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channel-group command

Could someone point me to a link about information about this command besides its options and such. If you dont know of a link but know the command, some of the questions I'm having include:

(1) what are the benefits of this command and why would you use this configuration?

(2) What platforms will this work for?

(3) If using between two switches and lets say you have four fiber links between the two, can you put three of the links into a channel-group and leave the other link out of the channel-group?

(4) Do the interfaces in the channel-group have to be in the same slot (same switching module) to be in a channel-group?

(5) Again, if you have two switches connected by four fiber links and you put each side of interfaces in a channel-group, do both sides have to be the same channel-group number?

(6) Neglecting that it is a hardware or line problem, what might be the reason for some of the interfaces in a channel-group to come up while others dont assuming that all the interfaces are configured exactly the same?

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Accepted Solutions

Re: channel-group command

Hello,

let me try to answer these questions from the heart:

(1) you can use this command to create a etherchannel (up to 8 links - fast or gigaethernet) grouped together to connect 2 switches and avoid that spanning tree block the links.

(2) I dont think this command is platform specific.

(3) I believe you can leave 1 link out, but you'll have STP acting on both interfaces (port-channel and the 4th link). So 1 interface will be blocked.

(4) No there's no need to put use interfaces on the same module.

(5) yes, all interfaces from both switches need to be on the same group.

(6) there are some configurations that need to be same in order to the interfaces to form the etherchannel (vlan, speed, duplex, etc).

Check the following links for full explanation:

http://www.cisco.com/en/US/tech/tk389/tk213/technologies_white_paper09186a0080092944.shtml#wp39660

http://www.cisco.com/univercd/cc/td/doc/product/lan/c3550/12225see/scg/swethchl.htm

HTH,

if it does, please rate this post.

Vlad

I always use 3550 as example (ccie studying reasons) but the same is valid for other switches.

3 REPLIES

Re: channel-group command

Hello,

let me try to answer these questions from the heart:

(1) you can use this command to create a etherchannel (up to 8 links - fast or gigaethernet) grouped together to connect 2 switches and avoid that spanning tree block the links.

(2) I dont think this command is platform specific.

(3) I believe you can leave 1 link out, but you'll have STP acting on both interfaces (port-channel and the 4th link). So 1 interface will be blocked.

(4) No there's no need to put use interfaces on the same module.

(5) yes, all interfaces from both switches need to be on the same group.

(6) there are some configurations that need to be same in order to the interfaces to form the etherchannel (vlan, speed, duplex, etc).

Check the following links for full explanation:

http://www.cisco.com/en/US/tech/tk389/tk213/technologies_white_paper09186a0080092944.shtml#wp39660

http://www.cisco.com/univercd/cc/td/doc/product/lan/c3550/12225see/scg/swethchl.htm

HTH,

if it does, please rate this post.

Vlad

I always use 3550 as example (ccie studying reasons) but the same is valid for other switches.

Re: channel-group command

(1) what are the benefits of this command and why would you use this configuration?

>> channel-group x mode is an interface configuration to group identical interfaces in a combined logical interface, think of it as small pipes being combined to create a bigger pipe so that you can have more flows.

(2) What platforms will this work for?

>> channel-group is specific to catalyst running IOS like 2950, 2955, 2960, 2970, 3550, 3560, 3750, cat4000 with sup II+ or above, cat6000 with sups running Native IOS. For catalyst running it will be "port channel" command.

(3) If using between two switches and lets say you have four fiber links between the two, can you put three of the links into a channel-group and leave the other link out of the channel-group?

>> yes, by virtue of channel it will have a lower cost than the single interface therefore STP will block the single interface because it has a higher path cost to the root (STP can be discuss in a separate discussion).

(4) Do the interfaces in the channel-group have to be in the same slot (same switching module) to be in a channel-group?

>> No it does not have to be but they have to be identical. You cannot channel two ports where one is 100 MB and the other is a gig port.

(5) Again, if you have two switches connected by four fiber links and you put each side of interfaces in a channel-group, do both sides have to be the same channel-group number?

>> NO, channel group have local significance. If on switch A port 1 and 2 are in channel-group 1 and port 1 and 2 are connecting to port 3 and 4 of switch B. In switch B port 3 and 4 can be in channel-group 2 or 3 or 1 if you want to keep them the same for administrative sanity.

(6) Neglecting that it is a hardware or line problem, what might be the reason for some of the interfaces in a channel-group to come up while others dont assuming that all the interfaces are configured exactly the same?

>> cabling not right, interfaces in the same channel group are not alike, problem with the other end, or bug but this seldom is the case.

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Re: channel-group command

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