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Port-channeling between router (2901) and switch (3750)

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Sep 20th, 2010
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I'm setting up a port-channel between a router (2901) and a switch (3750). I know that the port-channel configuration on the router side is layer 3. As for the port-channel on the switch side, does it have to be configured as layer 3 aswell or can it be layer 2? I've tried the switch's port-channel config as both a layer 3 and a layer 2 and both configuration appears to work. What is the recommend port-channel configuration when going from a router to a switch?


Thanks,

J.D

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Nathan Spitzer Mon, 09/20/2010 - 11:39
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I'll start by being obnoxiosly pendantic: When going between a router and a switch your only choice is layer-2.


However what I think you mean is "Should I use my WAN router as the site router, or should I use my layer-3 switch?" The answer to that question depends, however in general I dont like configuring VLAN subinterfaces on the WAN router if I can help it. I find it much cleaner to use a /30 transit network on a layer-3 physical interface (in your case a port-channel) between the router and L3-switch then use the L3 switch as the site router. At a small site with a flat network it really doesnt matter, just be consistent but should the site grow to have a couple of VLANS with significant cross-VLAN traffic and you use the WAN router as the gateway, all that traffic goes through the WAN router when it would be handled much better by the L3 switch.


I prefer to do it right the first time and have the L3 switch do its job. My 2 cents.

Jon Marshall Mon, 09/20/2010 - 12:13
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I'll start by being obnoxiosly pendantic: When going between a router and a switch your only choice is layer-2.


I might be wrong but i'm not sure this statement is correct. ISR routers support L3 etherchannel and so do 3750 switches ie. you don't need to use switchports at either end.


Jennifer, if you are using the 3750 switch to do the intervlan routing and you simply want to aggregate the bandwidth from the 3750 to the ISR then i would recommend using L3 personally. I always favour L3 over L2 if it can be done as this keeps your L2 boundary to the 3750 switch and doesn't extend it to the router.


Jon

Nathan Spitzer Mon, 09/20/2010 - 12:16
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But if you use the 3750 with a L3 etherchannel then BY DEFINITION it is acting as a router, not a switch. Thats why I said I was being obnoxiously pedantic. However it is a VERY important distinction and keeping the "switching" function seperate from the "routing" function in a L3 switch makes troubleshooting much easier. I once had a guy who SWORE for a switch to work right it needed an IP in every VLAN it had defined. Took me 2 hours toget hime clear on the difference between the layer-2 and layer-3 functions of a switch.

Jon Marshall Mon, 09/20/2010 - 12:21
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But if you use the 3750 with a L3 etherchannel then BY DEFINITION it is acting as a router, not a switch.


Ahh i understand what you mean but my turn to be pedantic.


If you use the 3750 with a L3 etherchannel then by DEFINITION it is acting as a L3 switch and, in addition, as a L2 switch


Edit - actually, even if you made the etherchannel L2 with switchports and used an SVI then it is still acting as a L3 switch so i'm not sure how using a L3 etherchannel determines it's use.


But i do understand your point.


Jon

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