Link Aggregation Control Protocol (LACP)

Link aggregation is bundling two or more links as one logical link for increasing bandwidth and availability. This can be done by one of the two protocols . One of them is the Cisco proprietary PAgP and the other is standard based LACPPAgP is the Port Aggregation Protocol. And the LACP is the Link Aggregation Control Protocol. These two protocols does not interoperate each other. You can use only one of them to configure link aggragation. Here, we will focus to the standard based, LACP protocol.The last Cisco devices are already use LACP instead of PagP.

To aggragete links to a single logical link, firstly we need more than two ports with the same characteristics such as data rate, duplex capability, etc.. The ports can be in different interfaces, this is not important.

There is also a limitation about ports in the bundle. In Cisco platforms the ports in a bundle is limited by 8ports . This can be change in some platforms. For example in Cisco 10000 series, only 4 port is supported in a bundle.

This bundle is called Link Aggregation(LAG) generally.

We can name the created logical name as “port channel”. Up to 64 port channels is supported per device. Here, do not confuse this limitations. Maximum 8 ports in a bundle and maximum 64 bundles in a device.

When we create a port channel witf ports, then if one of the ports is failed, then the traffic is switched to the other ports.

What about loops? Is is prevented with a mechanism and returning of a multicast or a broadcast packets to the other ports in the bundle, is blocked.

The link aggregation is created by LACP packets. And this packets are exchanged between ports in two different modes. These modes are active and passive modes.

A port in an active mode, can initiate the nagotiation with sending the LACP packets. But a port in a passive mode can not do this. It can only respond to th received LACP packet.

LACP Parameters

There are some parameters that we can use during configuration of the link aggregation. These parameters are:

LACP System Priority : This is configured per router.It is used with MAC address to create LACP System ID.

LACP System ID = LACP System Priority + MAC Address

LACP Port Priority : It is confiured per port. It is used to form Port Identifier with Port Number.

LACP Port Identifier = LACP Port Priority + Port Number

It is also used to determine which port should be in standby mode during an hardware limitation.

LACP Administrative Key : It is automatically calculated equal to the channel group identification number on each LACP configured port. It defines the ability of a port to aggregate with other ports. The aggragation abiliy is determined by, port characteristics and configuration restrictions.

LACP Max-bundle : It is the number of bundled ports in a bundle. As I mentined below it is maximum 8.But in some platforms it can be 4.

If all the compatible ports can not be aggregated by LACP, the the remaining ones go to teh standby state. When there is a failure of one of teh bundled ports, then they become active one by one.

Some Important Tips

• Every port that you want to add to the bundle must be identical.
• The ports in a bundle can be in different line cards.
• MTU can be configured on port channel and then it is propagated to the ports under it.
• QoS and CAR are configured at port level. But ACL is configured on port channel.
• MPLS ip must be enabled on ports that are bundled with the command mpls ip.
• MAC configurations only allowed in port channel.
• On port channel you should apply Unicast Reverse Path Forarding(uRPF), with the command ip verify unicast reverse-path.
• When an interface has an ip address, firstly you must disable the ip address and then you can add the interface to the bundle.
• Hold queue in command can be used on ports and hold queue out command is used on port channel.

In this article, we have focused on LACP. Im the following article, we will do the link aggregation configuration between two routers. To undertsand more about LACP, continue with the next link aggregation configuration article.

To learn more about Link Aggregation, check the following posts. In the following posts we will focus on how to configure LAG in various vendors’s (Cisco, Juniper and Alcatel-Lucent) devices.

Link Aggregation – Part 1 (Link Aggregation Control Protocol, LACP)
Link Aggregation – Part 2 (LACP Configuration on Cisco Devices)
Link Aggregation – Part 3 (LACP Configuration on Juniper Devices)
Link Aggregation – Part 4 (LAG Configuration on Alcatel-Lucent Devices)