Everyone knows the difference between a router and switch right? Good.. (for those that need a good refresher) this post is not going is not going to dive into that topic. What I want to talk about is Router vs Switching from a positioning standpoint. One question I often get asked working with customers is:
“Can I just use my layer 3 switch to terminate this <WAN,Internet,MPLS,etc> link?”
Most of the time technically speaking sure you could do that, but should you? With the speeds of circuits increasing its not uncommon to see enterprises moving towards multiple or even ten gigabit links for connectivity (WAN and Internet). A prime example of this is the adoption of cloud services from Amazon, Azure, etc.
If you begin with looking at things from a pure cost perspective. A high port density layer 3 capable switch looks pretty attractive. You can often get high 1/10gig port density, BGP, OSPF, PIM, etc in a neat 1RU footprint. If you compare that to a router the price is often significantly higher with much less density. Based on this why should anyone even look at routers anymore? The answer comes down to features and performance
When looking at either an upgrade or redesign of your routing edge you need to keep in mind the following.
What will this device need to support today? What might this device have to support tomorrow? Is this really the best tool to use for the job?
Lets use an example
Assume your current WAN connectivity is in the form of a managed MPLS from a large ISP. They install on premise routers that you connect using Ethernet into your local sites layer 3 switch. You peer to them using OSPF and then in turn they pass all the routes between sites.
Now lets say in the short future you roll out a company wide VoIP system. You have to now configure QoS at the locations to optimize that traffic.. At this point anyone who has ever works on switches will quickly see the challenges ahead. Sure you can do QoS to some extent. Here we can start to see some of the differences between what a router vs switch can do.
As your company begins to grow and expand the powers that be decide that the WAN needs redundancy. It is decided that each location will now have its own internet circuit and firewall installed. At first you configure static IPSEC tunnels between the firewalls to build the backup path and use floating static routes in case of a WAN failure.
After more time passes the company grows and acquires more sites and locations. Quickly the simple static tunnels becomes a nightmare to manage and keep track of as sites pop up. This is where dropping a router into your network would allow you to convert the manually created tunnels into a simple DMVPN solution for the backup link with dynamic routing to simplify the network and its management.
All of these previous examples show how features you might not have a use for today might be needed tomorrow. If you start making a checklist of features between the two you will quickly see the difference for yourself.
There is also the issue of performance, mostly related in terms of routing tables, forwarding rates, buffer sizes, etc. Be sure to review the data sheets on any device you are looking to insert into your network especially for larger networks. A 10g switch might only support 256k IPv4 routes while a 10g router might support over 1M