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I understand layer 3 switches are essentially routers, but often don't have the capabilities a router has, e.g. NAT. So I'm trying to work out where it would be necessary to have layer 3 switches vs routers

When is would use layer 3 switches oppose to routers for two buildings.

  • use layer 3 switches in both buildings

  • use layer 2 switches and in the buildings and connect these to 1 layer 3 switch

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  • Asking for opinions is off topic here. The choice of router vs layer 3 switch usually comes down to media type and cost. It's not clear to my why you would use router on a stick. Are there other factors you're not mentioning?
    – Ron Trunk
    Jun 23, 2020 at 17:57
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    You could ask this on chat, where questions like this are welcomed.
    – Ron Trunk
    Jun 23, 2020 at 17:59
  • re: router on a stick. I didn't initially think about layer 3 switches, so initially router on a sticker was the first option
    – The_Bear
    Jun 23, 2020 at 18:03

2 Answers 2

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Layer-3 switches in each building will give you the most flexibility.

Configuring an L2 trunk -- or even a StackWise or similar link -- between the buildings will make subnetting and management less complex. You can do this even if you select layer-3 switches as your equipment.

For small sites, it's worth evaluating the cost of using the same type of switch everywhere. You might find the extra spend makes sparing and operations easier.

Fiber type may be another choice you can make. Single-Mode Fiber (SMF) has advantages in range, support for higher-speed interfaces, and sometimes, compatibility with carrier connectivity (without extra devices like media converters.) Multi-Mode Fiber (MMF) is slightly less expensive upfront, and so are the transceivers you use with it. My personal experience is that standardizing on all SMF can simplify inventory and operations without creating too much cost for small sites (e.g. those without so many links that the price difference becomes large.)

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  • My guess is you don't need the router, or even benefit from its presence. The real question is, do you want your ASA to do all the layer-3 routing between your different VLANs (allowing you to use zone-based policies and stateful firewall to segregate parts of your network) or do you just use a layer-3 configuration on your switches? It's up to you and depends on your security and bandwidth needs.
    – Jeff Wheeler
    Jun 23, 2020 at 19:25
  • Regarding connecting the buildings together - If I used layer 3 switches in each building, with the layer 2 switches connected to these, is there are difference in connecting the building either via their layer 2 switches or via the layer 3 switches?
    – The_Bear
    Jun 23, 2020 at 19:43
  • There'd be no difference in your options for the inter-building connection, firewall topology, etc. Having all those other switches be cheaper layer-2 ones can be a good cost savings without giving up much in terms of network configuration flexibility.
    – Jeff Wheeler
    Jun 23, 2020 at 21:47
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To optimise network connectivity for this requirement can be even accomplished by

Router : 1 (example 1800 series) or any high end router, Switch 1: layer3 switch (example Cisco 3850) or any other vendors Switch 2: layer2 switches depends upon number of end users and servers hosted

Configurations overview

Configuration SVI in layer3 switch number of SVI configuration depend upon business requirements among this SVI one VLAN can be used for management for accessing layer3 switch and layer2 access switches . DHCP can be configured on layer3 switches for dynamic allocation of ip address to end-users. Use static addressing for DMZ VLAN. Restriction among VLANs can be controlled by a access-list configuration in layer3 . Connect access -switches with trunk links from layer3 switch . One layer2 switch is deployed in 1st building , second layer2 switch in 2nd building . Layer3 switch and router will be deployed in 1st building . If distance between two buildings are above 100 meters then use fiber cable for connecting layer3 switch to 2 building access-switch use media convertors at both end for ethernet to fiber convertors pupose if not go with sfp modules at layer3 end and media convertors at layer2 switch end. Configure default route in layer3 switch pointing towards router inside interface .

Router configuration

ISP internet ip should configure on router outside interface and configuration default routers in router

Assuming LAN supernet address : 10.180.0.0/19

Ip route 0.0.0.0 0.0.0.0 point towards ISP gateway ip route. 10.180.0.0 255.255.224.0 pointing towards layer3 egress interface /*connecting router and layer3 interface

Configure natting in router for internet access and static natting for accessing application from outside both for inbound and outbound traffic

Even firewall can be used in place of router as per your requirement If your setup wants to access any other DC resources from point to point link you can have feasibility connectivity from layer3 as L3 interface and configure static routing both ends.

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