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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter21 Configuring EtherChannel Understanding EtherChannel

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 dir ects t ran smi ssions base d o n that
knowledge. 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 sends packets to the source by using any of the interface s in the EtherChannel.
With aggregate-port learning, it is not important on whi ch phy sica l po rt t he pa ck et a rri ves.
PAgP cannot automatically detect when the partner device is a physical learner and the local device is
an aggregate-port learner. Therefore, you must manually set the learning method on the local devi ce or
source-based distribution by using the pagp learn-method interface configuration comman d. With
source-based distribution, any given source MAC address is always sent on the same physical port. You
can also configure a single interface within the group fo r all t ran smi ssion s and use o ther in terf ac es f or
hot standby. The unused interfaces in the group can be swapped into operatio n in just a fe w se conds i f
the selected single interface loses hardware-signal detection. You can configure which interface is
always selected for packet transmission by changing its pri ority by usin g t he pagp por t-priority
interface configuration command. The higher the priority, the more likely that the port will be selected.

PAgP Interaction with Other Features

The Dynamic Trunking Protocol (DTP) and Cisco Discovery Protocol (CDP) send and re ceive p acket s
over the physical interfaces in the EtherChannel. Trunk ports send an d receive PAgP protocol data units
(PDUs) on the lowest numbered VLAN.
STP sends packets over the first interface in the EtherChannel.
The MAC address of a Layer 3 EtherChannel is the M AC add ress o f th e fi r st i nte rfac e in t he
port-channel.
PAgP sends and receives PAgP PDUs only from interfaces that are up and have PAgP enabled for the
auto or desirable mode.
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 forwar ding.
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-M AC addr es s of t he in comi ng pac ket.
Therefore, to provide load balancing, packets from di fferent 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).
When source-MAC address forwarding is used, load dis tribution based on the source and destination IP
address is also enabled for routed IP traffic. All routed IP traffic chooses a port based on the source and
destination IP address. Packets between two IP hosts always use the same port in th e channel, and traf fi c
between any other pair of hosts can use a different 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. You configure the load balancing and
forwarding method by using the port-channel load-balance global configuration command.