Ether Channel Overview
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Cisco CCIE Prep v1.0—MoCule 5-38
An EtherChannel consists of individual Fast Ethernet or Gigabit Ethernet links bundled into a single logical link. The EtherChannel provides full-duplex bandwidth up to 800 Mbps (Fast EtherChannel) or 8 Gbps (Gigabit EtherChannel) between your switch and another switch or host. Each EtherChannel can consist of up to eight compatibly configured Ethernet interfaces. All interfaces in each EtherChannel must be the same speed, and all must be configured as either Layer 2 or Layer 3 interfaces.
Etherchannel can be deployed anywhere in the network where bottlenecks are likely to occur. A common example is deploying Etherchannel between the wiring closets and the data center to increase bandwidth to support the aggregate bandwidth requirements of clients. EtherChannel also 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.
On the Catalyst 3550 Etherchannels can be created on both Layer 2 (switch ports) and Layer 3 (router ports) interfaces. The configuration differs between the however both configurations involve logical interfaces.
■ With Layer 3 interfaces, you manually create the logical interface by using the interface port-channel global configuration command.
With Layer 2 interfaces, the logical interface is dynamically created.
■ With both Layer 3 and 2 interfaces, you manually assign an interface to the EtherChannel by using the channel-group interface configuration command. This command binds the physical and logical ports together.
Each EtherChannel has a logical port-channel interface numbered from 1 to 64. The channel groups are also numbered from 1 to 64.
After you configure an EtherChannel, configuration changes applied to the port-channel interface apply to all the physical interfaces assigned to the port-channel interface. Configuration changes applied to the physical interface affect only the interface where you apply the configuration. To change the parameters of all ports in an EtherChannel, apply configuration commands to the port-channel interface.
Port Aggregation Protocol (PAgP)
© 2002, Cisco Systems, Inc. All rights reserved. Cisco CCIE Prep v1.0—Module 5-39
The Port Aggregation Protocol (PAgP) facilitates the automatic creation of EtherChannels by exchanging packets between Ethernet interfaces. By using PAgP, the switch learns the identity of partners capable of supporting PAgP and learns the capabilities of each interface. It then dynamically groups similarly configured interfaces into a single logical link (channel or aggregate port); these interfaces are grouped based on hardware, administrative, and port parameter constraints. For example, PAgP groups the interfaces 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 switch port.
PAgP Modes
The table below shows the user-configurable EtherChannel modes for the channel-group interface configuration command: on, auto, and desirable. Switch interfaces exchange PAgP packets only with partner interfaces configured in the auto or desirable modes; interfaces configured in the on mode do not exchange PAgP packets.
|
EtherChannel Moded |
Description |
|
auto |
Places an interface into a passive negotiating state, in which the interface responds to PAgP packets it receives but does not start PAgP packet negotiation. This setting minimizes the transmission of PAgP packets. |
|
desirable |
Places an interface into an active negotiating state, in which the interface starts 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. |
Both the auto and desirable modes allow interfaces to negotiate with partner interfaces to determine if they can form an EtherChannel based on criteria such as interface speed and, for Layer 2 EtherChannels, trunking state and VLAN numbers.
Interfaces can form an EtherChannel when they are in different PAgP modes as long as the modes are compatible. For example:
An interface in the desirable mode can form an EtherChannel with another interface that is in the desirable or auto mode.
An interface in the auto mode can form an EtherChannel with another interface in the desirable mode.
An interface in the auto mode cannot form an EtherChannel with another interface that is also in the auto mode because neither interface starts PAgP negotiation.
An interface in the on mode that is added to a port channel is forced to have the same characteristics as the already existing on mode interfaces in the channel.
Note You should exercise care when setting the mode to on (manual configuration). All ports configured in the on mode are bundled in the same group and are forced to have similar characteristics. If the group is misconfigured, packet loss or spanning-tree loops might occur.
If your switch is connected to a partner that is PAgP-capable, you can configure the switch interface 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.
The silent mode is used 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 connected to a silent partner prevents that switch port from ever becoming operational; however, the silent setting allows PAgP to operate, to attach the interface to a channel group, and to use the interface for transmission.
3550(config)# int faO/18
3550(config-if)# switchport trunk encapsulation 3550(config-if)# switchport mode trunk 3550(config-if)# channel-group 1 mode auto
Creating a port-channel interface Port-channell 3550(config-if)# int faO/19
3550(config-if)# switchport trunk encapsulation 3550(config-if)# switchport mode trunk 3550(config-if)# channel-group 1 mode auto
3550(config-if)# int faO/2O
3550(config-if)# switchport trunk encapsulation 3550(config-if)# switchport mode trunk 3550(config-if)# channel-group 1 mode auto
3550(config-if)# int faO/19
3550(config-if)# switchport trunk encapsulation 3550(config-if)# switchport mode trunk 3550(config-if)# channel-group 1 mode auto
3550(config-if)# end 3550(config)#
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Cisco CCIE Prep v1.0—Module 5-41
You configure Layer 2 EtherChannels by configuring the Ethernet interfaces with the channel-group interface configuration command, which creates the port-channel logical interface.
Use the steps outlined in the following table to assign a Layer 2 Ethernet interface to a Layer 2 EtherChannel:
|
Command |
Purpose |
|
3550(config)# interface interface-id |
Enters interface configuration mode and specifies a physical interface to configure. Up to eight interfaces of the same type and speed can be configured for the same group. |
|
3550(config-if)# switchport mode {access | trunk} 3550(config-if)# switchport access vlan vlan-id |
Assigns the interface as a static-access port in one VLAN or configures it as a trunk. If you configure the interface as a static-access port, assign it to only one VLAN. The range is 1 to 4094. |
|
3550(config-if)# channel-group channel-group-number mode {auto [non-silent] | desirable [non-silent] | on} |
Assigns the interface to a channel group, and specifies the PAgP mode. The default mode is auto silent. For channel-group-number, the range is 1 to 64. Each EtherChannel can have of up to eight compatibly configured Ethernet interfaces. |
To remove an interface from the EtherChannel group, use the no channel-group interface
To remove an interface from the EtherChannel group, use the no channel-group interface configuration command.
Continue reading here: Configuring Layer 3 Ether Channels
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