Switch ASICs vs Router IOS

Xenogan

Question: If a L3 switch can perform routing at wire speed then why do we need routers?

This question was answered by Jeremy with CBT Nuggets by saying while switch ASICs are faster, Router IOS s necessary because it provides the necessary features such as NAT.

I get it... but don't switches have an IOS as well? Can someone provide a more clear answer?

docrice

I'm not the authority on this subject, but I'd assume the resources for a large routing table (TCAM) are not something addressed by ASICs in switches.

https://supportforums.cisco.com/document/60831/cam-content-addressable-memory-vs-tcam-ternary-content-addressable-memory

EdTheLad

A layer 3 switch is a router, high end routers like CRS's and ASR9K's have fast switching ASICs in there line cards.

Node Man

The differences betweem routers and switches are blurring as devices become more powerful, but remember that a switch is a layer 2 device and a router is a layer 3 device. That means that a layer two device does not have to perform any routing calculations and works within a single network. A layer 3 device internetworks and needs to perform routing calculations to send packets in the best way possible according to the routing protocol.

In regards to a layer 3 switch, the switch must determine to process a packet at a layer 2 level or 'punt' it up to layer 3 for routing.

slinuxuzer

Also, don't forget the need for modular expansion bays for various wan and voice modules that might not be available if you put a switch motherboard in the form factor.

theodoxa

Layer 3 switches don't generally support NAT or Serial Interfaces. While I don't see any reason these couldn't be supported (some 6500 Series can do NAT) packets requiring the switch to do anything other than basic routing/switching (No NAT, Encryption, etc...) and Non-IP/IPX packets can't be handled by CEF, losing any speed advantage a Layer 3 switch might offer.

fredrikjj

This is a pretty good read on this topic How did we ever get into this switching mess? « ipSpace.net by @ioshints

ccie14023

OK, here we go:

In the dark ages (around 1999 or so) switches were pure layer 2 devices with ASICs which made them run at blazingly fast speeds (100 Mbps). We used one or more switches as the layer 2 connectivity for devices on a common subnet, but we needed a way to cross subnets. Hence routers. These were layer 3 devices that tended to run on general purpose CPUs and not so many ASICs. For example, the Cisco 2500 series used Motorola 68K processors. Did I say blazingly fast? The 68K could do 16 MHz. Wow.

Then someone got an idea: why not build the layer 3 router function into the switch? After all, by this time, people weren't using separate switches for subnets but were using VLANs. So, I could build a big switch and stick a router into it and have the inter-VLAN routing done on box. The early incarnation of this, the Cat 5K with Route Switch Module (RSM) actually had two different CLIs. You logged into the RSM separately from the switch, and the RSM used IOS while the layer 2 portion used CatOS. Eventually Cisco and other vendors realized this was silly and started to integrate routing and switching CLI even while having separate hardware internally. So now you had a combined layer 2/layer 3 box which made administration easier but confused the heck out of future generations of network engineers, who were told that layer 2 and layer 3 are separate and then were told to administer a box where they weren't.

Meantime, Cisco decided to take its 6500 platform, stick a 7600 label on it, and sell it as a "router" since they thought service providers like to buy routers and not switches. At first the 6500 and 7600 platforms were totally identical. Here at Juniper we do something similar. Our EX 9200 switches are basically just MX routers.

Radia Perlman has suggested that maybe "switch" is just a generic term for a packet pushing box, and has lost a technically specific meaning. I tend to agree. I would disagree with a previous poster who said you can't do NAT on a switch. You can on many if not most. Switches these days have many of the same functions as routers, and routers have many of the functions of switches (e.g., fast packet pushing via ASIC.) The difference seems to be one of product placement. Switches are targeted at campus and data center, tend to have a high density of relatively homogenous port types (Ethernet mainly), and often have specialized features designed for high-density low-latency moving of packets. See Juniper's Virtual Chassis Fabric, for example. Routers tend to have a lower port density, support more interface types, and may have more features, although not always.

In short, if you're confused, you have every right to be. I've been in this business a long time, work for a vendor, and I'm confused. Welcome to networking!

powmia

Switches are cheaper for switching, Routers are cheaper for routing. They both use ASICs, but getting a box with enough ASIC to power high port density, along with enough TCAM (for a variety of features) is expensive. This is why you have commodity switches (Broadcom) that are essentially a switch on a chip. Once you branch out of what can be built into a generic ASIC, you get a more integrated system.