- EtherChannel Overview
- Channel Groups and Port-Channel Interfaces
- Port Aggregation Protocol
- Port Aggregation Protocol Modes
- Silent Mode
- Link Aggregation Control Protocol Modes
- Link Aggregation Control Protocol and Link Redundancy
- Link Aggregation Control Protocol Interaction with Other Features
- Link Aggregation Control Protocol Interaction with Other Features 1:1 Redundancy
- MAC Address Forwarding
- IP Address Forwarding
- Load-Balancing Advantages
- Switch Stack and Port Aggregation Protocol
- Switch Stacks and Link Aggregation Control Protocol
- Layer 2 EtherChannel Configuration Guidelines
- Layer 3 EtherChannel Configuration Guidelines
- Auto-LAG Configuration Guidelines
- Configuring Layer 2 EtherChannels
- Configuring Layer 3 EtherChannels
- (Optional) Configuring EtherChannel Load-Balancing
- (Optional) Configuring EtherChannel Extended Load-Balancing
- (Optional) Configuring the Port Aggregation Protocol Learn Method and Priority
- Configuring Link Aggregation Control Protocol Hot-Standby Ports
- Configuring the LACP Max Bundle
- Configuring Link Aggregation Control Protocol Port-Channel Standalone Disable
- Configuring the Link Aggregation Control Protocol Port Channel Min-Links
- (Optional) Configuring the Link Aggregation Control Protocol System Priority
- (Optional) Configuring the Link Aggregation Control Protocol Port Priority
- Configuring Link Aggregation Control Protocol 1:1 Redundancy
- Example: Configuring Layer 2 EtherChannels
- Example: Configuring Layer 3 EtherChannels
- Example: Configuring Link Aggregation Control Protocol Hot-Standby Ports
- Example: Configuring Link Aggregation Control Protocol 1:1 Redundancy
- Example: Configuring Auto LAG
Configuring EtherChannels
Restrictions for EtherChannels
The following are restrictions for EtherChannels:
- All ports in an EtherChannel must be assigned to the same VLAN or they must be configured as trunk port.
- The LACP 1:1 redundancy feature is supported on port channel interfaces only.
- On EtherChannel member port selection is software based in Layer 2 and Layer 3 multicast route. This means that all multicast traffic under a group will be routed via the same physical port of the EtherChannel. As a result, the distribution of multicast traffic load balance over etherchannels might not evenly spread across member ports.
Information About EtherChannels
The following sections provide information about EtherChannels and the various modes to configure EtherChannels.
EtherChannel Overview
EtherChannel provides fault-tolerant high-speed links between switches, routers, and servers. You can use the EtherChannel to increase the bandwidth between the wiring closets and the data center, and you can deploy it anywhere in the network where bottlenecks are likely to occur. EtherChannel provides automatic recovery for the loss of a link by redistributing the load across the remaining links. If a link fails, EtherChannel redirects traffic from the failed link to the remaining links in the channel without intervention.
An EtherChannel consists of individual Ethernet links that are bundled into a single logical link, and each EtherChannel can consist of up to eight compatibly configured Ethernet ports.
Channel Groups and Port-Channel Interfaces
An EtherChannel comprises a channel group and a port-channel interface. The channel group binds physical ports to the port-channel interface. Configuration changes applied to the port-channel interface apply to all the physical ports bound together in the channel group.
The channel-group command binds the physical port and the port-channel interface together. Each EtherChannel has a port-channel logical interface that is numbered from 1 to 128 . This port-channel interface number corresponds to the one specified with the channel-group interface configuration command.
- With Layer 2 ports, use the channel-group interface configuration command to dynamically create the port-channel interface. You also can use the interface port-channel port-channel-number global configuration command to manually create the port-channel interface, but then you must use the channel-group channel-group-number command to bind the logical interface to a physical port. The channel-group-number can be the same as the port-channel-number, or you can use a new number. If you use a new number, the channel-group command dynamically creates a new port channel.
- With Layer 3 ports, you should manually create the logical interface by using the interface port-channel global configuration command followed by the no switchport interface configuration command. You then manually assign an interface to the EtherChannel by using the channel-group interface configuration command.
- With Layer 3 ports, use the no switchport interface command to configure the interface as a Layer 3 interface, and then use the channel-group interface configuration command to dynamically create the port-channel interface.
Port Aggregation Protocol
The Port Aggregation Protocol (PAgP) is a Cisco-proprietary protocol that can be run only on Cisco devices and on those devices that are licensed by vendors to support PAgP. PAgP facilitates the automatic creation of EtherChannels by exchanging PAgP packets between Ethernet ports. PAgP can be enabled on cross-stack EtherChannels.
By using PAgP, the switch or switch stack learns the identity of partners capable of supporting PAgP and the capabilities of each port. It then dynamically groups similarly configured ports (on a single device in the stack) into a single logical link (channel or aggregate port). Similarly configured ports are grouped based on hardware, administrative, and port parameter constraints. For example, PAgP groups the ports with the same speed, duplex mode, native VLAN, VLAN range, and trunking status and type. After grouping the links into an EtherChannel, PAgP adds the group to the spanning tree as a single device port.
Port Aggregation Protocol Modes
PAgP modes specify whether a port can send PAgP packets, which start PAgP negotiations, or only respond to PAgP packets received.
Places a port into a passive negotiating state, in which the port responds to PAgP packets it receives but does not start PAgP packet negotiation. This setting minimizes the transmission of PAgP packets.
Places a port into an active negotiating state, in which the port starts negotiations with other ports by sending PAgP packets.
Switch ports exchange PAgP packets only with partner ports that are configured in the auto or desirable modes. Ports that are configured in the on mode do not exchange PAgP packets.
Both the auto and desirable modes enable ports to negotiate with partner ports to form an EtherChannel based on criteria such as port speed. and for Layer 2 EtherChannels, based on trunk state and VLAN numbers.
Ports can form an EtherChannel when they are in different PAgP modes as long as the modes are compatible. For example:
- A port in the desirable mode can form an EtherChannel with another port that is in the desirable or auto mode.
- A port in the auto mode can form an EtherChannel with another port in the desirable mode.
A port in the auto mode cannot form an EtherChannel with another port that is also in the auto mode because neither port starts PAgP negotiation.
Silent Mode
If your switch is connected to a partner that is PAgP-capable, you can configure the switch port for nonsilent operation by using the non-silent keyword. If you do not specify non-silent with the auto or desirable mode, silent mode is assumed.
Use the silent mode when the switch is connected to a device that is not PAgP-capable and seldom, if ever, sends packets. An example of a silent partner is a file server or a packet analyzer that is not generating traffic. In this case, running PAgP on a physical port that is connected to a silent partner prevents that switch port from ever becoming operational. However, the silent setting allows PAgP to operate, to attach the port to a channel group, and to use the port for transmission.
Port Aggregation Protocol Learn Method and Priority
Network devices are classified as PAgP physical learners or aggregate-port learners. A device is a physical learner if it learns addresses by physical ports and directs transmissions based on that knowledge. A device is an aggregate-port learner if it learns addresses by aggregate (logical) ports. The learn method must be configured the same at both ends of the link.
When a device and its partner are both aggregate-port learners, they learn the address on the logical port-channel. The device sends packets to the source by using any of the ports in the EtherChannel. With aggregate-port learning, it is not important on which physical port the packet arrives.
PAgP cannot automatically detect when the partner device is a physical learner and when the local device is an aggregate-port learner. Therefore, you must manually set the learning method on the local device to learn addresses by physical ports. You also must set the load-distribution method to source-based distribution, so that any given source MAC address is always sent on the same physical port.
You also can configure a single port within the group for all transmissions and use other ports for hot-standby. The unused ports in the group can be swapped into operation in just a few seconds if the selected single port loses hardware-signal detection. You can configure which port is always selected for packet transmission by changing its priority with the pagp port-priority interface configuration command. The higher the priority, the more likely that the port will be selected.
The device supports address learning only on aggregate ports even though the physical-port keyword is provided in the CLI. The pagp learn-method command and the pagp port-priority command have no effect on the device hardware, but they are required for PAgP interoperability with devices that only support address learning by physical ports, such as the Catalyst 1900 switch.
When the link partner of the device is a physical learner, we recommend that you configure the device as a physical-port learner by using the pagp learn-method physical-port interface configuration command. Set the load-distribution method based on the source MAC address by using the port-channel load-balance src-mac global configuration command. The device then sends packets to the physical learner using the same port in the EtherChannel from which it learned the source address. Only use the pagp learn-method command in this situation.
Port Aggregation Protocol Interaction with Other Features
The Dynamic Trunking Protocol (DTP) and the Cisco Discovery Protocol (CDP) send and receive packets over the physical ports in the EtherChannel. Trunk ports send and receive PAgP protocol data units (PDUs) on the lowest numbered VLAN.
In Layer 2 EtherChannels, the first port in the channel that comes up provides its MAC address to the EtherChannel. If this port is removed from the bundle, one of the remaining ports in the bundle provides its MAC address to the EtherChannel. For Layer 3 EtherChannels, the MAC address is allocated by the active device as soon as the interface is created (through the interface port-channel global configuration command).
PAgP sends and receives PAgP PDUs only from ports that are up and have PAgP enabled for the auto or desirable mode.
Link Aggregation Control Protocol
The LACP is defined in IEEE 802.3ad and enables Cisco devices to manage Ethernet channels between devices that conform to the IEEE 802.3ad protocol. LACP facilitates the automatic creation of EtherChannels by exchanging LACP packets between Ethernet ports.
By using LACP, the switch or switch stack learns the identity of partners capable of supporting LACP and the capabilities of each port. It then dynamically groups similarly configured ports into a single logical link (channel or aggregate port). Similarly configured ports are grouped based on hardware, administrative, and port parameter constraints. For example, LACP groups the ports with the same speed, duplex mode, native VLAN, VLAN range, and trunking status and type. After grouping the links into an EtherChannel, LACP adds the group to the spanning tree as a single device port.
The independent mode behavior of ports in a port channel is changed. With CSCtn96950, by default, standalone mode is enabled. When no response is received from an LACP peer, ports in the port channel are moved to suspended state.
Link Aggregation Control Protocol Modes
LACP modes specify whether a port can send LACP packets or only receive LACP packets.
Places a port into an active negotiating state in which the port starts negotiations with other ports by sending LACP packets.
Places a port into a passive negotiating state in which the port responds to LACP packets that it receives, but does not start LACP packet negotiation. This setting minimizes the transmission of LACP packets.
Both the active and passive LACP modes enable ports to negotiate with partner ports to an EtherChannel based on criteria such as port speed, and for Layer 2 EtherChannels, based on trunk state and VLAN numbers.
Ports can form an EtherChannel when they are in different LACP modes as long as the modes are compatible. For example:
- A port in the active mode can form an EtherChannel with another port that is in the active or passive mode.
- A port in the passive mode cannot form an EtherChannel with another port that is also in the passive mode because neither port starts LACP negotiation.
Link Aggregation Control Protocol and Link Redundancy
LACP port-channel operation, bandwidth availability, and link redundancy can be further refined with the LACP port-channel min-links and the LACP max-bundle features.
The LACP port-channel min-links feature:
- Configures the minimum number of ports that must be linked up and bundled in the LACP port channel.
- Prevents a low-bandwidth LACP port channel from becoming active.
- Causes an LACP port channel to become inactive if there are too few active members ports to supply the required minimum bandwidth.
The LACP max-bundle feature:
- Defines an upper limit on the number of bundled ports in an LACP port channel.
- Allows hot-standby ports with fewer bundled ports. For example, in an LACP port channel with five ports, you can specify a max-bundle of three, and the two remaining ports are designated as hot-standby ports.
Link Aggregation Control Protocol Interaction with Other Features
The DTP and the CDP send and receive packets over the physical ports in the EtherChannel. Trunk ports send and receive LACP PDUs on the lowest numbered VLAN.
In Layer 2 EtherChannels, the first port in the channel that comes up provides its MAC address to the EtherChannel. If this port is removed from the bundle, one of the remaining ports in the bundle provides its MAC address to the EtherChannel. For Layer 3 EtherChannels, the MAC address is allocated by the active device as soon as the interface is created through the interface port-channel global configuration command.
LACP sends and receives LACP PDUs only from ports that are up and have LACP enabled for the active or passive mode.
Link Aggregation Control Protocol Interaction with Other Features 1:1 Redundancy
The LACP 1:1 Redundancy feature supports an EtherChannel configuration with one active link, and fast switchover to a hot-standby link. The link that is connected to the port with the lower port priority number (and therefore, of a higher priority) will be the active link, and the other link will be in a hot-standby state. If the active link goes down, LACP performs a fast switchover to the hot-standby link to keep the EtherChannel up. When the failed link becomes operational again, LACP performs another fast switchover to revert to the original active link.
To allow the higher priority port to stabilize when it becomes active again after a higher-priority to lower-priority switchover, the LACP 1:1 Hot Standby Dampening feature configures a timer that delays switchover back to the higher priority port after higher priority port becomes active.
EtherChannel On Mode
EtherChannel on mode can be used to manually configure an EtherChannel. The on mode forces a port to join an EtherChannel without negotiations. The on mode can be useful if the remote device does not support PAgP or LACP. In the on mode, a usable EtherChannel exists only when the devices at both ends of the link are configured in the on mode.
Ports that are configured in the on mode in the same channel group must have compatible port characteristics, such as speed and duplex. Ports that are not compatible are suspended, even though they are configured in the on mode.
You should use care when using the on mode. This is a manual configuration, and ports on both ends of the EtherChannel must have the same configuration. If the group is misconfigured, packet loss or spanning-tree loops can occur.
Load-Balancing and Forwarding Methods
EtherChannel balances the traffic load across the links in a channel by reducing part of the binary pattern that is formed from the addresses in the frame to a numerical value that selects one of the links in the channel. You can specify one of several different load-balancing modes, including load distribution based on MAC addresses, IP addresses, source addresses, destination addresses, or both source and destination addresses. The selected mode applies to all EtherChannels configured on the device.
Layer 3 Equal-cost multi path (ECMP) load balancing is based on source IP address, destination IP address, source port, destination port, and layer 4 protocol. Fragmented packets will be treated on two different links based on the algorithm that is calculated using these parameters. Any changes in one of these parameters result in load balancing.
MAC Address Forwarding
With source-MAC address forwarding, when packets are forwarded to an EtherChannel, they are distributed across the ports in the channel based on the source-MAC address of the incoming packet. Therefore, to provide load-balancing, packets from different hosts use different ports in the channel, but packets from the same host use the same port in the channel.
With destination-MAC address forwarding, when packets are forwarded to an EtherChannel, they are distributed across the ports in the channel based on the destination host’s MAC address of the incoming packet. Therefore, packets to the same destination are forwarded over the same port, and packets to a different destination are sent on a different port in the channel.
With source-and-destination MAC address forwarding, when packets are forwarded to an EtherChannel, they are distributed across the ports in the channel based on both the source and destination MAC addresses. This forwarding method, a combination source-MAC and destination-MAC address forwarding methods of load distribution, can be used if it is not clear whether source-MAC or destination-MAC address forwarding is better suited on a particular device. With source-and-destination MAC-address forwarding, packets sent from host A to host B, host A to host C, and host C to host B could all use different ports in the channel.
IP Address Forwarding
With source-IP address-based forwarding, packets are distributed across the ports in the EtherChannel based on the source-IP address of the incoming packet. To provide load balancing, packets from different IP addresses use different ports in the channel, and packets from the same IP address use the same port in the channel.
With destination-IP address-based forwarding, packets are distributed across the ports in the EtherChannel based on the destination-IP address of the incoming packet. To provide load balancing, packets from the same IP source address that is sent to different IP destination addresses could be sent on different ports in the channel. Packets sent from different source IP addresses to the same destination IP address are always sent on the same port in the channel.
With source-and-destination IP address-based forwarding, packets are distributed across the ports in the EtherChannel based on both the source and destination IP addresses of the incoming packet. This forwarding method, a combination of source-IP and destination-IP address-based forwarding, can be used if it is not clear whether source-IP or destination-IP address-based forwarding is better suited on a particular device. In this method, packets sent from the IP address A to IP address B, from IP address A to IP address C, and from IP address C to IP address B could all use different ports in the channel.
Load-Balancing Advantages
Different load-balancing methods have different advantages, and the choice of a particular load-balancing method should be based on the position of the device in the network and the kind of traffic that needs to be load-distributed.
Use the option that provides the greatest variety in your configuration. For example, if the traffic on a channel is going only to a single MAC address, using the destination-MAC address always chooses the same link in the channel. Using source addresses or IP addresses might result in better load-balancing.
EtherChannel and Switch Stacks
If a stack member that has ports participating in an EtherChannel fails or leaves the stack, the active switch removes the failed stack member switch ports from the EtherChannel. The remaining ports of the EtherChannel, if any, continue to provide connectivity.
When a switch is added to an existing stack, the new switch receives the running configuration from the active switch and updates itself with the EtherChannel-related stack configuration. The stack member also receives the operational information (the list of ports that are up and are members of a channel).
When two stacks merge that have EtherChannels configured between them, self-looped ports result. Spanning tree detects this condition and acts accordingly. Any PAgP or LACP configuration on a winning switch stack is not affected, but the PAgP or LACP configuration on the losing switch stack is lost after the stack reboots.
Switch Stack and Port Aggregation Protocol
With PAgP, if the active switch fails or leaves the stack, the standby switch becomes the new active switch. The new active switch synchronizes the configuration of the stack members to that of the active switch. The PAgP configuration is not affected after an active switch change unless the EtherChannel has ports residing on the old active switch.
Switch Stacks and Link Aggregation Control Protocol
With LACP, the system ID uses the stack MAC address from the active switch. When an active switch fails or leaves the stack and the standby switch becomes the new active switch, the LACP system ID is unchanged. By default, the LACP configuration is not affected after the active switch changes.
Default EtherChannel Configuration
The default EtherChannel configuration is described in this table.
- Port Aggregation Protocol Modes