Switches can combine or bundle parallel links to form single logical link . This method is called Ether channel technology.
The benefit of bundling multiple links and forming an ether channel is we can have more bandwidth pass through the EtherChannel.
If we have multiple seperate links without forming an etherchannel then STP will block one port thus resulting in one link failure.
Hence we need etherchannel to allow more bandwidth data to pass through the link.
Upto eight interfaces can be grouped together to form a etherchannel.
In order to form etherchannel, the interfaces must belong to the same VLAN if they are access ports.
If we are forming Etherchannel for trunking then the interfaces must have the same trunking mode.
The interfaces must have the same speed, duplex setting and STP settings as well.
If one of the links in the etherchannel fail then the traffic will be distributed among the remaining links.
Etherchannel uses two protocols for its negotiation.One is Port Aggregation Protocol(PAgP) and the other is Link Aggregation Protocol(LACP).
Port Aggregation Protocol(PAgP):
PAgP is cisco proprietary protocol.
when we configure the switches in AUTO or DESIRABLE mode then they use PAgP protocol.
AUTO:Used in PAgP, it places the interface in a passive negotiation mode;It only responds to PAgP packets that it receives. In this mode the switch will not start the negotiation process; this setting minimizes the transmission of PAgP packets.
DESIRABLE:Used in PAgP, the switches will actively negotiate an Etherchannel link.
To configure switchport for PAgP negotiation use the following commands
Switch(config)#interface <type/number>
Switch(config-if)#channel-protocol pagp
Switch(config-if)#channel-group <number> mode {on|{{auto|desirable}{non-silent}}}
Link Aggregation Protocol(LACP):
LACP is an industry standard IEEE 802.3ad protocol.
ACTIVE:Used in LACP, the switches will actively negotiate an Etherchannel link.
PASSIVE: Used in LACP, it places the interface in a passive negotiation mode where it only responds to LACP packets that it receives. In this mode the switch will not start the negotiation process; this setting minimizes the transmission of LACP packets.
ON: Forces the interface into an Etherchannel without PAgP or LACP packets, both switches must be configured in ON mode for the EtherChannel to be established.
To configure switchport for PAgP negotiation use the following commands
Switch(config)# lacp system-priority <priority>
Switch(config)#interface <type/number>
Switch(config)#channel-protocol lacp
Switch(config-if)#channel-group <number> mode {on|passive|active}
Siwtch(config-if)#lacp port-priority <priority>
