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Cisco 3500 etherchannel port groups

Posted on 2011-02-12
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Last Modified: 2012-06-27
I have 3 Cisco 3500xl switches that I want to daisy chain together.  Switch A will connect to B, and B will connect to C.  

When configuring the port group, should I create 2 separate port groups?  For example if I have 'SwitchA' F0/1, F0/2, & 'SwitchB' F0/1 & F0/2 in 'port group 1', and SwitchB F0/3, F0/4 & SwitchC F0/3, & F0/4 in port group 2, is this the correct way to configure it?  

Should all ports be in one port group amongst all switches, or is the 'Port Group' number only locally significant?   I'm guessing on switch 2, there has to be 2 separate port groups, a different one going to each switch or you will get a switching loop.  
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Question by:B1izzard
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by:manni78
manni78 earned 150 total points
ID: 34881007
Yes you have create two difrrent channels for different switches connected to each other. Example Channel 1 for Switch A & B Port fa0/1-2; channel 2 for B & C Port fa0/3-4.
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by:B1izzard
ID: 34881090
If you do not, does it cause a switching loop I'm assuming?
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MAG03 earned 250 total points
ID: 34882136
Group numbers are local to the switch so you can have channel-group 1 with fa0/1 & fa 0/2 on one switch connecting to fa0/1 & fa0/2 in channel group 2 on another switch.  You can not have two channel groups of the same number on one switch.  All ports in a channel group must go to the same destination.

It will not create a loop but an inconsistancy will be created. When data exchang is initiated the switches elects a path/link for that traffic in the etherchannel for the duration. If a link that is going to a different switch is chosen the packets will never reach its destination.  There for in a non stack environment all channel groups must have links going to the same destination.

The way around this is to stack the switches at the other end of the link. Lets say that there are 3 switches that are stacked. Then one switch with channel group 1 can have 1 link going to each stacked switch within the same channel group as the stack is considered to be the same destination for the channel group.
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Expert Comment

by:manni78
ID: 34882360
B1izzard, It will not create loop but you channel will not work.....
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by:SeeMeShakinMyHead
SeeMeShakinMyHead earned 100 total points
ID: 34882546
If using VLANs other than just VLAN1, you will get VLAN mismatches as well.  For consistency, just name channel-group 1 on both ends for swA to swB and channel-group 2 for swB to swC.  Make sure encapsulation, vlans, and speed/duplex are the same on both ends.
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Author Closing Comment

by:B1izzard
ID: 34883199
Great explanations.  Thanks.
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