The Fast EtherChannel, Gigabit EtherChannel, Port Channel, Channel and Port
Group will all refer to EtherChannel, so the information given in this document
applies to all of the above EtherChannels.
From the STP point of view,
an EtherChannel is seen as a single port. This presents a danger of creating
forwarding loops if channeling ports are not consistent on both sides of the
channel.
For example, if switch A has two separate links and switch B
considers those same links as part of the channel, switch B will send a
broadcast or unknown unicast packet. That packet will be forwarded back to
switch B, as seen in the diagram.
This causes
packet duplication and changes the forwarding table on switch B to point in the
wrong direction.
Symptoms associated with misconfigured EtherChannels
include:
- Loss of connectivity due to switching loops.
- Increased backplane utilization may be noticed from the LEDs on a normally
lightly loaded switch.
- Console messages reporting that ports are rapidly relearning MAC addresses.
- Interfaces may be automatically placed into an "ErrDisable"
state.
When configuring EtherChannel, ensure that both sides of the link are
configured correctly. Ports will not form an EtherChannel unless they are
identically configured in terms of speed, duplex, native VLAN trunk etc.
The following is a summary of the default behavior for EtherChannel in a
switched and routed environment:
- Switches allocate links based on Layer 2 addresses.
- Routers allocate links based on Layer 3 addresses.
- Switches allocate links based on source address.
- Routers allocate links based on destination address.
Strictly speaking, this summary is only true of lower end switches and
routers such as 2500, 2600, 2950T and 3550. Higher end routers and switches may
take into account all of the above as well as TCP port numbers when making a
link forwarding decision.
Assuming that the routers and switches in the
network are using these defaults, then load balancing will work well in typical
topologies. In these situations, the switch sees many source MAC addresses and
the router sees many destination IP addresses. This allows traffic to be
effectively spread across the available links
However, consider the
situation in Figure
. The top
switch is forwarding traffic across the EtherChannel based on the source MAC
address. In this instance there is only one source MAC address, which is the
MAC address on the router. Consequently, traffic flowing from the router to the
PCs will use only a single Ethernet link. The solution is to configure the top
switch to allocate links based on the destination MAC address using the
port-channel load-balance {dst-mac | src-mac} global
configuration command:

switch(config)#port-channel load-balance
dst-mac
Wherever possible and whenever supported by
the network device, use a mechanism for distributing traffic that takes into
account many addresses when determining which link to use.