What can you do on a cisco router that you cant do on a layer 3 switch?

[limonKhan85423]

Hi I have a 3750G and would like to know what is the difference between L3 and proper routers as the lines have become somewhat blurred

I understand that routers are L3 only and they have the advantage of multiple routing protoclos like rip ospf eigrp & ISIS. Is there anything else i am misssing?

Thanks.

[Giuseppe Larosa]

Hello @limonKhan85423 ,

the main differences are in some features that needs to be performed in software like NAT Network Address Translation.

Only high end switches like C6500 with Sup720 could do NAT.

So to take an appropriate decision on what device to use you need to know what will be the role and duties of the device.

If acting as edge device connecting to the internet with a need for NAT you will need a router as your Catalyst 3750G does not support NAT.

Again if acting as edge device if you are interested in some security features like Zone Based Firewalling this is again a reason to choice a router over a switch.

Other features that might be useful is the use of NBAR for traffic classification in QoS and modular QoS that is not present in C3750G.

A SW based router has more features making it suitable at the edge / WAN , a multilayer switch on the other hand has much more performance making it ideal as core device for inter VLAN routing at wire speed.

Hope to help

Giuseppe

[Martin L]

Besides mentioned above NAT feature, difference between L3 switch and a router is type of physical connections, ports you can make or add to a router.  WANs vs LANs. Only Routers can directly connect to ISP via various add-on modules with ports or connection types like serial, DSL, Cable, etc. L3 switch has limited, and usually fixed ports and modules are fiber uplinks to other switches or routers.

Regards, ML
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[Richard Burts]

In addition to the points already made about differences between routers and switches you will find that sometimes there are aspects of a feature that are supported on a router but not on (at least some) switches. I have had several discussions in the community with users who were implementing Policy Based Routing on switches and found that while the verify-availability parameter worked on a router that the switch they were using did not support this capability.

[cyberconsultants]

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@Martin L wrote:

Only Routers can connect to ISP via various ports or connection types like serial, DSL, Cable, etc.

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This is not true. A modem is what modulates and demodulates non-digital ISP signals.

Combo modems/routers (which ISPs will often refer to as 'gateway devices' since they sit at the edge) are the likely source of confusion here.

Both routers and L3 switches can and do connect to modems.

[Martin L]

not directly, my point is that L3 switch has limited and usually fixed ports and modules are fiber uplinks to other switches or routers.

[Joseph W. Doherty]

BTW, "modern" cable (or DSL) modems are generally not actually modems, as the "WAN" side is digital too.

As to routers and/or L3 switches connecting to such devices, also today, most would likely have an Ethernet connection, which typically any router or L3 switch will support.

Finding a L3 switch that will accept a video coax cable or DSL link is much less likely than finding a router that can.  However, so much has moved to Ethernet hand-offs, many different WAN hardware connections options aren't nearly as an important as they once were, and in fact, "modern" routers, especially the "little ones" often don't support many hardware connections beyond Ethernet.

So, back to the OP's question, generally routers offer network features not provided by L3 switches, while the L3 switches often offer much more capacity/performance than like priced routers.  Again, it's also possible to provide router like features with L3 switch performance although, also again, generally at a much higher price than a "regular" router or "regular" L3 switch.

The OP's 3750G has 32 Gbps capacity, compare that against the current ASR series.

[Joseph W. Doherty]

The really big difference between a router and a L3 switch, the former's features are usually based on software using a "general" CPU while the latter is based on very specialized hardware.  This allows the former (a router) to do almost anything it wants while the latter is limited to what the hardware provides.  This distinction is roughly analogous to a PC's main CPU, running some OS (packed with "features") and the PC's graphic board (which does specific graphic functions, very, very fast).

As some of the other posts have noted, routers often support many more hardware options, than L3 switches, because routers' software can support them.  Yet, those hardware modules often have much lower bandwidth capacity, either individually, and/or in overall aggregate (because the overall hardware is much more performance limited).

As you note, the distinction between the two can blur, but usually more so on the "high-end" than the "low-end".  This because part of the "high-end" platforms cost is due to having both potent CPU and software, while having additional hardware for capacity/performance.  Still, even on the "high-end", it's often not difficult to note how such a platform might favor features over performance.  The former, usually is of some router linage, while the latter usually is of some L3 switch linage.

Possibly of interest was one of the earlier Cisco platforms, the 6500 and 7600 series.  The former was a L3 switch series, and the latter a "router".  Basically, the chassis was the same and they could use many of the same line cards, but not all the same cards.  Also, early on (I recall?) they could even run the same IOS image.

[Jonathan Cuthbert]

The lines are blurring, but they still exist.  It's a question of ASIC-based (switches) or QFP/CPU-Based (routers) platforms in modern releases.  When it comes to Layer 3 features, routers, all things being equal, have higher scale.  Switches have bigger port density. 
One thing that routers can do that switches cannot is crypto-features.  So if you want an IPSec tunnel, you need routers.  There are other distinctions and great points made throughout these posts.  The ultimate question is "What problem are you trying to solve?"  That's how we begin the positioning discussion. 

[Richard Burts]

Excellent explanation (and a well deserved +5). I have for a long time regarded address translation as the major distinguisher of router vs switch. Do not know why I was slow to recognize that ipsec was also a distinguisher. 

And I really like the what problem are you trying to solve approach. That really is the key.

[Joseph W. Doherty]

"One thing that routers can do that switches cannot is crypto-features."

MACsec?

"When it comes to Layer 3 features, routers, all things being equal, have higher scale."

Could you expand on this a bit?

Perhaps you're referring to something like Cisco's "Mass Scale Networking", and series like the ASR 9Ks, 8K series or the CRSs.

If so, these "routers" are generally chuck full of hardware to provide their massive capacity, but I thought they too gave up some flexibility/features of (much) smaller routers because of their hardware.  For example, do all these series support IPSec tunnels?  Do they support ISR QoS features like NBAR and CBWFQ and its many options?  Other ISR features?

(NB: my experience with massive routers is dated, going back to the CRS-1s and CRS-3s, but if they had such features, they certainly weren't important, or used, by us, as you correctly write such features did not address the problem we were trying to solve.)

I completely agree lines can blur between what's a L3 switch and what's a router, especially as newer hardware continues to support more and more features that once could only be done by software.  Yet to me, the fundamental difference is a L3 switch is restricted by its hardware while a router is restricted by its software.

Also, to me, what a vendor "calls" a device, may really have more to due with "marketing" rather than the underlying architectural implementation.  This is why in an earlier post I mentioned the 6500 and 7600 series being interesting, as at one point (I recall?) you could basically use the same hardware and IOS running in/on either, but one was a "L3 switch" and the other a "router".  The initial distinction between the two, and later divergence, I thought had more to do with Cisco's marketing approach rather then technical implementation.

Anyway, to the OP, to better view what's a L3 switch vs. a router, first it might help to understand the significance of "switching" (frames) vs. routing (packets), also why MPLS's predecessor was developed.

it might help to also study an earlier Cisco approach to creating a L3 switch by actually using a L2 switch, the Catalyst 5x00 with an external Cisco router (a router switch module could also be used, but the external router really "shows" the approach); the result being MLS, multilayer switching.