Ether channels - Cisco Switching PAgP, LACP

Ether channels

The ability to configure multiple physical links into one logical link is called an ether channel. Ether channels are a link aggregation technology and are also referred to a as port channel architecture. Once the ether channel has been configured the resulting virtual interface is called a port channel. The benefits of this are we can combine physical links to combine bandwidth, logically speaking two 100Mbps links combine into a 200Mbps link. An ether channel provides redundancy, if one port fails the other one is still active. An ether channel must be between 2 and 8 physical links

Below we will configure an ether channel in Cisco Packet tracer;

Link Aggregation Control Protocol (LACP)

LACP is an open standard which can be used by most vendors. Switch ports can form an ether channel in different LACP modes following the below criteria; 

-A port in the active mode can form an Ether Channel with another port that is in the active or passive mode.

-A port in the passive mode cannot form an Ether Channel with another port that is also in the passive mode because neither port starts LACP negotiation.

A port in active mode negotiates with the other side to form the Ether channel. The interface in passive mode indicates using LACP, but responds to requests only and does not send any requests.

LACP negotiation modes for channel establishment

LACP configurations;

Here are the configurations for LACP notice the mode is set to active. On a second switch the command are the same just replace active with passive.

Manual On Ether Channel mode

When using manual on mode an ether channel will be created only when another interface is using manual on mode. They forcefully form an ether channel without negotiation. 

Port aggregation protocol (PAgP)

PAgP is a Cisco proprietary ether channel technology. Switch Ports can form an Ether Channel when they are in different PAgP modes as per below criteria;

-A port in the desirable mode can form an Ether Channel with another port that is in the desirable or auto mode.

-A port in the auto mode can form an Ether Channel with another port in the desirable mode.

The port in desirable mode is one which sends requests to the other side to see if it is also using PAgP. The port in auto mode defines using PAgP but does not send requests. 

PAgP negotiation modes for channel establishment

PAgp configurations

Notice here we don't need to specify the protocol as this is default on Cisco switches. On another switch the mode would be set to auto