Layer 3 Switch Versus Layer 3 Router as a Routing Engine documents or Best Practices

[patrick.hurley]

I have a customer that has layer 2 LAN switching devices.  They are using the edge router for layer 3 routing between 3 vlans.  What complicates this is that they actually have two layer 3 edge routers, one Cisco, the other Edgewater backing each other up.  When the Cisco router went down there was a missing route in the Edgewater.  In this mode of course no vlans were being routed properly.

My question is are there documents that show the best practice of using layer 3 switching in the LAN core rather than using edge routers for layer 3?

[marce1000]

                                   >the best practice of using layer 3 switching

 On first notice that is a bit contradictory switching is switching (layer 2) and routing happens on layer 3

 M.

[Jon Marshall]

Terminology really ie. L3 switching is usually understood to be routing in hardware as opposed to software although the distinction has become somewhat blurred these days. 

Jon

[Joseph W. Doherty]

Hmm, an interesting question.  Don't recall ever seeing a L3 switch recommended over router for LAN routing as a best practice.

It's often a typical practice, though, because L3 switches tend to support LAN routing performance much, much (much) more economically than a router providing the same level of performance.  Further, LAN routing often doesn't require all the features a router provides, which are often more applicable to WANs.

Two exceptions, though.  First, for LANs were most of the traffic is going off site (i.e. mostly north/south), via a router, the router might be used for local LAN routing too, as there's very little local site routing (i.e. little to none east/west).

Second, some Enterprise level smart/enhanced L2 switches support a very limited L3 routing capability.  These might be used for local site LAN routing avoiding both a router doing that routing and the purchase of a "true" L3 switch.

BTW, in the case you describe, a L3 switch wouldn't necessarily improve the situation for redundancy.  They too can be misconfigured, and as often configured as the "core" of a small LAN, if they fail, they might be a single point of failure.

[patrick.hurley]

Layer 3 routing on a switch is really what I am talking about rather than run the routing on a WAN edge device that doesn't have the layer 3 routing/throughput capability.   

[Joseph W. Doherty]

Yes, I understood that.  But, just because you can do something doesn't mean you should.

What I was trying to get across was there are situations where using a L3 switch provides no real benefit.  If not, it would explain why always having one is not a best practice.

[Giuseppe Larosa]

Hello @patrick.hurley ,

L3 routing on switch is well suited for inter VLAN routing.

NAT and VPN have to be performed on a router as most switches are unable to perform NAT and to manage site to site IPSEc tunnels.

If you choice to move inter vlan routing to switches you will need additional subnets for communication between the L3 switches and the Cisco router and the L3 switches and the other vendor router.

To achieve full redundancy and fault tolerance at node level you would need two L3 switches to perform inter VLAN routing as noted by  Joseph and these two have to communicate with the two routers for exiting to outside world.

A dynamic routing protocol between the four devices would be the ideal solution.

Hope to help

Giuseppe