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Examining How EtherChannel Works

EtherChannel allows you to take ports on a Cisco switch and combine them to give you a larger data pipe. In this way, you can double, triple, even quadruple, the amount of data you send between two devices. EtherChannel offers great advantages in high-bandwidth situations where you need more throughput.

Today people are still looking for the fastest way to implement inter-switch links. After the release of Fast Ethernet (100 Mbps), you could expect to have a few of these more expensive ports on your switch, allowing you to use them for inter-switch links. The same thing happened after the prices dropped on Fast Ethernet ports, and 24-port and 48-port Fast Ethernet switches started becoming common.

Gigabit Ethernet came around as a more expensive offering, and rather than getting a switch with 24 or 48 expensive gigabit ports, you could get a switch with two or four Gigabit Ethernet ports to use for inter-switch links. The same process is going on with Gigabit Ethernet switches, which now have ten Gigabit Ethernet ports for supporting inter-switch links.

As long as there is a faster but more expensive connection type, you will see it used as a method of interconnecting switches. And when that technology becomes inexpensive, and manufacturers are using it for all their switch ports, there will be new, faster, and more expensive technologies released and used where they can do the most good.

If you have 24 devices running at 100 Mbps on a 24-port switch, they can combine and send up to 2,400 Mbps (2.4 Gbps) — if by chance they all need to send their maximum bandwidth to a remote system at the same time. Additionally, interconnecting with other switches at a lowly 100 Mbps causes a severe bandwidth deficit (2.4 Gbps versus 100 Mbps) when trying to send data.

If you add a single Gigabit Ethernet port to the switch, you still have a bandwidth deficit (2.4 Gbps versus 1 Gbps) that is less severe. This is a deficit only if all 24 devices are sending data at their maximum speed, which will not occur normally.

Enter EtherChannel, which allows you to take multiple ports on a pair of switches and interconnect them as a single link. This situation is different from STP, which allows you to connect multiple ports but then blocks traffic on all but one port. With EtherChannel, all ports function as a single combined link.

Therefore, if you interconnect two Gigabit Ethernet ports, you get 2 Gbps of throughput, which is slightly lower than the combined potential speed of 2.4 Gbps.

Switches avoid collisions by having only a single host on each collision domain, allowing data transfer at nearly wire speed.

To show a connection deficit situation, I set up a Cisco 2960 switch (WS-C2960-48TT-L) with 48 Fast Ethernet (100 Mbps) ports, and two 1 Gigabit Ethernet copper ports, as well as a Cisco Catalyst 3750G switch (Catalyst 3750G-24T) with twenty-four 1 Gigabit Ethernet ports. During this example, I will adjust the load put on the switch by the client computers.

  • In the following figure, you see a network with a single client computer streaming data from a video server on a Gigabit Ethernet switch. Having one client on two expensive switches is not typical. When only one client streams the data, the client’s computer receives the data at a rate of 60 Mbps.

    Because a single client computer is on an unencumbered network, you can deduce that 60 Mbps is the maximum speed at which the application can receive data.

    image0.jpg
  • A typical network, with 24 clients are shown, all streaming video from the server. In this situation, each client receives streamed data at approximately 33 Mbps (1000 Mbps uplink multiplied by 80 percent [network overhead] divided by 24 clients).

    This is the fastest speed at which clients could possibly get the data. With video streaming at a slower than maximum rate, clients may experience interruptions in their video playback.

    image1.jpg
  • In the following figure, a two-port EtherChannel group is enabled between the two switches, which doubles the bandwidth available on the uplink, bringing the link speed up to 2 Gbps. With the inter-switch link at 2 Gbps, the transfer rate for each client goes up to the maximum speed of 60 Mbps per client.

    To support all 48 clients at that rate, you need two more gigabit ports to add an EtherChannel group — not possible on this switch, but possible with a Cisco Catalyst 3750G switch, which has 48 gigabit ports.

    image2.jpg

You are not limited to combining two ports. You can use EtherChannel to get up to eight active ports in a single channel group and up to six port groups on a switch. Later sections of this chapter walk you through the process of setting up these EtherChannel links.

EtherChannel comes with a cost. The cost is the loss of extra ports. If you have two 48-port gigabit switches, and you decide you need an 8 Gbps interconnect, you lose eight ports per switch, whereas you may have budgeted for only one port per switch based on your port counts for servers, and so on. Now your available device ports go from 94 host connections to 80 host connections.

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