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| 3.9 |  | EtherChannel |
| 3.9.4 |  | Port Aggregation Protocol (PAgP) |

Port Aggregation Protocol (PAgP) is a Cisco-proprietary technology that facilitates the automatic creation of EtherChannels by exchanging packets between Ethernet interfaces. By using PAgP, the switch learns the identity of partners that can support PagP and learns the capabilities of each interface. It then dynamically groups similarly configured interfaces into a single logical link (channel or aggregate port). These interfaces are grouped based on hardware, administrative, and port parameter constraints. For example, PAgP groups the interfaces with the same speed, duplex mode, native VLAN, VLAN range, and trunking status and type. After grouping the links into an Ether-Channel, PAgP adds the group to the spanning tree as a single switch port.

The various PAgP modes and their definitions are spelled out in Figure . EtherChannel includes four user-configurable modes. They are on, off, auto, and desirable. Only auto and desirable are PAgP modes. The auto and desirable modes can be modified with the silent and non-silent keywords. By default, ports are in auto-silent mode. Both the auto and desirable modes allow ports to negotiate with connected ports to determine if they can form an EtherChannel, based on criteria such as port speed, trunking state, and VLAN numbers.

The On mode forces the port to channel without PAgP. With the on mode, a usable EtherChannel exists only when a port group in on mode is connected to another port group in on mode. The Off mode prevents the port from channeling. The Auto mode places a port into a passive negotiating state, in which the port responds to PAgP packets it receives but does not initiate PAgP packet negotiation. This is the default setting. The Desirable mode places a port into an active negotiating state, in which the port initiates negotiations with other ports by sending PAgP packets.

Ports can form an EtherChannel when they are in different PAgP modes as long as the modes are compatible. For example, a port in desirable mode can form an EtherChannel successfully with another port that is in desirable or auto mode. A port in auto mode can form an EtherChannel with another port in desirable mode. A port in auto mode cannot form an EtherChannel with another port that is also in auto mode, because neither port initiates negotiation.

On CatOS switches, configuring an EtherChannel automatically creates an administrative group, designated by an integer between 1 and 1024, to which the EtherChannel belongs. If preferred, specify the administrative group number. Forming a channel without specifying an administrative group number creates a new automatically numbered administrative group. An administrative group can contain a maximum of eight ports. In addition, on CatOS switches, each EtherChannel is automatically assigned a unique EtherChannel ID.