LACP vs PAgP: What's the Difference?

Quick Definition: Link Aggregation Control Protocol (LACP) and Port Aggregation Protocol (PAgP) are layer-2 protocols used to implement link aggregation. LACP is an industry-standard protocol supported by most network vendors, offering flexibility and cross-stack aggregation capabilities.

PAgP, a Cisco proprietary protocol, is designed to work exclusively with Cisco devices. While both protocols serve similar functions, LACP's vendor-neutral support and cross-stack capabilities make it a more versatile choice in mixed network environments.

The Significance of Link Aggregation

Link aggregation techniques have been used since the 1990s, initially with proprietary implementations from vendors such as Cisco, Avaya, and Juniper. Link aggregation was designed to help increase effective bandwidth and improve redundancy and failover capabilities.

Packets sent through the logical pipe see higher bandwidth than with individual component links, and traffic can be balanced across multiple links in the group, thereby reducing congestion. The links must all have identical speed ratings.

Redundancy and failover capabilities are also improved, as other links in the LAG can be used in the event of an individual link failure. Redundancy and resilience can be further improved if the links in the group are “cross-stack” – that is, spread over multiple switches. 

Overview of LACP (Link Aggregation Control Protocol) and PAgP (Port Aggregation Protocol)

PAgP was one of the first proprietary link or port aggregation protocols. Using PAgP, Cisco switches can automatically create an EthernetChannel port aggregation group between two PAgP-compliant switches.

PAgP’s design allows an initiating switch to interrogate and confirm whether the distant end-port supports PAgP before creating the aggregated port group. Since PAgP is a proprietary protocol, it is only used in network connections between Cisco switches (or switches from vendors licensed by Cisco to use PAgP).

Following the development of PAgP and port aggregation, the industry mobilized to define a link aggregation control protocol standard. LACP was initially defined in the IEEE 802.3ad standard – now known as IEEE 802.1AX. LACP is supported by most vendors – including Cisco – so any vendor’s LACP-compliant switch can be included and managed in a link aggregation group (AKA port aggregation group).

PAgP and LACP can co-exist in a network, BUT not within the same aggregation group. Unlike PAgP, LACP supports cross-stack aggregation – regardless of vendor switch – thus making it much more flexible than the Cisco-only PAgP. Note, however, that Cisco does support LACP, so LACP-compliant ports on Cisco switches can be part of both stack-to-stack and cross-stack aggregation groupings. 

What are the Key Differences Between LACP and PAgP?

LACP and PAgP basically perform the same function – grouping together physical links/ports into a single logical entity, although there are a number of differences. 

Proprietary vs Industry Standard

For starters, PAgP is a proprietary specification designed and authored by Cisco, whereas LACP is an industry-standard (now IEEE 802.1AX) published under the auspices of the IEEE.

As an IEEE standard, LACP has been adopted and implemented by most major switching equipment vendors, and their switches will, therefore, interoperate in link aggregation group mode. Cisco switches operating the PAgP protocol will only aggregate links with similarly configured Cisco switches. Since Cisco also supports the LACP, that standard offers a good deal more flexibility and scalability than PAgP.

In essence, any switch – Cisco or otherwise – on an LACP-compliant network might be capable of operating in link aggregation group mode. For example, check out our blog post on how to deploy LACP Between Cisco’s IOS operating system and Juniper Networks’ Junos.

Operating Modes

LACP and PAgP each have a number of operating modes, which are associated with the request to initiate link aggregation operation. 

LACP modes are Active and Passive. The default mode is Passive! In Active Mode, the initiating end of the link – known as the “actor” – sends an LACP protocol data unit (PDU) packet to the distant end of the link – the “partner.” The partner can be in either Active or Passive Mode, and when it receives the PDU, it will respond positively, and the link will be included in an active link aggregation group.

If both ends of the link are set to Passive Mode, then link aggregation cannot take place. A netadmin must set at least one end of each link to Active Mode in order for link aggregation negotiation to begin.

PAgP operating modes are Auto and Desirable. Desirable Mode is similar to LACP’s Active Mode. Actors in Desirable Mode can initiate link/port aggregation negotiation by sending a PAgP packet to the remote partner.

The partner must be configured as either Auto or Desirable. In this case, the partner will respond positively to the PAgP packet, and the formation of a PAgP EtherChannel will begin. If both ends of the link are in Auto Mode, then an EtherChannel cannot be formed.

And Finally, Cross-Stack Support

Cross-stack link aggregation group configurations provide for improved network redundancy as well as reduce administrative overhead. LACP supports cross-stack operations, while PAgP does not.

Conclusion

Your choice of link aggregation protocol will be mostly driven by the switch you have in your network. For Cisco-based networks, PAgP is probably the easiest and most efficient to set up. With a network of non-Cisco switches, LACP is the only choice. Since Cisco does support LACP as well as PAgP, if you plan to implement a cross-stack link aggregation, LACP is the only choice.

Either way, setting up and managing link/port aggregation are key skills needed by network engineers and administrators. CBT Nuggets offers several online training courses that will help you learn those skills. Start with the Cisco CCNA 200-301 certification training before moving on to troubleshooting Cisco EtherChannel and Link Aggregation Group Ports.

 If you’re working with Juniper Network switches, then learn how to implement LACP by taking our Juniper Link Aggregation online training course.

If you’re not already signed up for CBT Nuggets’ online training courses, then we invite you to take a free 7-day trial.