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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 27 Configuring Eth erChannels Understanding EtherChannels

Physical Learners and Aggregate-Port Learners

Network devices are cla ssified as PAgP physi cal l earn ers o r agg regate- port le ar ners. A d evice is a
physical learn er i f i t lea rns a ddre sses by physic al po rt s and dir ects t rans missi ons ba sed o n that
knowledge. A device is an aggregate-port learner if it learns addresses by aggregate (logical) ports.
When a device and its pa rtner ar e both aggregate-por t lear ners, they lear n the ad dress on the logica l
port-channel. The device sends packets to the source by using any of the interfaces in the EtherChannel.
With aggre ga te- port lea rni ng, i t i s not impo rt ant on whi ch phy sica l po rt t he pa ck et a rri ves.
The switc h uses sourc e-MAC address distribution fo r a chan nel if it is conn ected to a physical l earner
even if the user configur es destinati on-MAC address distribution.
These frame distribution mechanisms are possible for frame transmission:
•Port selection based on the source-MAC address of the packet
•Port selectio n based on the destina tion- MAC address of the packet
The switc h supports up t o eight po rts in a PAgP group.

PAgP Interaction with Other Features

The Dyna mic Trunking Protocol (DT P) and Cisco Discovery Protocol (CD P) send and re ceive packets
over the physical interfaces in the EtherChannel. Trunk ports send and receiv e PAgP protocol data units
(PDUs) on the lowest numbered VLAN.
Spanning tree sends packets over a single physical interface in the EtherChannel. Spanning tree regards
the Ether Channel as one port.
PAgP sen ds and rece ives PAgP PDUs onl y from interfaces that are up and have PAgP en abled fo r the
auto or desirable mode.

Understanding Load Balancing and Forwarding Methods

EtherChannel balances the traf fic 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.
EtherC hannel loa d balancing can use eit her sourc e-MAC or destination -MAC address fo rwarding.
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.
There for e, to provid e lo ad b ala nc in g, p acket s f rom d iff er en t ho st s u s e d iff er ent por ts in the ch a nnel, but
pack ets from the same hos t use the sam e port in th e channel ( and the MA C address le arne d by the switch
does not ch ange).
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 desti natio n are sent on a different po rt in the chann el. You configur e the load ba lancing and
forwarding m eth od by u sing th e port-channel load-balance global configuratio n command .
In Figure 27-3, an EtherChannel of four workstations communicates with a router. Because the router is
a single-M AC-addres s device, source-base d forwarding on the switch Ethe rChan nel ensur es that the
switch use s all available bandwi dth to the rou ter. The router is configured fo r destina tion- based
forwarding because the large number of workstations ensures that the traffic is evenly distribu ted from
the router EtherChannel.