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Design thoughts: Replacing a L2 aggregation switch


I have purchased a 4507R switch to replace a 2924M-XL switch that acts as an aggregation switch in our network. Let me explain further what I plan to do.

I have 20 remote sites connected point to point via 100 Mbps dark fibre to the 2924M-XL. Most of the sites have only a handful of users but 5 of them are bigger (ie. 20-70 users). Some of the larger remote sites (small campuses really) have 2-5 switches in a star topology with the "hub" switch connecting back to the 2924M-XL. Each site has 1 or 2 user VLANs and a management VLAN. The 2924M-XL trunks all VLANs back to a 6513 at the core of our network.

I will be connecting the 4507R along 2 seperate dark fibre runs (for layer 1 redundancy) to 2 6513s in our core. This will give us fault-tolerence should our primary 6513 fail.

My problem is I'm struggling with the decision to go layer 2 or layer 3 between the 4507R and the 6513s. Layer 2 would be alot easier to implement and support (I'm the sole administrator of this rather large network) but then I'd have RSTP to deal with among the 2 6513s and 4507. I'm comfortable with RSTP since I run it between 2950G switches dual connected to the 6513s but my gut feeling is that I should be putting in layer 3 between the 6513s and the 4507.

We will be implementing VoIP is the next 2 years and I'm unsure how that affects my decision.

One last comment. Would layer 2 trunking of VLANs from the 4507 to the 6513s WITHOUT trunking these VLANs between the 6513s be a viable optionand would HSRP between the 2 6513s still work OK for layer 3 redundany? The remotes sites are setup with unique user VLANs but there is a special use VLAN that spans 4 of the sites and my manegement VLAN spans all the sites (I'm planning to change this).

Thanks everyone for your thought/opinions.



Re: Design thoughts: Replacing a L2 aggregation switch

Hi there Ian,

I'm a big fan of routing over switching, which I read is becoming Cisco's recommended way of doing things.

I would route between the 2 x 6513's and the 4507 as it will not only give you fault tolerance, but also load balancing, plus cutting down on broadcast domains and all those other nice things.

As far as configuration goes, onec you've got it up and running, then it'll just keep running. It seems like you will only need straight forward routing here and nothing too complex. Setting it up would be a simple affair.

VoIP, in my experience, is much better implemented over a routed network than a switched one. There are loads more things that you can do at layer 3 than you can at layer 2. Think about all the QoS that you'll be able to implement, with shaping and policing, etc. Much more security can be built in at layer 3 too. You'll get the likes of NBAR and all other features that you'll be able to (over time) tweak you network with.

As for performance, you'll never spot a difference. The 4507 will be lots faster than the 2924 and using cef, the 4507 will keep a forwarding table for ip's the same way a 2900 keeps a mac table.

You will not regret routing it.

Hope this helps - if so, please give it a rating.


Re: Design thoughts: Replacing a L2 aggregation switch

hi lan,,,

yes you will have lots of benifite by deploying layer 3 redundancy and for voice you will have lots of option...can we go for the GLBP to utilise the resources also...insted of using HSRP as in HSRP one router will be in standby condition...ya in HSRP aslo we will have the load balancing by varing priority and making multiple HSRP group...but what do you think about GLBP...?