Use Case for Routing Protocols on Routers?

[jrberger111]

This is probably a dumb question, but what is the use case for a routing protocol (EIGRP, OSPF, IS-IS, etc) for a typical average business? Why would a business need more than one router (excluding GLBP)? Why wouldn't a building just have multiple multilayer switches connecting everything in a building and run routing protocols through them? I can understand why the government or an ISP may have tons of routers, but why would, for example, a bank or a school?

[Richard Burts · Accepted solution]

The title of this post asks for use case for "routing protocols" on routers. It seems to me that the post pretty much provides the answer when it talks about running routing protocols on multilayered switches on the inside network. You also need the routing protocol running on the router so that the router knows about the networks that are present inside the network.

It seems to me that the real question the original poster wants to ask is about having multiple routers. If we do back some years there was a clear case for multiple routers because there were WAN technologies that ran on routers but not on switches. In current environments that is not so true. There are a few things that can be done only on routers and that could be a reason for multiple routers. The obvious one that comes to mind is Network Address Translation. From an architectural perspective we might want to harden the device that sits at the edge of a network, or the device that sits at the connection of a remote site to a main site. It is probably easier to harden a router (fewer interfaces, vlans, subnets) as compared to a switch.

[jrberger111]

Thank You! I'm sorry that my post was kind of confusing. I mainly thought of these questions because many of the labs I see have many routers linked back to back. I was trying to wrap my head around the realistic application of such a concept. I had known that routers are generally used as edge devices, but I had completely forgotten about NAT.

[Joseph W. Doherty · Accepted solution]

To your first question, why "a typical average business" needs a routing protocol, it's basically the same answer for whether to use a routing protocol or not, which is you need to inform a L3 routing device, whether a router or L3 switch, about not directly connected networks and where to forward traffic to them.  Informing a routing device might be done statically or using a routing protocol.  The first tends to become more and more difficult to maintain as the network topology grows in size and/or complexity and the latter offers other advantages the former normally does not, such as avoiding forwarding traffic to networks that are not, currently, reachable.

To your second question, why a business might need more than a single router.  Well, sometimes a business doesn't need any router for routing, as a L3 switch might be used.  However, when a router is needed for its features, which a L3 switch doesn't support, perhaps one common reason to having more than one, at a location, is for redundancy.  (E.g. years ago I worked at a company where our minimum branch network was pair of 2811s, connected to a dual stack of 3750s.  BTW, each 2811 was connected to a different WAN vendor, which we had to insure did not collate with the other, including not using the same POP or have a fiber run that could be cut with the other vendor by a backhoe.)

To your third question, why wouldn't a building just have L3 switches, using a routing protocol on them.  Well, actually, that's fairly common.  Again, routers are used where you need router features, mostly as the edge of the LAN and WAN.

Lastly, you mention banks and schools, but often such have multiple branches, and for the connections to those branches, a router or L3 switch might be used.  Again, which, depends on feature needs.  Even if there are no other L3 devices as the site, the site edge is generally routing between itself and a hub/HQ and/or other sites.