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Networking / Network Switch / Understanding Link Aggregation Control Protocol

Understanding Link Aggregation Control Protocol

Posted on By FS.COM

What Is LACP (Link Aggregation Control Protocol)?

LACP is a subcomponent of IEEE 802.3ad (Link Aggregation) standard. The standard prescribes that LACP can be a method to bundle multiple physical links between network devices into a single logical link. As a result, links that LACP enabled, can increase its logical bandwidth and network reliability without changing any network infrastructure. Moreover, even if one link fails, under the LACP mode, the other available link members in the same LACP group will balance the load.


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How LACP Works?

At the moment LACP enabled between the two switches, they will send LACPDUs (LACP data units) to each other. After receiving LACPDUs from each other, the two switches will compare with each other to check which side’s system priority is higher. Then they will have a negotiation with each other to choose the higher one to be the Actor and the lower one being the Partner. If the two switches have similar system priority, the switch with smaller MAC address will be the Actor.


After the Actor selected, the two switches will choose active ports based on the port priorities of Actor’s port. However, if the Actor’s ports have the same priorities, the ports with smaller port numbers will be chosen to be active ports. After the corresponding ports of two switches being selected, the port-channel (LACP group) is established. Then active links will load balance data to do communication.

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Confusing Questions About LACP


1. How to Configure Link Aggregation Switch?


Link aggregation switch, or LACP switch, refer to set up or configure the switch to achieve the link aggregation technology. Link aggregation switch can be any switch like Gigabit Ethernet switches or 10 Gigabit switches that supports LACP. Generally, there are six steps to configure link aggregation switches:


Step 1: Add member interfaces to the channel-group.


Step 2: Set the LACP system priority and determine the Actor so that the Partner selects active interfaces based on the Actor interface priority.


Step 3: Set the upper threshold for the number of active interfaces to improve reliability. (This step is only optionally applied in CLI dynamic configuration commands.)


Step 4: Set LACP interface priorities and determine active interfaces so that interfaces with higher priorities are selected as active interfaces. (This step is only optionally applied in CLI dynamic configuration commands.)


Step 5: Create VLANs and add interfaces to the VLANs. (This step is only optionally applied in CLI dynamic configuration commands.)


Step 6: Verify the LACP configuration.


More details about how to configure the LACP switch or link aggregation switch, you can refer to LACP Configuration on FS S3900 Series Switches.


2. LAG vs LACP: What’s the Difference?


LAG (link aggregation group) refers to the initial technology to realize link bundling and load balancing without any protocol involved. It is also called as the manual mode because of its working process that users need to manually create a port-channel and add member interfaces to that port-channel. After the aggregation links being established, all those links are active links to forward data packets. If one active link fails, the other remaining active links will load balance the traffic. However, this mode can only detect disconnections of its member links, but cannot detect other faults such as link-layer faults and incorrect link connections.


While LACP is a protocol for auto-configuring and maintaining LAG. Under the mode of LACP, the port-channel is created based on LACP. The LACP provides a standard negotiation mechanism for a switching device so that the switching device can automatically form and start the aggregated link according to its configuration. After the aggregated link is formed, LACP is responsible for maintaining the link status. When the link aggregation condition is changed, LACP adjusts or removes the aggregated link. If one active link fails, the system selects a link among backup links as the active link. Therefore, the number of links participating in data forwarding remains unchanged. In addition, this mode cannot only detect disconnections of its member links, but also other faults such as link-layer faults and incorrect link connections.


3. LACP vs PAGP: How They Differ From Each Other?


Both LACP and PAGP protocols are used for link aggregation. They are aimed at bundling links and balancing traffic across the member links to provide aggregated throughput. PAGP provides the same negotiation benefits as the LACP. Both LACP and PAGP packets are exchanged between switches over port-channel enabled ports. The most significant difference is the vendors supported for them. LACP is an open standard and supported by most vendors, while PAGP is Cisco proprietary only used between Cisco devices or switches licensed by vendors to support PAGP. Besides, LACP can support cross-stack and cross-MLAG, while PAGP can’t since it does not support participating interfaces on different physical switches. Thus if you need to form the port-channel on stack switch, it is better to choose LACP instead of PAGP.


Parameters LACP PAGP
Origin IEEE passed 802.3ad (LACP) in the year 2000 Invented in the early 1990s
Vendors Supported Open Standard Cisco Proprietary
FS Ethernet Switches Supported PoE+ Series; S3700/3900 Series; S5800/5900/8050 Series, N Series N/A
Mode Passive: This mode places a port in a passive negotiating state. In this type of mode, the port responds to LACP packets that it receives but does not initiate LACP packet negotiation. Active(The default mode for LACP) : This mode places a port in an active negotiation state in which the port initiates negotiations with other ports by sending LACP packets. Auto: This mode places an interface in a passive negotiating state in which the interface responds to PAGP packets that it receives but does not initiate PAGP negotiation. Desirable (The default mode for PAGP): This mode places an interface at an active negotiating state in which the interface initiates negotiations with other interfaces by sending PAGP packets.




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