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| 3.9 |  | EtherChannel |
| 3.9.7 |  | EtherChannel configuration guidelines |

There are a host of guidelines for configuring EtherChannel. These relate to trunks, dynamic VLANs, STP, and otherwise. If improperly configured, some EtherChannel interfaces are automatically disabled to avoid network loops and other problems.

To minimize time spent troubleshooting EtherChannel, follow these recommendations.

Assign all ports in an EtherChannel to the same VLAN, or configure them as trunk ports. If the EtherChannel is configured as a trunk, configure the same trunk mode on all the ports in the EtherChannel. Configuring ports in an EtherChannel indifferent trunk modes can have unexpected results. An EtherChannel supports the same allowed range of VLANs on all the ports in a trunking EtherChannel. If the allowed range of VLANs is not the same for a port list, the ports do not form an EtherChannel even when set to auto or desirable mode.

Do not configure the ports in an EtherChannel as dynamic VLAN ports. Doing so can adversely affect switch performance. An EtherChannel will not form with ports that have different GARP, GVRP, or QoS configurations. If a broadcast limit is configured 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 1second when the broadcast limit is exceeded.

An EtherChannel will not form with ports that have the port security feature enabled. The port security feature cannot be enabled for ports in an EtherChannel. If IGMP multicast filtering is using one port in an EtherChannel, set the EtherChannel mode for both PAgP and LACP to off. No other mode can be used.

An EtherChannel will not form if one of the ports is a Switched Port Analyzer (SPAN) destination port. An EtherChannel will not form if protocol filtering is set differently on any of the ports. Each EtherChannel can have up to eight compatibly configured Ethernet interfaces. Configure all interfaces in an EtherChannel to operate at the same speed and duplex modes.

Enable all interfaces in an EtherChannel. If an interface in an EtherChannel is shutdown, it is treated as a link failure, and its traffic is transferred to one of the remaining interfaces in the EtherChannel.

Interfaces with different STP Port Costs can form an EtherChannel as long they are otherwise compatibly configured. Setting different STP Port Costs does not, by itself, make interfaces incompatible for the formation of an EtherChannel. However, it is preferable to set STP port costs to be equal for all ports in an EtherChannel.

Beyond the restrictions listed here, the only remaining concern is what ports can be used when configuring an EtherChannel. This question is addressed next. Catalyst 4000 and 6000 line cards have newer chipsets that allow use of an even or odd number of links in the EtherChannel. The ports do not have to be contiguous or even on the same line card, as is true with some Catalyst devices and line modules. The newer chips on the Catalyst 4000s and 6000s are not available on all Catalyst hardware.

Older Catalyst switches use an Ethernet Bundle Controller (EBC) to manage aggregated EtherChannel ports (for example, performing frame distribution). The EBC communicates with the Enhanced Address Recognition Logic (EARL) Application-Specific Integrated Circuit (ASIC). The EARL is responsible for, among other things, learning MAC addresses. For example, the EARL ages out all addresses learned on a link in an EtherChannel bundle when it fails. After a link failure, the EBC and the EARL work together to recalculate in hardware the “lost” source-destination address pairs on a different link. On the older switches, when selecting ports to group for an EtherChannel process, ports that belong to the same EBC must be selected. On a 24-port module, there are three groups of eight ports. On a 12-port module, there are three groups of four ports.

Be sure to check the hardware documentation before attempting to create EtherChannel bundles. Older Catalyst switches restrict which ports are used in a given bundle. The remainder of the material in this section pertains primarily to Catalyst 5000 line modules.

In a 12-port module up to two dual-segment EtherChannel configurations can be created within each group of four ports, as illustrated in Example A. One dual-segment EtherChannel configuration can be created within each group, as in Example B. Example C illustrates a four-segment and a two-segment EtherChannel configuration.

Some EtherChannel configurations must be avoided on older Catalyst devices, including most Catalyst 5000 line cards. Example D 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 the first group has to be used. 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, and 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 (EBC). This EtherChannel configuration uses two ports from the first group and two ports from the second group.

Finally, Example G is an invalid EtherChannel configuration because it does not use the first ports on the module to start the EtherChannel process (like Example D). The EtherChannel process cannot be started with middle ports on the line module. All the examples in Figure apply to 24-port modules as well. The only difference between 12 and 24 port modules is the number of EtherChannel segments that can be formed within a group. The 12 port module allows only two (two-port) Ether-Channel segments in a group, whereas the 24-port module supports up to four (two-port) EtherChannel segments per group.