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| 4.3 |  | Troubleshooting Switched Ethernet Networks |
| 4.3.8 |  | Troubleshooting EtherChannel |

Fast EtherChannel allows multiple physical Fast Ethernet links to be combined into one logical channel. This allows load sharing of traffic among the links in the channel as well as redundancy in the event that one or more links in the channel should fail. Fast EtherChannel can be used to interconnect LAN switches, routers, servers, and clients via unshielded twisted-pair (UTP) wiring or single mode and multimode fiber. 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 goal of EtherChannel is to distribute traffic over several physical links to enhance performance and availability. One area that is often not considered when configuring EtherChannel is how EtherChannel determines the physical link over which the frame will be sent.

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.

If the switches or routers support only minimal addresses for making the distribution decision, take particular care to consider the Layer 2 data flow destined for, or sourced from, single nodes such as Routers, Servers and storage- area network (SAN) devices.