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L2 or L3 Core?

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Apr 11th, 2004
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I have approx 35 L2 switches connecting via Trunks to one 6513 that has the MSFC2. I want to take the customer away from this flat bridged network. In building a true Core, in best practices when do you move away from a L2 Core into a L3 core because of STP and Peering.


I need detailed specifics, I have pdf's showing pictures but no specifics as to why.


Thank you,

John

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Craig Norborg Tue, 04/13/2004 - 10:39
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Well, alot of it has to do with how the network is configured. First off, if they are running multiple VLAN's right now, but not using their MSFC's to route between them (ie: using a router instead), moving to L3 switching with the MSFC will give them quite an improvement.


If you aren't using multiple VLAN's and don't intend to, going to L3 switching won't really gain you a thing. However, there are some good reasons to have multiple VLAN's. Such as if they could use a server VLAN. Doing this with a router vs. a L3 switch isn't a good idea because of the # of packets going from the clients VLAN to the server VLAN.


Now why would you want multiple VLAN's?

Isolating the broadcast domain for one, or creating multiple smaller broadcast domains

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Minimizing the impact that a STP hit would have on each VLAN. ie: If you have a separate server VLAN, you can do work on the client VLAN without affecting those switches on the server VLAN.


Another reason would be the ability to put ACL's on one VLAN while not having it affect clients on another.


But, I should stress that if there isn't any reason to have separate VLAN's, putting them in for the sake of doing so will just complicate the clients network. Yes, it is a good idea to get away from flat-bridged networks, but doing so without a reason or a plan is not a good thing to do...

jgreer429 Thu, 07/29/2004 - 19:10
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"Minimizing the impact that a STP hit would have on each VLAN."


L3 switching will allow an active network port between switches to switch to another network port. With L2 STP when our active connection goes down the whole segment/switch is disconnected until the port is in forward mode. With L3 routing if you have an alternate route you should experience no disconnected users.

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