Layer 2 vs. Layer 3 links

Unanswered Question
May 11th, 2007

I have 2 4510R's at the core and in a remote building I am putting a 4507R. I will be uplinking the 4507 to each of the 4510's using 10 gig modules. Would it be better for me to use layer 2 or layer 3 uplinks? Also can I load balance across these 2 links?

I have this problem too.
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med_ddevlin Fri, 05/11/2007 - 11:42

I'd use Layer3 uplinks using a seperate VLANs creating seperate broadcast domains. It depends on how large the network is though and how much it would expand in the future. I'd go with Layer3 though.

For load balancing 2 trunk links, you could EtherChannel to perform that using a port-channel.

Please rate if this helps

royalblues Fri, 05/11/2007 - 12:11

This would depend upon your requirement.

Layer 3 links are preferable as you reduce your broadcast domain size.

But if it is a requirement to extend the same vlans across the 2 buildings, then you will have to use L2 links and configure them as trunks.

Also you cannot use etherchannel for redundancy across 2 different chassis.

Though MHSRP can be deployed and you can load balance certain vlans on one link and the rest on another with fallback.

If you are running L3 links, you can use a dynamic routing protocol like EIGRP & OSPF which would by default loadbalance between these 2 links.

HTH, rate if it does


med_ddevlin Fri, 05/11/2007 - 12:22

Wait, and please correct me if I'm wrong here but, you can use EtherChannel between 2 seperate switches. It's just a logical grouping of up to 8 physical ports.

All that needs to be done is have all the ports you want to include in your channel-group in the same vlan, trunk the vlan then create the channel-group on the interfaces.

Jon Marshall Fri, 05/11/2007 - 13:05


As far as i am aware you can't unless you are talking about a 3750 stack for example. On a 4500 you use ports from different modules within the same chassis but you cannot have an etherchannel that has one of its ends on two switches.



This is incorrect. You can not create a port channel between two seperate 4510 chassis. Nortel products can do this and they coin it as Split MLT.

Cisco does have plans in the works to support this on the 6500 series switch and it may even be available today but I am positive you can not do this on a 4510 switch. We have approx. 25 of these in production.

I don't feel there is a clear answer to your L2/L3 question. It is truly a question of what you want on your network. There are pros and cons for each.

L2 - your 4510 is the Gateway for your VLAN's and you are introducing spanning tree to your network. This is a collapsed core model and it is a Cisco design model.

You can only load balance by using PVST and splitting your VLAN priority on the 4510's. If you only have one VLAN then there is no option for split MLT in the Cisco world.

L3 - your 4507 is the gateway for your VLAN's and you are eliminating spanning tree and introducing routing.

You can load balance by using OSPF or static routes with equal costs. If you want to do this I suggest you research load balancing over L3 links.


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