5.5 Redundant Links
5.5.3 EtherChannel rules of the road
If improperly configured, some Fast EtherChannel ports are disabled automatically to avoid network loops and other problems. Use the following guidelines to avoid configuration problems:
  • Assign all ports in a channel to the same VLAN, or configure the ports as trunk ports.
  • If you configure the channel as a trunk, configure the same trunk mode on all the ports in the channel, on both ends of the link. Configuring ports in a channel in different trunk modes can have unexpected results.
  • Configure all ports in a channel to operate at the same speed and duplex mode (full or half duplex).
  • If configuring a broadcast limit on the ports, configure the broadcast limit as a percentage limit for the channeled ports. With a packets-per-second broadcast limit, unicast packets might get dropped for one second when the broadcast limit is exceeded.
  • If the channel is composed of trunk ports, you must configure the same allowed VLAN range on all the ports. When the allowed VLAN range is not the same for all trunks in a channel, trunk ports on which a particular VLAN is not allowed will drop the packets for that VLAN, while ports on which the VLAN is allowed will transmit the traffic. If the allowed VLAN range is not the same on all ports in the channel, the ports do not form a channel when set to the auto or desirable mode with the set port channel command.
  • Do not configure the ports in a channel as dynamic VLAN ports. Doing so can adversely affect switch performance.
  • Make sure port security is disabled on channeled ports. If you enable port security on a channeled port, the port shuts down when it receives packets with source addresses that do not match the secure address of the port.

These rules are generally applicable to most EtherChannel technology-capable modules; however, some exceptions exist with later modules. For example, the Catalyst 6000 line cards do not constrain you to use even numbers of links. Three link bundles can be created. Nor do the ports have to be contiguous, or even on the same line card, as is true with some Catalyst devices and line modules. The exceptions to the EtherChannel rules for the Catalyst 6000 are as a result of the newer chipsets on the line modules. These newer chips are not present on all hardware. Be sure to check your hardware features before attempting to create any of these other bundle types.

The EBC of earlier EtherChannel technology-capable modules manages aggregated EtherChannel ports. For example, the EBC manages traffic distribution across each segment in the bundled link. The distribution mechanism is described later in this section.

When selecting ports to group for an EtherChannel process, you must select ports that belong to the same EBC. On a 24-port EtherChannel technology-capable module, there are three groups of eight ports. On a 12-port EtherChannel technology-capable module, there are three groups of four ports.

For example, in a 12-port module, you can create up to two dual-segment EtherChannel configurations within each group, as illustrated in Example A of Figure . Or, you can create one dual-segment EtherChannel configuration within each group, as in Example B of Figure . Example C illustrates a four-segment and a two-segment EtherChannel configuration.

You must avoid some EtherChannel configurations on early Catalyst 5000 equipment. Example D of Figure illustrates an invalid two-segment EtherChannel configuration using Ports 3 and 4 of a group. The EBC must start its bundling with the first ports of a group. This does not mean that you have to use the first group. In contrast, a valid dual-segment EtherChannel configuration can use Ports 5 and 6 with no EtherChannel segment on the first group.

Example E illustrates another invalid configuration. In this example, two EtherChannel segments are formed. One is a dual-segment EtherChannel configuration, the other is a four-segment EtherChannel configuration. The dual-segment EtherChannel configuration is valid. The four-segment EtherChannel configuration, however, violates the rule that all ports must belong to the same group. This EtherChannel configuration uses two ports from the first group and two ports from the second group.

Example F shows an invalid configuration where an EtherChannel configuration is formed with discontiguous segments. You must use adjacent ports to form an EtherChannel configuration.

Finally, Example G shows an invalid EtherChannel configuration because it does not use the first ports on the module to start the EtherChannel process. You cannot start the EtherChannel process with middle ports on the line module.

All the examples in Figure apply to the 24-port modules too. The only difference between a 12- and 24-port module is the number of EtherChannel segments that can be formed within a group. The 12-port module allows only two EtherChannel segments in a group, whereas the 24-port module supports up to four EtherChannel segments per group.