connected servers with 1-Gigabit Ethernet uplink interfaces.

Figure 3. Dual Homed Fabric Extender vPC Topology

The Cisco Nexus device can support up to 12 configured dual homed Fabric Extenders with this topology. A maximum of 576 single homed servers can be connected to this configuration.

To create a vPC domain, you must first create a vPC domain ID on each vPC peer switch using a number from 1 to 1000. This ID must be the same on a set of vPC peer devices.

You can configure the EtherChannels and vPC peer links by using LACP or no protocol. When possible, we recommend that you use LACP on the peer-link, because LACP provides configuration checks against a configuration mismatch on the EtherChannel.

The vPC peer switches use the vPC domain ID that you configure to automatically assign a unique vPC system MAC address. Each vPC domain has a unique MAC address that is used as a unique identifier for the specific vPC-related operations, although the switches use the vPC system MAC addresses only for link-scope operations, such as LACP. We recommend that you create each vPC domain within the contiguous network with a unique domain ID. You can also configure a specific MAC address for the vPC domain, rather than having the Cisco NX-OS software assign the address.

The vPC peer switches use the vPC domain ID that you configure to automatically assign a unique vPC system MAC address. The switches use the vPC system MAC addresses only for link-scope operations, such as LACP or BPDUs. You can also configure a specific MAC address for the vPC domain.

We recommend that you configure the same VPC domain ID on both peers and, the domain ID should be unique in the network. For example, if there are two different VPCs (one in access and one in aggregation) then each vPC should have a unique domain ID.

After you create a vPC domain, the Cisco NX-OS software automatically creates a system priority for the vPC domain. You can also manually configure a specific system priority for the vPC domain.

Note	If you manually configure the system priority, you must ensure that you assign the same priority value on both vPC peer switches. If the vPC peer switches have different system priority values, the vPC will not come up.

The Cisco NX-OS software uses a peer-keepalive link between the vPC peers to transmit periodic, configurable keepalive messages. You must have Layer 3 connectivity between the peer switches to transmit these messages; the system cannot bring up the vPC peer link unless a peer-keepalive link is already up and running.

You can configure a hold-timeout and a timeout value simultaneously.

Hold-timeout value—The hold-timeout value range is between 3 to 10 seconds, with a default value of 3 seconds. This timer starts when the vPC peer link goes down. The purpose of the hold-timeout period is to prevent false-positive cases.

If you configure a hold-timeout value that is lower than the timeout value, then the vPC system ignores vPC peer-keepalive messages for the hold-timeout period and considers messages for the reminder of the timeout period. If no keepalive message is received for this period, the vPC secondary device takes over the role of the primary device. For example, if the hold-timeout value is 3 seconds and the timeout value is 5 seconds, for the first 3 seconds vPC keepalive messages are ignored (such as, when accommodating a supervisor failure for a few seconds after peer link failure) and keepalive messages are considered for the remaining timeout period of 2 seconds. After this period, the vPC secondary device takes over as the primary device, in case there is no keep alive message.

Timeout value—The timeout value range is between 3 to 20 seconds, with a default value of 5 seconds. This timer starts at the end of the hold-timeout interval. If you configure a timeout value that is lower than or equal to the hold-timeout value, then the timeout duration is initiated after the hold-timeout period. For example, if the timeout value is 3 seconds and the hold-timeout value is 5 seconds, the timeout period starts after 5 seconds.

Note	We recommend that you configure the vPC peer-keepalive link on the Cisco Nexus device to run in the management VRF using the mgmt 0 interfaces. If you configure the default VRF, ensure that the vPC peer link is not used to carry the vPC peer-keepalive messages.

Many configuration and operational parameters must be identical on all interfaces in the vPC. After you enable the vPC feature and configure the peer link on both vPC peer switches, Cisco Fabric Services (CFS) messages provide a copy of the configuration on the local vPC peer switch configuration to the remote vPC peer switch. The system then determines whether any of the crucial configuration parameters differ on the two switches.

Enter the show vpc consistency-parameters command to display the configured values on all interfaces in the vPC. The displayed configurations are only those configurations that would limit the vPC peer link and vPC from coming up.

The compatibility check process for vPCs differs from the compatibility check for regular EtherChannels.

A vPC peer link is the link that is used to synchronize the states between the vPC peer devices.

Note	You must configure the peer-keepalive link before you configure the vPC peer link or the peer link will not come up.

Once you have created the vPC domain ID and the vPC peer link, you can create EtherChannels to attach the downstream switch to each vPC peer switch. That is, you create one single EtherChannel on the downstream switch with half of the ports to the primary vPC peer switch and the other half of the ports to the secondary peer switch.

On each vPC peer switch, you assign the same vPC number to the EtherChannel that connects to the downstream switch. You will experience minimal traffic disruption when you are creating vPCs. To simplify the configuration, you can assign the vPC ID number for each EtherChannel to be the same as the EtherChannel itself (that is, vPC ID 10 for EtherChannel 10).

Note	The vPC number that you assign to the EtherChannel that connects to the downstream switch from the vPC peer switch must be identical on both vPC peer switches.

Configure the command line interface by using a track object and a track list that is associated with the Layer 3 link to the core and on all vPC peer links on both vPC peer devices. You use this configuration to avoid dropping traffic if that particular module goes down because when all the tracked objects on the track list go down, the system does the following:

• Stops the vPC primary peer device sending peer-keepalive messages which forces the vPC secondary peer device to take over.

• Brings down all the downstream vPCs on that vPC peer device, which forces all the traffic to be rerouted in the access switch toward the other vPC peer device.

Once you configure this feature and if the module fails, the system automatically suspends all the vPC links on the primary vPC peer device and stops the peer-keepalive messages. This action forces the vPC secondary device to take over the primary role and all the vPC traffic to go to this new vPC primary device until the system stabilizes.

Create a track list that contains all the links to the core and all the vPC peer links as its object. Enable tracking for the specified vPC domain for this track list. Apply this same configuration to the other vPC peer device.

The Link Aggregation Control Protocol (LACP) uses the system MAC address of the vPC domain to form the LACP Aggregation Group (LAG) ID for the vPC.

You can use LACP on all the vPC EtherChannels, including those channels from the downstream switch. We recommend that you configure LACP with active mode on the interfaces on each EtherChannel on the vPC peer switches. This configuration allows you to more easily detect compatibility between switches, unidirectional links, and multihop connections, and provides dynamic reaction to run-time changes and link failures.

The vPC peer link supports 16 EtherChannel interfaces.

Note	When you manually configure the system priority, you must ensure that you assign the same priority value on both vPC peer switches. If the vPC peer switches have different system priority values, vPC does not come up.

Note

During a vPC role change (where secondary becomes primary), if the ports are in LACP-individual state on FEX, the ports will flap and this will cause brief connectivity loss before the primary is up. To avoid this issue, ensure that port-channel member ports are not configured as LACP-individual state or LACP is not used (On mode).

When you first bring up the vPC functionality, STP reconverges. STP treats the vPC peer link as a special link and always includes the vPC peer link in the STP active topology.

We recommend that you set all the vPC peer link interfaces to the STP network port type so that Bridge Assurance is automatically enabled on all vPC peer links. We also recommend that you do not enable any of the STP enhancement features on VPC peer links.

You must configure a list of parameters to be identical on the vPC peer switches on both sides of the vPC peer link.

STP is distributed; that is, the protocol continues running on both vPC peer switches. However, the configuration on the vPC peer switch elected as the primary switch controls the STP process for the vPC interfaces on the secondary vPC peer switch.

The primary vPC switch synchronizes the STP state on the vPC secondary peer switch using Cisco Fabric Services over Ethernet (CFSoE).

The vPC manager performs a proposal/handshake agreement between the vPC peer switches that sets the primary and secondary switches and coordinates the two switches for STP. The primary vPC peer switch then controls the STP protocol for vPC interfaces on both the primary and secondary switches.

The Bridge Protocol Data Units (BPDUs) use the MAC address set for the vPC for the STP bridge ID in the designated bridge ID field. The vPC primary switch sends these BPDUs on the vPC interfaces.

Note	Display the configuration on both sides of the vPC peer link to ensure that the settings are identical. Use the show spanning-tree command to display information about the vPC.

The Cisco Fabric Services over Ethernet (CFSoE) is a reliable state transport mechanism that you can use to synchronize the actions of the vPC peer devices. CFSoE carries messages and packets for many features linked with vPC, such as STP and IGMP. Information is carried in CFS/CFSoE protocol data units (PDUs).

When you enable the vPC feature, the device automatically enables CFSoE, and you do not have to configure anything. CFSoE distributions for vPCs do not need the capabilities to distribute over IP or the CFS regions. You do not need to configure anything for the CFSoE feature to work correctly on vPCs.

You can use the show mac address-table command to display the MAC addresses that CFSoE synchronizes for the vPC peer link.

Note	Do not enter the no cfs eth distribute or the no cfs distribute command. CFSoE must be enabled for vPC functionality. If you do enter either of these commands when vPC is enabled, the system displays an error message.

When you enter the show cfs application command, the output displays "Physical-eth," which shows the applications that are using CFSoE.

vPC has the following configuration guidelines and limitations:

• You must enable the vPC feature before you can configure vPC peer-link and vPC interfaces.

• You must configure the peer-keepalive link before the system can form the vPC peer link.

• The vPC peer-link needs to be formed using a minimum of two 10-Gigabit Ethernet interfaces.

• You can connect a pair of Cisco Nexus 5000 Series switches or a pair of Cisco Nexus 5500 Series switches in a vPC directly to another switch or to a server. vPC peer switches must be of the same type, for example, you can connect a pair of Nexus 5000 series switches or a pair of Nexus 5500 Series switches but you cannot connect a Nexus 5000 Series switch to a Nexus 5500 Series switch in a vPC topology.

• Only port channels can be in vPCs. A vPC can be configured on a normal port channel (switch-to-switch vPC topology), on a port channel fabric interface (fabric extender vPC topology), and on a port channel host interface (host interface vPC topology).

• A Fabric Extender can be a member of a Host Interface vPC topology or a Fabric Extender vPC topology but not both simultaneously.

• You must configure both vPC peer switches; the configuration is not automatically synchronized between the vPC peer devices.

• Check that the necessary configuration parameters are compatible on both sides of the vPC peer link.

• You may experience minimal traffic disruption while configuring vPCs.

• You should configure all the port channels in the vPC using LACP with the interfaces in active mode.

• When the peer-switch command is configured and vPC keepalive messages exchanged through an SVI instead of a management interface, additional Spanning Tree Protocol (STP) configuration is required. STP needs to be disabled on the dedicated link that carries the keepalive traffic between the vPC peers. You can disable STP on the dedicated link by configuring STP BPDUfilter on the both ends of the dedicated link. We recommend that the VLAN of the vPC keepalive SVI be allowed on only the interconnecting dedicated link and disallowed on all other links, including the peer link.

• You cannot have a link for non-vPC traffic in parallel with a vPC topology. This can cause errors with the traffic forwarding logic resulting in duplicate or missed packets.

• You cannot configure non-vPC interfaces across host ports on two different FEXs.

You must enable the vPC feature before you can configure and use vPCs.

You can disable the vPC feature.

Note	When you disable the vPC feature, the Cisco Nexus device clears all the vPC configurations.

You must create identical vPC domain IDs on both the vPC peer devices. This domain ID is used to automatically form the vPC system MAC address.

You can configure the destination IP for the peer-keepalive link that carries the keepalive messages. Optionally, you can configure other parameters for the keepalive messages.

The Cisco NX-OS software uses the peer-keepalive link between the vPC peers to transmit periodic, configurable keepalive messages. You must have Layer 3 connectivity between the peer devices to transmit these messages. The system cannot bring up the vPC peer link unless the peer-keepalive link is already up and running.

Ensure that both the source and destination IP addresses used for the peer-keepalive message are unique in your network and these IP addresses are reachable from the Virtual Routing and Forwarding (VRF) instance associated with the vPC peer-keepalive link.

Note

We recommend that you configure a separate VRF instance and put a Layer 3 port from each vPC peer switch into that VRF instance for the vPC peer-keepalive link. Do not use the peer link itself to send vPC peer-keepalive messages. For information on creating and configuring VRFs, see the Unicast Routing Configuration Guide for your device.

You can create a vPC peer link by designating the EtherChannel that you want on each switch as the peer link for the specified vPC domain. We recommend that you configure the EtherChannels that you are designating as the vPC peer link in trunk mode and that you use two ports on separate modules on each vPC peer switch for redundancy.

Beginning with Cisco NX-OS Release 5.0(3)N1(1), youYou can configure a restore timer that delays the vPC from coming back up until after the peer adjacency forms and the VLAN interfaces are back up. This feature avoids packet drops if the routing tables fail to converge before the vPC is once again passing traffic.

When a vPC peer-link is lost, the vPC secondary switch suspends its vPC member ports and its switch virtual interface (SVI) interfaces. All Layer 3 forwarding is disabled for all VLANs on the vPC secondary switch. You can exclude specific SVI interfaces so that they are not suspended.

The switch services, such as ping, ssh, telnet, radius, are VRF aware. You must configure the VRF name in order for the correct routing table to be used.

You can specify the VRF name.

You can bind a VRF instance to a vPC. One reserved VLAN is required for each VRF. Without this command, the receivers in a non-VPC VLAN and the receivers connected to a Layer 3 interface might not receive multicast traffic. The non-vPC VLANs are the VLANs that are not trunked over a peerlink.

The vPC peer-gateway feature allows a vPC switch to act as the active gateway for packets that are addressed to the router MAC address of the vPC peer. You can enable local forwarding without the need to cross the vPC peer-link. In this scenario, the feature optimizes use of the peer-link and avoids potential traffic loss.

You can enable Layer 3 forwarding for packets destined to the gateway MAC address of the virtual Port Channel (vPC).

Note	You must configure this feature on both vPC peer switches.

You can suspend a non-virtual port channel (vPC) port when a vPC secondary peer link goes down. A non-vPC port, also known as an orphaned port, is a port that is not part of a vPC.

Note	When a port is configured as an orphan port, the port will flap. This occurs because the system reevaluates whether the port can be brought up, given the constraints of the orphan port. For example, MCT needs to be up and election needs to be complete.

To connect to a downstream server from a Cisco Nexus Fabric Extender you can create a EtherChannel host interface. An EtherChannel host interface can have only one host interface as a member depending on the fabric extender model. The Cisco Nexus 2148T allows only one interface member per fabric extender, newer fabric extenders allow up to 8 members of the same port-channel on a single fabric extender. You need to create an EtherChannel host interface to configure a vPC on it that uses the Fabric Extender topology.

When you create a vPC domain, the system automatically creates a vPC system priority. However, you can also manually configure a system priority for the vPC domain.

By default, the Cisco NX-OS software elects a primary and secondary vPC peer switch after you configure the vPC domain and both sides of the vPC peer link. However, you may want to elect a specific vPC peer switch as the primary switch for the vPC. Then, you would manually configure the role value for the vPC peer switch that you want as the primary switch to be lower than the other vPC peer switch.

vPC does not support role preemption. If the primary vPC peer switch fails, the secondary vPC peer switch takes over to become operationally the vPC primary switch. However, the original operational roles are not restored when the formerly primary vPC comes up again.

You can configure a hybrid vPC and non-vPC peer switch topology by using the spanning-tree pseudo-information command to change the designated bridge ID so that it meets the STP VLAN-based load-balancing criteria and then change the root bridge ID priority to a value that is better than the best bridge priority. You then enable the peer switch. For more information, see the command reference for your device.

Note	If you previously configured global spanning tree parameters and you subsequently configure spanning tree pseudo information parameters, be aware that the pseudo information parameters take precedence over the global parameters.

Use the following commands to display vPC configuration information:

For information about the switch output, see the Command Reference for your Cisco Nexus Series switch.

The following example shows how to configure the dual homed Fabric Extender vPC topology using the management VRF to carry the peer-keepalive messages on switch CiscoNexus-1 as shown in following figure:

Figure 4. vPC Configuration Example

The following example shows how to configure the single homed Fabric Extender vPC topology using the default VRF to carry the peer-keepalive messages on switch CiscoNexus-1 as shown in following figure:

Figure 5. vPC Configuration Example

Note	The following example only shows the configuration of CiscoNexus-1 which is connected to the Fabric Extender NX-2000-100. You must repeat these steps on its vPC peer, CiscoNexus-2, which is connected to the Fabric Extender NX-2000-101.

The following table lists the default settings for vPC parameters.