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 and can only be used between Cisco switches and/or switches from licensed vendors. A similar purpose protocol known as LACP— released by the IEEE and 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.

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 multicast group MAC address of CDP

Limitations

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 platforms, remove this limitation using Virtual Switching System (VSS),[1] which allows port channels to be split between two chassis.

HP is doing it similarly through IRF technology but without limitation of just 2 boxes (they are allowing 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. Also Extreme Networks may do this functionality via M-LAG Multilink Aggregation. Cisco Nexus switches also allow this, using Virtual Port Channel.

Advantages

With Port Aggregation Protocol "[t]he line speed of an agport is the total of the line speeds of each of its physical ports."[2] 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.

This might have been an idea of the study group but it is not the way it has been implemented. at least not since the onset of CEF.

The way EtherChannel load-balancing works is the switch assigns 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).[3] They are then divided out over the available links. Therefore no single flow can exceed the speed of a physical port. There is no practical advantage of PAgP over LACP.

This is not the case with Link Aggregate Control Protocol which "[d]oes not increase the bandwidth for a single conversation.".[4] 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.

See also

References

  1. ↑ "Catalyst 6500 Virtual Switching System 1440 - Products & Services". Cisco. Retrieved 2016-01-29.
  2. ↑ "Port Aggregation Protocol" (PDF). Ieee802.org. Retrieved 2015-01-30.
  3. ↑ "Catalyst 6500, 4500, and 3750 Series Switches EtherChannel Load-Balancing". Cisco. Retrieved 2015-12-28.
  4. ↑ "IEEE 802.3ad Link Aggregation (LAG)" (PDF). Ieee802.org. Retrieved 2015-01-30.

External links

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