Pagp And Lacp Interaction With Other Features - Cisco Catalyst 3550 series Software Configuration Manual

Understanding EtherChannels

PAgP and LACP 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 and LACP protocol
data units (PDUs) on the lowest numbered VLAN.
Spanning tree sends packets over the first interface in the EtherChannel.
The MAC address of a Layer 3 EtherChannel is the MAC address of the first interface in the
port-channel.
PAgP sends and receives PAgP PDUs only from interfaces that have PAgP enabled for the auto or
desirable mode. LACP sends and receives LACP PDUs only from interfaces that have LACP enabled
for the active or passive mode.
Understanding Load Balancing and Forwarding Methods
EtherChannel balances the traffic load across the links in a channel by randomly associating a newly
learned MAC address with one of the links in the channel.
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).
When source-MAC address forwarding is used, load distribution 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 the channel, and traffic
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.
In Figure
switch to the router. Source-based load balancing is used on the switch end of the EtherChannel to ensure
that the switch efficiently uses the bandwidth of the router by distributing traffic from the workstation
across the physical links. Since the router is a single MAC address device, it uses destination-based load
balancing to efficiently spread the traffic to the workstations across the physical links in the
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 or IP addresses might result in better load balancing.
Catalyst 3550 Multilayer Switch Software Configuration Guide
30-6
30-3, multiple workstations are connected to a switch, and an EtherChannel connects the
Chapter 30 Configuring EtherChannels
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