On Fri, 27 Sep 2002, Stephen J. Wilcox wrote:

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When designing an all IP network requiring mostly Ethernet interfaces, the logical conclusion is to specify layer 3 switches (instead of routers). The cost per port and functionality requirements make a layer 3 switch the perfect choice.

First questions would be how large is this network in terms of interfaces and traffic flows and how is it distributed. If its small use cheap L2 switches, if its on one or a couple of sites with not many hosts why do you need L3 in there at all, stick to L2..

Agree with the not many hosts part. You especially don't want customer hosts to connect directly to your layer 2 core because it gets very messy. But the main problem with layer 2 is that it handles redundant links so badly: spanning tree simply disables them.

If its larger and more distributed then you need to aggregate up anyway so I'd imagine its cheaper to use plain L3 routers connecting the L2 LAN across intersite WANs

Needs more quantifying to find any conclusion but I dont see that an "all IP network" requires a L3 switch network!

Presumably, you'll need at least one router (I mean a device configured to do layer 3 forwarding, whatever it may be called) to connect to your transit(s).

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However, the rule of thumb in the IP community is that routers are superior to layer 3 switches and should be utilized instead, especially when considering core type functionality.

I think we have a terminolgy issue here.. assuming a L3 switch is a device which uses routing decisions to influence a switching process then you get this on current Cisco routers.. the L3 makes the routing decision on the first packet in the stream but then additional frames are switched.

This is traditional "layer 4 (or multilayer) switching" or flow-based routing/switching. Be very careful with this because it can blow up in your face if there are very many new flows every second, which is typically the case for any serious level of WWW traffic.

But that aside I think you mean what vendors call "L3 switches or L4 switches" which are like a L2 switch but go into higher layer protocols to influence the switching decision and perform other features most commonly load balancing.

The terms are used so loosely these days that you really need to investigate and not infer functionality from the name alone.

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Does this rule of thumb still apply considering the modern layer 3 switches available? If not, why? What makes a layer 3 switch sub-standard to a pure router? Any quantitative analysis you could provide would be greatly appreciated.

Most commonly seems to be interoperability, the switches do their own job fine in their own isolated environment but they cant act as a "ISP router".. in my experience then tend to have odd bugs and behave slightly unexpectedly when say for example routing OSPF or BGP. Altho this is probably a chicken and egg - if more people tried to use them perhaps the vendors would fix the code!

I've been running OSPF on a pretty old Extreme switch for years now in a small network (hand full of routers, several hundred routes) and no problems at all. I've had mixed reports on BGP on Extreme and Riverstone, but on Foundry it seems to work well for at least several people I've talked with. All of these boxes will route IP very fast and relatively cheap. If money is really tight, you could consider PC's running your favorite Unix flavor and Zebra. The functionality is fine, but the hard- and software fails more often so you need to put in more redundancy. Another good way to go would be just a couple of large Cisco or Juniper boxes for all the layer 3 stuff (limiting broadcast domains, filtering and routing protocols) and connect everything else using switches and VLANs.

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I realize your answer may depend on device position within the network. I am comparing a router to a layer 3 switch as a core routing device, an EBGP border router and access device. Remember, my network is comprised of mostly Ethernet interfaces (FW, GE) and the occasional DS1 and DS3 interface.

Core routers typically don't do any filtering and the BGP setup (if any) is straightforward, so switch-like routers are good here. For access you need filtering, which many layer 2 switches won't do. Multilayer stuff is also good here, since it gives you many ports and good performance. If you don't want to have these boxes talk BGP to your customers you can simply backhaul BGP customer subnets over a VLAN to one or more "real" routers elsewhere. I wouldn't necessarily recommend routers with a switching heritage as border routers since this tends to stress the BGP implementations the most. In any case, you'll be taking the road less travelled so test your stuff real good before deployment.

On Fri, 27 Sep 2002, Richard A Steenbergen wrote:

On Fri, Sep 27, 2002 at 11:28:39AM +0200, Iljitsch van Beijnum wrote:

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Core routers typically don't do any filtering and the BGP setup (if any) is straightforward, so switch-like routers are good here.

May god have mercy on your core.

Thank you. But what exactly necessitates devine leniency? You aren't taking my remarks to mean that it's a good idea to redistribute a full BGP view into an IGP, are you? What I'm getting at is a small setup where all transit and peering links are in the same location. The border routers at this location can inject a default into the IGP so the number of routes in the non-border routers stays nice and small.