Configuring Ether Channel

Default EtherChannel Configuration

Table 25-2 Default EtherChannel Configuration 

Feature	Default Setting
Channel groups	None assigned.
PAgP mode	No default.
PAgP learn method	Aggregate-port learning on all interfaces.
PAgP priority	128 on all interfaces. (Changing this value on Catalyst 2950 switches has no effect.)
Load balancing	Load distribution on the switch is based on the source-MAC address of the incoming packet.

EtherChannel Configuration Guidelines

If improperly configured, some EtherChannel interfaces are automatically disabled to avoid network loops and other problems. Follow these guidelines to avoid configuration problems:

•Each EtherChannel can have up to eight compatibly configured Ethernet interfaces.

•Configure all interfaces in an EtherChannel to operate at the same speeds and duplex modes.

•Enable all interfaces in an EtherChannel. If you shut down an interface in an EtherChannel, it is treated as a link failure, and its traffic is transferred to one of the remaining interfaces in the EtherChannel.

•When a group is first created, all ports follow the parameters set for the first port to be added to the group. If you change the configuration of one of these parameters, you must also make the changes to all ports in the group:

–Allowed-VLAN list

–STP path cost for each VLAN

–STP port priority for each VLAN

–STP Port Fast setting

•If you configure SPAN on a port that is a member of the EtherChannel, it leaves the EtherChannel.

•For EtherChannels:

–Assign all interfaces in the EtherChannel to the same VLAN, or configure them as trunks.

–If you configure an EtherChannel from trunk interfaces, verify that the trunking mode (802.1Q) is the same on all the trunks. Inconsistent trunk modes on EtherChannel interfaces can have unexpected results.

–An EtherChannel supports the same allowed range of VLANs on all the interfaces in a trunking Layer 2 EtherChannel. If the allowed range of VLANs is not the same, the interfaces do not form an EtherChannel even when PAgP is set to the auto or desirable mode.

–Interfaces with different STP path costs can form an EtherChannel as long they are otherwise compatibly configured. Setting different STP path costs does not, by itself, make interfaces incompatible for the formation of an EtherChannel.

Configuring EtherChannels

You configure Layer 2 EtherChannels by configuring the Ethernet interfaces with the channel-group interface configuration command, which creates the port-channel logical interface.

Beginning in privileged EXEC mode, follow these steps to assign a Layer 2 Ethernet interface to a Layer 2 EtherChannel:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	interfaceinterface-id	Enter interface configuration mode, and specify a physical interface to configure. Valid interfaces include physical interfaces. Up to eight interfaces of the same type and speed can be configured for the same group.
Step 3	channel-groupchannel-group-numbermode {auto[non-silent] |desirable[non-silent] |on}	Assign the interface to a channel group, and specify the PAgP mode. The default mode is auto silent. For channel-group-number, the range is 1 to 6. Each EtherChannel can have up to eight compatibly configured Ethernet interfaces. For mode, select one of these keywords: •auto—Enables PAgP only if a PAgP device is detected. It places an interface into a passive negotiating state, in which the interface responds to PAgP packets it receives but does not initiate PAgP packet negotiation. •desirable—Unconditionally enables PAgP. It places an interface into an active negotiating state, in which the interface initiates negotiations with other interfaces by sending PAgP packets. •on—Forces the interface to channel without PAgP. With the on mode, a usable EtherChannel exists only when an interface group in the on mode is connected to another interface group in the on mode. •non-silent—If your switch is connected to a partner that is PAgP-capable, you can configure the switch interface for non-silent operation. You can configure an interface with the non-silent keyword for use with the auto or desirable mode. If you do not specify non-silent with the auto or desirable mode, silent is assumed. The silent setting is for connections to file servers or packet analyzers; this setting allows PAgP to operate, to attach the interface to a channel group, and to use the interface for transmission. For information on compatible PAgP modes for the switch and its partner.
Step 4	end	Return to privileged EXEC mode.
Step 5	show running-config	Verify your entries.
Step 6	copy running-config startup-config	(Optional) Save your entries in the configuration file.

If you delete the EtherChannel is by using the no interface port-channel global configuration command without removing the physical interfaces, the physical interfaces are shutdown. If you do not want the member physical interfaces to shut down, remove the physical interfaces before deleting the EtherChannel.

To remove an interface from the EtherChannel group, use the no channel-group interface configuration command.

This example shows how to assign Gigabit Ethernet interfaces 0/1 and 0/2 with PAgP mode desirable:

Switch# configure terminal 

Switch(config)# interface range gigabitethernet0/1 -2 

Switch(config-if)# channel-group 5 mode desirable 

Switch(config-if)# end 

Configuring EtherChannel Load Balancing

This section describes how to configure EtherChannel load balancing by using source-based or destination-based forwarding methods. For more information

Beginning in privileged EXEC mode, follow these steps to configure EtherChannel load balancing:

	Command	Purpose
Step 1	configure terminal	Enter global configuration mode.
Step 2	port-channel load-balance{dst-mac | src-mac}	Configure an EtherChannel load-balancing method. The default is src-mac. Select one of these keywords to determine the load-distribution method: •dst-mac—Load distribution is based on the destination-host MAC address of the incoming packet. Packets to the same destination are sent on the same port, but packets to different destinations are sent on different ports in the channel. •src-mac—Load distribution is based on the source-MAC address of the incoming packet. Packets from different hosts use different ports in the channel, but packets from the same host use the same port in the channel. If the link partner to the switch is a physical learner, set the load-distribution method to one of these ways: •If the channel-group interface configuration command is set to auto or desirable, the switch automatically uses the load distribution method based on the source-MAC address, regardless of the configured load-distribution method. •If the channel-group interface configuration command is set to on, set the load-distribution method based on the source-MAC address by using the port-channel load-balance src-mac global configuration command.
Step 3	end	Return to privileged EXEC mode.
Step 4	show etherchannel load-balance	Verify your entries.
Step 5	copy running-config startup-config	(Optional) Save your entries in the configuration file.

To return EtherChannel load balancing to the default configuration, use the no port-channel load-balance global configuration command.

Configuring the PAgP 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 ports.

For compatibility with Catalyst 1900 series switches, configure the Catalyst 2950 switches for source-MAC load distribution.

The Catalyst 2950 supports address learning only on aggregate ports even though the physical-port keyword is provided in the command-line interface (CLI). The pagp learn-method and the pagp port-priority interface configuration command have no effect on the switch hardware.

If the link partner to the switch is a physical learner that has the channel-group interface configuration command set to auto or desirable, the switch automatically uses the load-distribution method based on the source MAC address, regardless of the configured load distribution method.

If the link partner to the Catalyst 2950 switch is a physical learner that has the channel-group interface configuration command set to on, set the load-distribution method based on the source MAC address by using the port-channel load-balance src-mac global configuration command.

Displaying EtherChannel and PAgP Status

You can use the user EXEC commands described in Table 25-3 to display EtherChannel and PAgP status information:

Table 25-3 Commands for Displaying EtherChannel and PAgP Status 

Command	Description
show etherchannel [channel-group-number] {brief | detail | load-balance | port |port-channel | summary}	Displays EtherChannel information in a brief, detailed, and one-line summary form. Also displays the load-balance or frame-distribution scheme, port, and port-channel information.
show pagp {channel-group-number} {counters | internal | neighbor}1	Displays PAgP information such as traffic information, the internal PAgP configuration, and neighbor information.