Understanding the EtherChannel Port-Channel Interfaces Port Aggregation Protocol

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The EtherChannel is composed of individual Fast Ethernet or Gigabit Ethernet links bundled into a

single logical link as shown in Figure 10-1. The EtherChannel provides full-duplex bandwidth up to

800 Mbps (Fast EtherChannel) or 2 Gbps (Gigabit EtherChannel) between your switch and another

switch or host.

Note The network device to which your switch is connected can impose its own limits on the number of

interfaces in the EtherChannel. For Catalyst 2950 switches, the number of EtherChannels is limited to

six with eight ports per EtherChannel.

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 Layer 2 interfaces.

If a link within an EtherChannel fails, traffic previously carried over that failed link changes to the

remaining links within the EtherChannel. A trap is sent for a failure, identifying the switch, the

EtherChannel, and the failed link. Inbound broadcast and multicast packets on one link in an

EtherChannel are blocked from returning on any other link of the EtherChannel.

Understanding Port-Channel Interfaces

When you create an EtherChannel for Layer 2 interfaces, a logical interface is dynamically created. You

then manually assign an interface to the EtherChannel by using the channel-group interface

configuration command as shown in Figure 10-2.

Each EtherChannel has a logical port-channel interface numbered from 1 to 6.

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, for

example, Spanning Tree Protocol (STP) commands or commands to configure a Layer 2 EtherChannel

as a trunk.

Understanding the Port Aggregation Protocol

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

Table 10-1 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.

Table 10-1 EtherChannel Modes

Mode Description

auto

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. This setting minimizes

the transmission of PAgP packets.

desirable

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.

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 desirable mode can form an EtherChannel with another interface that is in desirable

or auto mode.

• An interface in auto mode can form an EtherChannel with another interface in desirable mode.

• An interface in auto mode cannot form an EtherChannel with another interface that is also in auto

mode because neither interface initiates 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.

Caution: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 STP 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, transmits 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.

Physical Learners and Aggregate-Port Learners

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 learning.

A device is an aggregate-port learner if it learns addresses by aggregate (logical) ports.

When a device and its partner are both aggregate-port learners, they learn the address on the logical

port-channel. The device transmits packets to the source by using any of the interfaces in the

EtherChannel. With aggregate-port learning, it is not important on which physical port the packet

arrives.

The Catalyst 2950 switch uses source-MAC address distribution for a channel if it is connected to a

physical learner even if the user configures destination-MAC address distribution.

These frame distribution mechanisms are possible for frame transmission:

• Port selection based on the source-MAC address of the packet

• Port selection based on the destination- MAC address of the packet

Catalyst 2950 switches support a maximum of eight ports to a PAgP group.

PAgP Interaction with Other Features

The Dynamic Trunking Protocol (DTP) and Cisco Discovery Protocol (CDP) send and receive packets

over the physical interfaces in the EtherChannel. Trunk ports send and receive PAgP protocol data units

(PDUs) on the lowest numbered VLAN.

STP sends packets over a single physical interface in the EtherChannel. Spanning tree regards the

EtherChannel as one port.

PAgP sends and receives PAgP PDUs only from interfaces that are up and have PAgP enabled for auto

or desirable modes.

Understanding Load Balancing and Forwarding Methods

EtherChannel balances the traffic load across the links in a 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.

EtherChannel load balancing can use either source-MAC or destination-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 (and the MAC address learned by the switch

does not change).

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. You configure the load balancing and

forwarding method by using the port-channel load-balance global configuration command.

In Figure 10-3, an EtherChannel of four workstations communicates with a router. Because the router is

a single-MAC-address device, source-based forwarding on the switch EtherChannel ensures that the

switch uses all available bandwidth to the router. The router is configured for destination-based

forwarding because the large number of workstations ensures that the traffic is evenly distributed from

the router EtherChannel.

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 might result in better load balancing.