EtherChannel is a link aggregation technology used primarily on Cisco switches. EtherChannel enables bundling of multiple physical Ethernet links to create one logical Ethernet link which provides high throughout and resilient links. EtherChannel can be provisioned across Cisco Switches and Routers.Upto 8 active links can be bundled to form EtherChannel for Fast Ethernet, Gigabit Ethernet and 10-GigabitEthernet links.
We can create an EtherChannel on a standalone switch, on a single switch in the stack, or on multiple switches in the stack (known as cross-stack EtherChannel). EtherChannel configuration can be in one of below enlisted 3 modes
Lets understand each one of EtherChannel modes in detail. Below diagram will be referred to while forming and configuring EtherChannel in PAgP, LACP and ON modes –
PAGP (PORT-AGGREGATION PROTOCOL)
PAgP is Cisco proprietary Etherchannel technology. It uses the multicast address of 01-00-0C-CC-CC-CC for communication.
Switch/Router Ports can form an EtherChannel when they are in different PAgP modes as per below criteria –
- A port in the desirable mode can form an EtherChannel with another port that is in the desirable or auto mode.
- A port in the auto mode can form an EtherChannel 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.
EtherChannel Configuration between Switch1 and Switch2 using PAgP is shown below –
LACP (LINK AGGREGATION CONTROL PROTOCOL)
LACP is an open standard protocol and published under the 802.3ad specification. It uses the multicast address of 01-80-c2-00-00-02.
Switch/Router Ports can form an EtherChannel when they are in different LACP modes as per below criteria –
- A port in the active mode can form an EtherChannel with another port that is in the active or passive mode.
- A port in the passive mode cannot form an EtherChannel with another port that is also in the passive mode because neither port starts LACP negotiation.
The port in active mode negotiates with the other side to form Etherchannel while interface in passive mode indicates using LACP, but responds to requests only and does not sending any request.
EtherChannel Configuration between Switch1 and Switch2 using LACP is shown below –
MANUAL “ON” ETHERCHANNEL MODE
When using a EtherChannel “ON” mode, EtherChannel will be created only when another interface group is in EtherChannel “on” mode. One major drawback of using manually configured “On” EtherChannel Mode is that any layer one device like media convertor or modem between 2 Etherchannel devices will not be able to diagnose link failure and keep on sending the traffic while PAgp/LACP configured devices will detect the failure and respond to it.
EtherChannel Configuration between Switch1 and Switch2 using “ON” configuration mode is shown below –
Note – Both Switch1 and Switch2 are configured under ON mode to forcefully form EtherChannel without negotiation.