The device on the other end of the EtherChannel is usually the determining factor in how the EtherChannel is configured. One design rule that must always be applied is that each of the links participating in an EtherChannel must have the same configuration. The descriptions can be different, but each of the physical links must be the same type and speed, and they must all be in the same VLAN. If they are trunks, they must all be configured with the same trunk parameters.
EtherChannel will negotiate with the device on the other side of the link. Two
protocols are supported on Cisco devices. The first is the Link Aggregation Control
Protocol (LACP), which is defined in IEEE specification 802.3ad. LACP is used when
connecting to non-Cisco devices, such as servers. As an example, Solaris will negotiate
with a Cisco switch via LACP. The other protocol used in negotiating EtherChannel links
is the Port Aggregation Control Protocol (PAgP), which is a Cisco-proprietary protocol.
Since PAgP is Cisco-proprietary, it is used only when connecting two Cisco devices via
an EtherChannel. Each protocol supports two modes: a passive mode (
auto in PAgP and
in LACP), and an active mode (
desirable in PAgP and
active in LACP). Alternatively, you can set the
on, thus forcing the creation of the
EtherChannel. The available protocols and modes are outlined in Figure 7-6.
Figure 7-6. EtherChannel protocols and their modes