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 ether channel. The PAgP is proprietary to Cisco Systems. A similar protocol known as LACP— released by the IEEE and known as 802.3ad — is an industry standard and is not tied to a 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 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.
PAgP messages are always sent to the well known Cisco multicast address 01-00-0C-CC-CC-CC with protocol type code 0X0104. PAgP uses the same multicast 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 the 4500E platforms, remove this limitation using Virtual Switching System (VSS), which allows port channels to be split between two chassis.
HP uses IRF technology without the 2 box limitation (they allow 4 to 9 boxes in single stack). 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. Extreme Networks may do this functionality via M-LAG Multilink Aggregation. Cisco Nexus switches also allow this, using Virtual Port Channel.
With Port Aggregation Protocol "[t]he line speed of an agport is the total of the line speeds of each of its physical ports." This means a PAgP group consisting of two 1Gbit/s physical ports will have a maximum transfer rate of 2Gbit/s (minus overhead) per transfer.
EtherChannel load-balancing works by having the switch assign a hash result from 0-7 based on the configured hash method (load-balancing algorithm) for the type of traffic. This hash result is commonly called a Result Bundle Hash (RBH). They are then divided out over the available links. Therefore no single flow can exceed the speed of a physical port.
This is not the case with Link Aggregate Control Protocol which "[d]oes not increase the bandwidth for a single conversation.". This means an LACP group consisting of two 1Gbit/s physical ports will have a maximum transfer rate of 1Gbit/s (minus overhead) per transfer.
- "Catalyst 6500 Virtual Switching System 1440 - Products & Services". Cisco. Retrieved 2016-01-29.
- "Port Aggregation Protocol" (PDF). Ieee802.org. Retrieved 2015-01-30.
- "Catalyst 6500, 4500, and 3750 Series Switches EtherChannel Load-Balancing". Cisco. Retrieved 2015-12-28.
- "IEEE 802.3ad Link Aggregation (LAG)" (PDF). Ieee802.org. Retrieved 2015-01-30.