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difference between Etherchannel port and a trunk port

Answered Question
Oct 2nd, 2003
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Can someone please explain the difference between an Etherchannel port and a trunk port or refer me do a doc where I can read up on it?


Thanks

The beauty of etherchannel is that it is considered one link. So say you two switches and you want redundant links, so you connect two cables between the switches. STP will not allow traffic over one of the links so you are only going to use one link at a time.


Now if you configure an etherchannel group between the two switches with two connections, STP thinks of the etherchannel group as one link and so both links will transmit/receive traffic in a more or less load balanced fashion. If one of the links in the etherchannel group fails, STP does nothing because it still looks at the etherchannel group as one link. You will just have 1/2 the bandwidth on the etherchannel group.


It is often good to use etherchannel group connections between switches because:

1. You get more bandwidth between switches.

2. You get redundancy in case one of the connections fails, the etherchannel group will continue to use the connections that remain.

3. You don't have to wory about STP.




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An etherchannel group is when you want to connect two devices with multiple links. A typical use is connecting two switches via 4 100Mbps connections and then "bundle" them in essence yielding 400Mbps (800Mbps full duplex) between the switches. Etherchannel ports don't care about VLANs. They don't know anything about VLANs. Trunk ports; however, are VLAN-driven:


A trunk port is a connection between a switch or a router that carries VLAN information, typically used when you have multiple VLANs. A typical use is to configure a trunk port (1 ethernet connection) between two switches so a user on switch 1 on VLAN 1 can talk to a guy on switch 2 on VLAN 1 and a guy on switch 1 VLAN2 can talk to a guy on switch 2 VLAN2.

Also, a trunk port is connected from a switch to a router so the router can route between the VLANs.


Often trunk ports are used on top of etherchannel groups. For example, say you have two switches and 10 vlans. There are ports on both switches in the vlans so you need a trunk port to allow interswitch VLAN info. Often people will create this trunk port on an existing etherchannel group so they can get the 400Mbps bandwidth between the swiches.


Think of the etherchannel group as a "big pipe" between cisco devices and the trunk port as a VLAN-driven connection between cisco devices that can also be an existing etherchannel group.

-HTH


konigl Thu, 10/02/2003 - 11:32
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Where some of the confusion might come from is that some vendors used to refer to "link aggregation", or bundling multiple physical connections together to act as one logical connection, as trunking.


EtherChannel was the first incarnation of link aggregation for Ethernet networking, and was developed by a company called Kalpana (which Cisco acquired). Kalpana developed the first Ethernet switches, which they named, appropriately, the EtherSwitch. After Cisco acquired Kalpana, they extended the EtherChannel concept to Fast Ethernet and Gigabit Ethernet.


The rest of the networking equipment manufacturers liked the idea, but didn't like that Cisco had it first, so of course they had to develop their own completely different way of doing the exact same thing. Eventually they worked out an interoperability standard that they would all support, IEEE 802.3ad "link aggregation". (Couldn't call it EtherChannel.)


Cisco's EtherChannels used to be proprietary. But now, on some of their switches the EtherChannels can also be configured to connect with other switches (including non-Cisco ones) that adhere to IEEE 802.3ad.


Strictly speaking, when you talk about trunking in a Cisco switch, you're referring to VLAN trunking, which was covered in the previous post.


Hope that helps.

axfalk Thu, 10/02/2003 - 11:51
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Thanks for your very eloquent response.

What happens in terms of the Spanning Tree when the Etherchannel connection fails? Does it go through the spanning tree?


Thanks again

Correct Answer

The beauty of etherchannel is that it is considered one link. So say you two switches and you want redundant links, so you connect two cables between the switches. STP will not allow traffic over one of the links so you are only going to use one link at a time.


Now if you configure an etherchannel group between the two switches with two connections, STP thinks of the etherchannel group as one link and so both links will transmit/receive traffic in a more or less load balanced fashion. If one of the links in the etherchannel group fails, STP does nothing because it still looks at the etherchannel group as one link. You will just have 1/2 the bandwidth on the etherchannel group.


It is often good to use etherchannel group connections between switches because:

1. You get more bandwidth between switches.

2. You get redundancy in case one of the connections fails, the etherchannel group will continue to use the connections that remain.

3. You don't have to wory about STP.




konigl Thu, 10/02/2003 - 12:45
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Also with STP enabled, if you have redundant connections between two Cisco switches, where one connection is an EtherChannel and the other is a non-EtherChannel, and the links are all of the same type (that is, all Fast Ethernet or all Gigabit Ethernet), then the EtherChannel will be the link that STP prefers. The non-EtherChannel link will be blocked.


This is because STP will give the EtherChannel a lower Port Cost according to a formula that can be found in the following thread:


http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&CommCmd=MB%3Fcmd%3Dpass_through%26location%3Doutline%40%5E1%40.ee9c473


FYI, that thread is the only place I have ever seen information on how STP assigns a Port Cost to an EtherChannel connection. It's not covered in the Cisco Press book "Cisco LAN Switching", or in any of the BCMSN books I've read. I've never seen anything in print either in Cisco's product documentation.


briedcan Thu, 10/16/2003 - 09:47
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How do I show the port cost of a Port channel? I have been looking through some different commands and can't seem to find it?


bc

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