Let’s start the New Year with understanding EtherChannel which is having more than one link connected to a server, switch, or router and treating it like one logical link. This is a fault-tolerant technique that is becoming a common setup in highly available, core devices. Cisco has come up with their own proprietary version call EtherChannel. The open standard that IEEE approved is 802.3ad which works with other venders and is often called LAG for short.
Why would I use EtherChannel? One big benefit is bandwidth you can combine up to eight parallel links and depending on the port speed and the physical cable you could in theory get up to 80 Gbit/s per second! (Using SPF+ links which can get up 10G a piece in theory) Etherchannel aggregates traffic across all available active ports which make it look like one logical cable.
Although this sounds wonderful and it is there are some things to keep in mind. EtherChannel does not always distribute the traffic equally across all available links it depends on the number of links you have connected but even that does not always guarantee equal traffic. EtherChannel hands-out values 0-7 (that’s why you can only have up to eight links) on each link. Some of those links may have more than one value assigned to them this makes balancing the traffic not always equal. So looking at the table below if we were using five physical links and every link was in theory in perfect distribution three of those links would be receiving twice as much traffic than the other two links.
If every link was in perfect distribution you could have physical links of two, four, or eight. But this depends on the usage of a particular device because Etherchannel does not load balance between physical links if one link becomes oversaturated it will drop packets. Etherchannel can look at the following methods to decide which physical link to send data over.
- The source MAC address
- The destination MAC address
- The source and destination MAC address
- The source IP address
- The destination IP address
- The source and destination IP address
- The source port
- The destination port
- The source and destination port
All of these methods depend on the hardware and software you are running but they will put a value between 0-7 on each connected link. Remember each link can have more than one value assigned to it. I hope this information is helpful and you are welcome to ask questions or comment. Also post some ideas that you would like to see posted that deals with either ICND1 or ICND2.
- Cisco Nexus vPC Configuration and Spanning-Tree Elimination (techbookshelf.com)
- SPAN Port Configuration (learnnetworkingwithme.wordpress.com)
- STP loop prevention protocol (hazima.wordpress.com)
- Journey to CCIE LAB Exam – Part 20 – EtherChannels (mungauwamaseghe.wordpress.com)