First you need to decide what load-balancing method you will use. If HSRP/GLBP?VRRP then you will need to have L2 connecting all switches and that is either single switch connected to all 3 for faster HSRP propogation or L2 links between all 3 cores.
Other thing is that you can use these links for a failover - e.g. if single link between one core and WAN router fails it can converge routing protocols to send traffic across other two.
If you can share your disign topology with access/distributio/core and WAN routers that can help to consider the benigits/drawbacks better.
I wanted to use L3 links to connect all three core's leolaohoo mentioned above.
So here is my setup for Core/Distro/Access
3 core Switches, each are 6509 switches not interconnected with each other currently
6 distrbution blocks, each block contains 2x 6509 Switches that are interconnected to each other via L2 running HSRP. All these switches are connected to each core via L2/L3 SVI Point-To-Point links. These distro's also contain their own appropriate SVI's for the access switches they connect down to.
30+ Access switches per distrbution block, these consist of 4507's and 3560's that connect to their appropriate distro via L2 links.
WAN Router, 7206 router which connects to the core via a L3 Point-to-Point link.
I think you should use ether channel on atleast two ports each connected between the Core/Distro/Access. It would increase your network throughput aswell as prove more convergence if some link fails. This etherchanneling can be on L2 or L3. In my EDN network we are following this infra design.
The only rason to have it I see is that if you have some L3 network connected to a single core switch only. And you want to distribute it among the other cores as well. Than having links between the cores traffic to this link will take the better path and will not overload distribution switches.
If there are no separate L3 network originated on one of the cores and they all maintain same netwroks learnt from WAN or distribution when this connection will not be useful. As it will not help much for failover as it would be done by OSPF and distirbution will stopp sending traffic to the core which had route to WAN failed .
I have attached what we currently do in our environment. The access layer obviously has the end devices. The distro layer has interface VLAN's for the access layer. The Distro's connect to each core via P-t-P Layer 3 link. The only difference is that we have 3 core switches. So would meshing all three of those core's make sense looking at the diagram below?
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