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 33.2 below shows the fields contained within a PAgP frame as seen on the wire:
Figure 33.2 – PAgP Ethernet Header
Although going into detail on the PAgP packet format is beyond the scope of the CCNA exam requirements, Figure 33.3 below shows the fields contained in a typical PAgP packet. Some of the fields contained within the PAgP packet are of relevance to the CCNA exam and will be described in detail as we progress through this module:
Figure 33.3 – The Port Aggregation Protocol Frame