Using port channels, Cisco NX-OS provides wider bandwidth, redundancy, and load balancing across the channels.

You can collect up to 16 ports into a static port channel or you can enable the Link Aggregation Control Protocol (LACP). Configuring port channels with LACP requires slightly different steps than configuring static port channels.

Note	Cisco NX-OS does not support Port Aggregation Protocol (PAgP) for port channels.

A port channel bundles individual links into a channel group to create a single logical link that provides the aggregate bandwidth of up to 16 physical links. If a member port within a port channel fails, traffic previously carried over the failed link switches to the remaining member ports within the port channel.

Each port can be in only one port channel. All the ports in an port channel must be compatible; they must use the same speed and operate in full-duplex mode. When you are running static port channels, without LACP, the individual links are all in the on channel mode; you cannot change this mode without enabling LACP.

Note	You cannot change the mode from ON to Active or from ON to Passive.

You can create a port channel directly by creating the port-channel interface, or you can create a channel group that acts to aggregate individual ports into a bundle. When you associate an interface with a channel group, Cisco NX-OS creates a matching port channel automatically if the port channel does not already exist. You can also create the port channel first. In this instance, Cisco NX-OS creates an empty channel group with the same channel number as the port channel and takes the default configuration.

Note	A port channel is operationally up when at least one of the member ports is up and that port’s status is channeling. The port channel is operationally down when all member ports are operationally down.

When you add an interface to a port channel group, Cisco NX-OS checks certain interface attributes to ensure that the interface is compatible with the channel group. Cisco NX-OS also checks a number of operational attributes for an interface before allowing that interface to participate in the port-channel aggregation.

The compatibility check includes the following operational attributes:

• Port mode
• Access VLAN
• Trunk native VLAN
• Allowed VLAN list
• Speed
• 802.3x flow control setting
• MTU
• Broadcast/Unicast/Multicast Storm Control setting
• Priority-Flow-Control
• Untagged CoS

Use the show port-channel compatibility-parameters command to see the full list of compatibility checks that Cisco NX-OS uses.

You can only add interfaces configured with the channel mode set to on to static port channels. You can also only add interfaces configured with the channel mode as active or passive to port channels that are running LACP. You can configure these attributes on an individual member port.

When the interface joins a port channel, the following individual parameters are replaced with the values on the port channel:

• Bandwidth
• MAC address
• Spanning Tree Protocol

The following interface parameters remain unaffected when the interface joins a port channel:

• Description
• CDP
• LACP port priority
• Debounce

After you enable forcing a port to be added to a channel group by entering the channel-group force command, the following two conditions occur:

• When an interface joins a port channel the following parameters are removed and they are operationally replaced with the values on the port channel; however, this change will not be reflected in the running-configuration for the interface:

  • QoS
  • Bandwidth
  • Delay
  • STP
  • Service policy
  • ACLs
• When an interface joins or leaves a port channel, the following parameters remain unaffected:

  • Beacon
  • Description
  • CDP
  • LACP port priority
  • Debounce
  • UDLD
  • Shutdown
  • SNMP traps

Cisco NX-OS load balances traffic across all operational interfaces in a port channel by reducing part of the binary pattern formed from the addresses in the frame to a numerical value that selects one of the links in the channel. Port channels provide load balancing by default and the basic configuration uses the following criteria to select the link:

• For a Layer 2 frame, it uses the source and destination MAC addresses.
• For a Layer 3 frame, it uses the source and destination MAC addresses and the source and destination IP addresses.
• For a Layer 4 frame, it uses the source and destination MAC addresses and the source and destination IP addresses.

  Note	You have the option to include the source and destination port number for the Layer 4 frame.

You can configure the switch to use one of the following methods to load balance across the port channel:

• Destination MAC address
• Source MAC address
• Source and destination MAC address
• Destination IP address
• Source IP address
• Source and destination IP address
• Destination TCP/UDP port number
• Source TCP/UDP port number
• Source and destination TCP/UDP port number

Use the option that provides the balance criteria with the greatest variety in your configuration. For example, if the traffic on a port channel is going only to a single MAC address and you use the destination MAC address as the basis of port-channel load balancing, the port channel always chooses the same link in that port channel; using source addresses or IP addresses might result in better load balancing.

The following figure shows how individual links can be combined into LACP port channels and channel groups as well as function as individual links.

Figure 1. Individual Links Combined into a Port channel

With LACP, just like with static port-channels, you can bundle up to 16 interfaces in a channel group.

Note	When you delete the port channel, Cisco NX-OS automatically deletes the associated channel group. All member interfaces revert to their previous configuration.

You cannot disable LACP while any LACP configurations are present.

LACP uses the following parameters:

• LACP system priority—Each system that runs LACP has an LACP system priority value. You can accept the default value of 32768 for this parameter, or you can configure a value between 1 and 65535. LACP uses the system priority with the MAC address to form the system ID and also uses the system priority during negotiation with other devices. A higher system priority value means a lower priority.

Note	The LACP system ID is the combination of the LACP system priority value and the MAC address.

• LACP port priority—Each port configured to use LACP has an LACP port priority. You can accept the default value of 32768 for the LACP port priority, or you can configure a value between 1 and 65535. LACP uses the port priority with the port number to form the port identifier. LACP uses the port priority to decide which ports should be put in standby mode when there is a limitation that prevents all compatible ports from aggregating and which ports should be put into active mode. A higher port priority value means a lower priority for LACP. You can configure the port priority so that specified ports have a lower priority for LACP and are most likely to be chosen as active links, rather than hot-standby links.
• LACP administrative key—LACP automatically configures an administrative key value equal to the channel-group number on each port configured to use LACP. The administrative key defines the ability of a port to aggregate with other ports. A port’s ability to aggregate with other ports is determined by these factors:
  • Port physical characteristics, such as the data rate, the duplex capability, and the point-to-point or shared medium state
  • Configuration restrictions that you establish

Individual interfaces in port channels are configured with channel modes. When you run static port channels, with no protocol, the channel mode is always set to on. After you enable LACP globally on the device, you enable LACP for each channel by setting the channel mode for each interface to active or passive. You can configure either channel mode for individual links in the LACP channel group.

Note	You must enable LACP globally before you can configure an interface in either the active or passive channel mode.

The following table describes the channel modes.

Both the passive and active modes allow LACP to negotiate between ports to determine if they can form a port channel, based on criteria such as the port speed and the trunking state. The passive mode is useful when you do not know whether the remote system, or partner, supports LACP.

Ports can form an LACP port channel when they are in different LACP modes as long as the modes are compatible as in the following examples:

• A port in active mode can form a port channel successfully with another port that is in active mode.
• A port in active mode can form a port channel with another port in passive mode.
• A port in passive mode cannot form a port channel with another port that is also in passive mode because neither port will initiate negotiation.
• A port in on mode is not running LACP.

Using port channels, data traffic may be dynamically redistributed due to either a link failure or load balancing. LACP uses the Marker Protocol to ensure that frames are not duplicated or reordered because of this redistribution. Cisco NX-OS supports only Marker Responders.

The following table provides a brief summary of major differences between port channels with LACP enabled and static port channels.