Port Aggregation Protocol (PAgP) is a Cisco proprietary link aggregation protocol that enables the automatic creation of EtherChannels. By default, PAgP packets are sent between EtherChannel-capable ports in order to negotiate the forming of an EtherChannel. These packets are sent to the destination Multicast MAC address 01-00-0C-CC-CC-CC, which is also the same Multicast address that is used by CDP, UDLD, VTP, and DTP. Figure 5-2 below shows the fields contained within a PAgP frame as seen on the wire:
Fig. 5-2. PAgP Ethernet Header
Although going into detail on the PAgP packet format is beyond the scope of the SWITCH exam requirements, Figure 5-3 below shows the fields contained in a typical PAgP packet. Some of the fields contained within the PAgP packet are of relevance to the SWITCH exam and will be described in detail as we progress through this chapter:
Fig. 5-3. The Port Aggregation Protocol Frame