I am looking to design the network in the image posted and from all the 3 tier model images I have seen they have not included a router. What's the deal with this? I know a router is required but how does one configure a router to have two lan ports on the same subnet? This would be needed because for load balancing and redundancy a link will need to be made to each core switch from the router? Please help
Those switches are multilayer so they do the routing.
You can't configure a router with two routed ports in the same subnet. You can however configure two switched ports in the same subnet by using the same VLAN.
For redundacy, routed ports use routing (dynamic or static) and switched ports can use spanning-tree, etherchannel and /or first-hop redundancy protocols.
Hope that helps.
Please could you explain how I could make use of first hop redundancy protocol with two switches and a router with 3 ports (lan1, lan1, wan)?
I undestood that you need to connect 2 interfaces to 2 different Core Switch with same vlan.
You can use BVI (Bridge Virtual Interface) for IOS devices or BDI (Bridge Domain Interface) for IOS-XE devices.
Using the same BVI or BDI on 2 Interfaces will let you use 1 IP address to connect 2 physical interfaces.
You can use this document.
Let me ask a question about the original post. It includes a diagram showing a 3 tier network which appears to be implemented with layer 3 switches for distribution and for core. Then it asserts that you need a router. My question is why would you "need" a router? It seems to me that this network could be implemented as shown with layer 3 switches and function quite well.
Perhaps the original poster wants to ask a somewhat different question, which might be whether it is possible to implement the network shown in the diagram using routers as the core devices (and perhaps as distribution though it probably is more common to have switches at distribution and routers at core). The answer is that yes it is possible to replace the switches at the core layer with routers. The routers can not have two interfaces in the same vlan or same subnet. But the routers can have vlan sub interfaces such that each router has a sub interface in each vlan and so there are two router interfaces processing the vlan (one on each router). The vlans connection from the router would go through the switches at the lower level. This would allow the router to run HSRP on each vlan sub interface and would provide the redundancy and possibly load sharing that the original poster asks about.