LACP abbreviation in networking stands for Link Aggregation Control Protocol which is widely used in a switching infrastructure.LACP is a open standards based protocol for link aggregation alternative to PAGP (Cisco Proprietary).
LACP packets flow over the the ether channel capable ports.It assigns roles to EtherChannel endpoints.
There are two parameters that define the role os endpoint and which ports will be active in an Etherchannel:
If no changes are made to the priority value then the switch with lowest MAC address will become decision maker as to what ports actively participate in EtherChannel at given time.
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.
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 far end to negotiate an ether channel and doesnt actively asks to negotiate an ether channel.
Now let us take and example as below to understand the configuration of LACP based ether-channel between two switches :
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.