What are LACP and PAGP? What are the differences?

In order to create maximum availability and stability for the network, enterprises and data centers emphasize link redundancy or redundant links when deploying networks. By providing automatic recovery from link loss, Ethernet channel technology is the most effective way to achieve network redundancy.

When configuring Etherchannel across network devices such as switches or routers, the two most widely used negotiation protocols are LACP (Link Aggregation Control Protocol) and PAGP (Port Aggregation Protocol).

How do they differ when it comes to aggregating links and redistributing the load in case of link failure in any network?

What is LACP?

LACP is a data link layer protocol defined in the IEEE 802.3ad standard. It provides a method to control the bundling of multiple physical ports to form a single logical channel. It enables network devices (usually data switches) to negotiate automatic bundling of links by sending LACP packets to peers.

In fact, the LACP protocol serves the general principle of link aggregation, which describes the work of establishing a parallel network structure to provide redundancy or improve performance. FS S3800 series Gigabit Ethernet switches can support the LACP protocol. However, they do not support entering the aggregated port to configure various parameters. They must be configured through the interface range command.

How does LACP work?

When LACP configuration is enabled, the local LAG cannot transmit packets unless a LAG with LACP is also configured on the remote end of the link. If LACP configuration is not enabled, the local LAG may try to transmit packets to the remote single interface, which may cause communication failure. LACP is used in conjunction with MLAG (Multi-Chassis Link Aggregation). The LACP protocol is typically used to negotiate North and South (between a host and an MLAG virtual switch or between MLAG virtual switches).

What is PAGP?

PAGP is a Cisco proprietary protocol that can only run on Cisco switches or vendor-licensed PAGP-enabled switches. PAGP helps to automatically create Etherchannels by exchanging PAGP packets between Ethernet ports. PAGP packets are exchanged between switches through Etherchannel-enabled ports. Ports with the same neighbor device ID and port group capabilities are bundled together to become bidirectional point-to-point Etherchannel links.

By using PAGP, a switch can learn the identities of its PAGP-capable partners and then dynamically group similarly configured ports into a single logical link (channel or aggregate port).

As shown in the following figure, PAGP is used to run on a Cisco Virtual Switching System (VSS), which consists of two physical Catalyst 6500 Series switches acting as a single logical switch. In a VSS, one switch is selected as the active switch, while the other is selected as the standby switch. Both the active and standby switches are linked to the access switch through PAGP. In this case, if the Etherchannel between the two Catalyst 6500 switches fails, the VSS can still communicate with the access switch through PAGP negotiation.

What's the difference?

Both LACP and PAGP protocols are used for link aggregation. They are designed to bundle links and balance traffic between member links to provide aggregated throughput. PAGP offers the same negotiation benefits as LACP. Both LACP and PAGP packets are exchanged between switches through Etherchannel-enabled ports. The most notable difference is the vendors that support them.

LACP is an open standard and supported by most vendors, while PAGP is Cisco proprietary and is only used between Cisco devices. In addition, LACP can support cross-stack, while PAGP does not because it does not support participating interfaces on different physical switches.

Therefore, if you need to form an Etherchannel on stacked switches, it is better to choose LACP instead of PAGP.

in conclusion

LACP and PAGP protocols are similar, but the configuration mode and aggregation mechanism are different. How to choose between LACP and PAGP. You must find out which protocols the device interface supports. Each interface on a network device should have the appropriate protocol (PAGP or LACP) identified and configured whether it should be negotiated.

Because LACP is an IEEE-based protocol, it is more commonly used to bundle links to achieve maximum throughput between wiring closets and data centers. However, PAGP is also required when Cisco equipment is embedded in your network and your network architecture can support PAGP negotiation.