The Cisco EtherChannel implementations provide a method to increase the bandwidth between two systems by bundling Fast Ethernet, Gigabit Ethernet, or 10 Gigabit Ethernet links. When bundling Fast Ethernet links, use Fast EtherChannel. Gigabit EtherChannel bundles Gigabit Ethernet links. EtherChannel port bundles enable you to group multiple ports into a single logical transmission path between a switch and a router, a switch and a host, or a switch and another switch. EtherChannels provide increased bandwidth, load sharing, and redundancy. If a link in the bundle fails, the other links take on the traffic load. You can configure EtherChannel bundles as trunk links.
Depending on your hardware, you can form an EtherChannel with up to eight compatibly configured ports on the switch. The participating ports in an EtherChannel trunk must have the same speed and duplex mode and belong to the same VLAN. Cisco’s proprietary hash algorithm calculates the way load balancing occurs, as shown in Table 6-6.
Table 6-6 EtherChannel Load Balancing
| Number of Ports in EtherChannel | Load Balancing Between Ports |
| 8 | 1:1:1:1:1:1:1:1 |
| 7 | 2:1:1:1:1:1:1 |
| 6 | 2:2:1:1:1:1 |
| 5 | 2:2:2:1:1 |
| 4 | 2:2:2:2 |
| 3 | 3:3:2 |
| 2 | 4:4 |
Port Aggregation Considerations
When EtherChannel is configured to bundle Layer 2 links, it aggregates the bandwidth of these links and changes Spanning Tree Protocol behavior because all links are treated as one link and thus are all in the Spanning Tree Protocol forwarding state.
When EtherChannel is configured for Layer 3 links, it aggregates the bandwidth of multiple Layer 3 links and optimizes routing because there is only one neighbor relationship per switch interconnection.
EtherChannel can be established by using three mechanisms:
- LACP: Link Aggregation Control Protocol (LACP) is defined in IEEE 802.3ad. It protects against misconfiguration but adds overhead and delay when setting up a bundle.
- PAgP: Port Aggregation Protocol (PAgP) is a Cisco-proprietary negation protocol. PAgP aids in the automatic creation of EtherChannel links.
- Static persistence configuration: This configuration does not add overhead as LACP does, but it can cause problems if not configured properly.
PAgP
PAgP aids in the automatic creation of EtherChannel links. PAgP packets are sent between EtherChannel-capable ports in order to negotiate the formation of a channel. PAgP requires that all ports in the channel belong to the same VLAN or be configured as trunk ports. When a bundle already exists and a VLAN of a port is modified, all ports in the bundle are modified to match that VLAN. If ports are configured for dynamic VLANs, PAgP does not form a bundle.
PAgP modes are off, auto, desirable, and on. Only the combinations auto/desirable, desirable/desirable, and on/on allow the formation of a channel. The device on the other side must have PAgP set to on if a device on one side of the channel, such as a router, does not support PAgP. Note that PAgP does not group ports that operate at different speeds or port duplex. If speed and duplex change when a bundle exists, PAgP changes the port speed and duplex for all ports in the bundle.