Pagp Interaction With Other Features; Understanding Load Balancing And Forwarding Methods - Cisco Catalyst 2950 Software Configuration Manual

Chapter 25 Configuring EtherChannels
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:
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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
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.
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Port selection based on the source-MAC address of the packet
Port selection based on the destination- MAC address of the packet
25-3, an EtherChannel of four workstations communicates with a router. Because the router is
Catalyst 2950 Desktop Switch Software Configuration Guide
Understanding EtherChannels
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