PAgP Port Modes

PAgP supports different port modes that determine whether an EtherChannel will be formed between two PAgP-capable switches. Before we delve into the two PAgP port modes, one particular mode deserves special attention. This mode (the “on” mode) is sometimes incorrectly referenced as a PAgP mode. The truth, however, is that it is not a PAgP port mode.

The on mode forces a port to be placed into a channel unconditionally. The channel will only be created if another switch port is connected and is configured in the on mode. When this mode is enabled, there is no negotiation of the channel performed by the local EtherChannel protocol. In other words, this effectively disables EtherChannel negotiation and forces the port to the channel. The operation of this mode is similar to the operation of the switchport nonegotiate command on trunk links. It is important to remember that switch interfaces that are configured in the on mode do not exchange PAgP packets.

Switch EtherChannels using PAgP may be configured to operate in one of two modes: auto or desirable. These two PAgP modes of operation are described in the following sections.

Auto Mode

Auto mode is a PAgP mode that will negotiate with another PAgP port only if the port receives a PAgP packet. When this mode is enabled, the port(s) will never initiate PAgP communications but will instead listen passively for any received PAgP packets before creating an EtherChannel with the neighbouring switch.

Desirable Mode

Desirable mode is a PAgP mode that causes the port to initiate PAgP negotiation for a channel with another PAgP port. In other words, in this mode, the port actively attempts to establish an EtherChannel with another switch running PAgP.

In summation, it is important to remember that switch interfaces configured in the on mode do not exchange PAgP packets, but they do exchange PAgP packets with partner interfaces configured in the auto or desirable modes. Table 33.1 shows the different PAgP combinations and the result of their use in establishing an EtherChannel:

Table 33.1 – EtherChannel Formation Using Different PAgP Modes