Port Aggregation Protocol
Port Aggregation Protocol (PAgP) is a Cisco Systems proprietary networking protocol, which is used for the automated, logical aggregation of Ethernet switch ports, known as an etherchannel. This means it can only be used between Cisco switches and/or switches from licensed vendors. A similar purpose protocol known as LACP, released by the IEEE known as 802.3ad, is an industry standard and is not tied to any specific vendor.
PAgP can be configured on a Cisco switch to operate in three different modes.
- auto - passive negotiation of the channel
- desirable - active negotiation of the channel
- on - no protocols are used: it assumes the other side has also enabled link aggregation
On Cisco network devices running CatOS, a single switch module may only be configured to run in either LACP or PAgP modes. Cisco devices that run IOS (native and/or non-hybrid mode boxes) support individual port configuration for LACP and are not restricted to per module settings as with CatOS.
1) PAgP messages are always sent to well known cisco multicast address 01-00-0C-CC-CC-CC with protocol type code 0X0104 2) PAgP uses the same mulicast group MAC address of CDP
A limitation of Port Aggregation Protocol is that all the physical ports in the aggregation group must reside on the same switch. Cisco's 6500 and soon the 4500E platform's remove this limitation using Virtual Switching System (VSS), which allows port channels to be split between two chassis. Similarly Avaya's SMLT protocol also removes this limitation by allowing the physical ports to be split between two switches in a triangle configuration or 4 or more switches in a mesh configuration. Also Extreme Networks may do this functionality vía M-LAG Multilink Agreggation. Cisco Nexus switches also allow this, using Virtual Port Channel.