LACP Modes – Link Aggregation Beginners Guide

LACP –

LACP abbreviation in networking stands for Link Aggregation Control Protocol which is widely used in a switching infrastructure.

LACP is an open standards-based protocol for link aggregation alternative to PAGP (Cisco Proprietary).

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LACP packets flow over the ether channel capable ports. It assigns roles to EtherChannel endpoints.

There are two parameters that define the role of os endpoint and which ports will be active in an Etherchannel:

System Priority = 2 byte priority value + 6 byte switch MAC address default System priority : 32768

Lowest System Priority is preferred

If no changes are made to the priority value then the switch with the lowest MAC address will become decision-maker as to what ports actively participate in EtherChannel at a given time.

Port Priority = 2 byte priority + 2 byte port number default priority : 32768

Lowest Port Priority is preferred

In LACP we may bundle as much as 16 ports together and only maximum of 8 are active at any given point in time and rest are on standby just in case if any active port fails.

Related – PAGP VS LACP

If no changes are made to the port priority value then first 8 ports (Assuming bundle is of 16 ports) are chosen as active ports in the bundle based on their low port number.

LACP MODES OF OPERATION :

Active Mode: In this, the switch actively asks the far end to negotiate an ether channel.

Passive Mode: In this mode, the switch waits to hear from the far end to negotiate an ether channel and doesn’t actively ask to negotiate an ether channel.

Now let us take an example as below to understand the configuration of LACP based ether-channel between two switches :

lacp modes

Switch 1 has MAC of “aa.aa.aa.aa.aa.aa” and switch 2 has MAC of  “bb.bb.bb.bb.bb.bb”

Switch 1 forms ether-channel with switch 2 using LACP and switch 1 will have system priority of 100 to make it the decision-maker.

Ports Gi 2/1 – Gi 2/4 & Gi 3/1 – Gi3/4 become the active ports as we configure the port priority of 100 on the ether channel and ports Gi 2/5 – Gi 2/8 & Gi 3/5 – Gi3/8 from will become the standby ports as they will have default port priority value of 32768.

 

CONFIGURATION

switch1(config)# lacp system priority 100switch1(config)# interface range Gi2/1 – /4 , Gi3/1 – 4

switch1(config)#channel protocol lacp

switch1(config)# channel-group 1 mode active

switch1(config)# lacp port priority 100

switch1(config)#exit

switch1(config)# interface range Gi2/5 – 8 , Gi3/5 – 8

switch1(config)#channel protocol lacp

switch1(config)#channel-group 1 mode active

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