LACP vs PAgP: What is the Difference and Relationship?

LACP (Link Aggregation Control Protocol) and PAgP (Port Aggregation Protocol) are two protocols used to combine multiple physical ports into one logical port, referred to as link aggregation or channel port. The purpose of link aggregation is to increase bandwidth, redundancy, and network availability. Link aggregation is also known as EtherChannel, trunking, bundling, or bonding.

LACP and PAgP have some similarities and differences. This article will discuss what LACP and PAgP are, what are the differences and relationships between the two, and when to use one of them.

What is LACP?

LACP is a standard protocol defined by IEEE 802.3ad. LACP allows switches or other network devices to automatically configure and manage link aggregation. LACP sends LACPDU (Link Aggregation Control Protocol Data Unit) frames between ports involved in link aggregation to negotiate and synchronize port status and parameters. LACP can also detect link failures and adjust link aggregation configuration dynamically.

LACP has two modes of operation: active and passive. Active mode means the port will always send LACPDU frames to start link aggregation negotiations. Passive mode means the port will only respond to LACPDU frames received from other ports, but will not send its own LACPDU frames. To establish link aggregation with LACP, at least one port must be in active mode.

What is PAgP?

PAgP is a proprietary protocol developed by Cisco. PAgP is like LACP but only works on Cisco devices that support it. PAgP also sends PAgPDU (Port Aggregation Protocol Data Unit) frames between ports involved in link aggregation to negotiate and synchronize the port states and parameters. PAgP can also detect link failures and adjust link aggregation configuration dynamically.

PAgP has two modes of operation: desirable and auto. Desirable mode means the port will always send PAgPDU frames to start link aggregation negotiations. Auto mode means the port will only respond to PAgPDU frames received from other ports, but will not send PAgPDU frames itself. To establish link aggregation with PAgP, at least one port must be in desirable mode.

What is the Difference between LACP and PAgP?

LACP and PAgP have several differences, including:

• LACP is a standard protocol that can work on network devices from different vendors, while PAgP is a proprietary protocol that can only work on Cisco devices that support it.
• LACP uses LACPDU frames, while PAgP uses PAgPDU frames. LACPDU and PAgPDU frames have different formats and are not compatible with each other.
• LACP has active and passive modes, while PAgP has desirable and auto modes. Active and desirable modes work the same, sending frames to start link aggregation negotiations. Passive and auto modes also work the same, they only respond to received frames. However, the name of the mode differs between LACP and PAgP.
• LACP can support up to 16 ports in a single link aggregation, but only 8 ports can be active simultaneously. PAgP can support up to 8 ports in a single aggregation link, and all ports can be active simultaneously.

Here is a table comparing LACP and PAgP:

What is the Relationship between LACP and PAgP?

LACP and PAgP have a mutually exclusive relationship, that is, they cannot be used simultaneously on the same port. If a port is configured to use LACP, it cannot use PAgP, and vice versa. If a port that uses LACP is connected to a port that uses PAgP, then link aggregation will not be formed, because LACPDU and PAgPDU frames cannot recognize each other.

LACP and PAgP also have a complementary relationship, that is, they can be used interchangeably on different ports. If the network device supports both protocols, then different ports can be configured to use LACP or PAgP, depending on needs and preferences. For example, ports connected to network devices from different vendors can use LACP, while ports connected to Cisco devices can use PAgP.

When to Use LACP or PAgP?

The choice between LACP or PAgP depends on several factors, including:

• Network device compatibility. If the network device only supports LACP, then LACP must be used. If the network device only supports PAgP, then PAgP should be used. If the network device supports both protocols, then the choice can be adjusted to your needs and preferences.
• Network device vendors. If the connected network device is from a different vendor, then LACP must be used, as LACP is a standard protocol that can work on network devices from different vendors. If the connected network device is from Cisco, then PAgP can be used, because PAgP is a proprietary protocol that can only work on Cisco devices that support it.
• Number of ports needed. If the number of ports required for link aggregation is more than 8, then LACP must be used, because LACP can support up to 16 ports in a single link aggregation. If the number of ports required for link aggregation is no more than 8, then PAgP can be used, because PAgP can support up to 8 ports in a single link aggregation.

Conclusion

LACP and PAgP are two protocols used to combine multiple physical ports into one logical port, referred to as link aggregation. The purpose of link aggregation is to increase bandwidth, redundancy, and network availability.

LACP is a standard protocol defined by IEEE 802.3ad, while PAgP is a proprietary protocol developed by Cisco. LACP and PAgP have some differences, among others in terms of compatibility, frame format, mode of operation, and number of ports supported.

LACP and PAgP also have a mutually exclusive and complementary relationship. LACP and PAgP cannot be used simultaneously at the same port but can be used interchangeably in different ports.

The choice between LACP or PAgP depends on several factors, including network device compatibility, network device vendor, and number of ports needed.