Cisco Catalyst 2975 Software Configuration Manual page 841
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Chapter 37
Configuring EtherChannels and Link-State Tracking
Understanding EtherChannels
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.
With source-and-destination MAC address forwarding, when packets are forwarded to an EtherChannel,
they are distributed across the ports in the channel based on both the source and destination MAC
addresses. This forwarding method, a combination source-MAC and destination-MAC address
forwarding methods of load distribution, can be used if it is not clear whether source-MAC or
destination-MAC address forwarding is better suited on a particular switch. With source-and-destination
MAC-address forwarding, packets sent from host A to host B, host A to host C, and host C to host B
could all use different ports in the channel.
With source-IP address-based forwarding, when packets are forwarded to an EtherChannel, they are
distributed across the ports in the EtherChannel based on the source-IP address of the incoming packet.
Therefore, to provide load-balancing, packets from different IP addresses use different ports in the
channel, but packets from the same IP address use the same port in the channel.
With destination-IP address-based forwarding, when packets are forwarded to an EtherChannel, they are
distributed across the ports in the EtherChannel based on the destination-IP address of the incoming
packet. Therefore, to provide load-balancing, packets from the same IP source address sent to different
IP destination addresses could be sent on different ports in the channel. But packets sent from different
source IP addresses to the same destination IP address are always sent on the same port in the channel.
With source-and-destination IP address-based forwarding, packets are sent to an EtherChannel and
distributed across the EtherChannel ports, based on both the source and destination IP addresses of the
incoming packet. This forwarding method, a combination of source-IP and destination-IP address-based
forwarding, can be used if it is not clear whether source-IP or destination-IP address-based forwarding
is better suited on a particular switch. In this method, packets sent from the IP address A to IP address
B, from IP address A to IP address C, and from IP address C to IP address B could all use different ports
in the channel.
Different load-balancing methods have different advantages, and the choice of a particular
load-balancing method should be based on the position of the switch in the network and the kind of
traffic that needs to be load-distributed. In
Figure
37-5, an EtherChannel from a switch that is
aggregating data from 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 only going to a single MAC address, using the destination-MAC address always chooses the
same link in the channel. Using source addresses or IP addresses might result in better load balancing.
Catalyst 2975 Switch Software Configuration Guide
37-9
OL-19720-02
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