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Network Re-design L2->L3 Advice


I have a small network consisting of a 4510, 2 stacks of 3750s, single 3750s and single 3560s. Please see the attached diagram. The 4510 and one 3750 stack act as the core switches in the network. The rest of the 3750s and 3560s are access switches. Currently we run a L2 STP (MST based) topology with around 20 VLANs. sw-dc1-c handles the VLAN routing with sw-alfdc-c acting as the backup using HSRP. We generally trunk 5-10 VLANs to each access switch depending on the VLANs necessary on the access switches.  All access switches run ipbase and the 4510(s) and core 3750 stack run advservices IOS.

We have another 4510 set to go into production and essentially take the place of sw-alfdc-c. We run an all Cisco VoIP (CUCM) system and play to convert our old analog CCTV system to an IP based CCTV system this coming year. I would like to convert our network from an L2 STP implementation to an L3 routed implementation. I am looking for advice on what the best thing is to do as I've consulted several people and received several different responses. Some suggested leaving as-is with STP in place and there is no need to go routed. Others suggested converting over to OSPF and yet another suggested EIGRP.

I'm looking for any advice and input anyone may have so I can finally get a handle on exactly what I want to do with our network.


  • LAN Switching and Routing
VIP Super Bronze

Network Re-design L2->L3 Advice


Have a look a this design document.  It it very informative and useful when designing campus networks.  It provides a lot of good info regarding layer-2 and layer-3 designs.


New Member

Network Re-design L2->L3 Advice

Hi Reza,

Thanks for the response! I've seen and read most of the document. I was hoping that someone could provide some "real-world" insight and advice on my particular issue.

Network Re-design L2->L3 Advice

From looking at your topology, I like the Etherchannel you have configured between your core router/switch and backup router/switch. I see most of your access switches have redundant connections going to each Primary core and backup core.

You could put some redundancy on sw-lec-a1, sw-it15-a1, sw-i325-a1, and sw-sat-a1. These looks to have one link each to the core switch, which if this is the case, if that link goes down, so does the switch. As far as L2/L3 redesign, you're going to have a few differences.



You will have to worry about routing loops, since you have redundant connections. You can also do load-balancing and use multiple links on your switches.



You will have to worry about routing loops. It will also add processing, CPU, and memory stress on your L3 switches. But if you have enough power to handle that, it won't really be an issue.

To be honest with you, since this is a small network, I would leave it at L2 and just add some more redundancy to be honest with you. But, that's just my view.

Network Re-design L2->L3 Advice

In addition to suggestions from the experts, you may want to look int this as well...

-->4510's as core switches with one of them as STP root and another as Secondary. (with etherchannel)

--> Make your access switches STP priority higher (than 4510s)

-->Create HSRP groups for each Vlan on 4510s and run two trunks from each access switch to each core switch.

--> As the Vlans are all connected directly connected, you may not need any routing protocol but if need be go with EIGRP.



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