difference between layer-3 switch and router

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Nov 18th, 2005
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can router be replaced by layer-3 switch?

what is the major difference between layer-3 switch and router and if router is replaced by layer-3 switch, can the network function properly?...plz ans to these ques are very important to me

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Georg Pauwen Fri, 11/18/2005 - 23:16
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basically, a Layer 3 switch combines routing (Layer 3) and switching (Layer 2) functionality in one box. Layer 3 switches are mainly used for local inter-VLAN routing; replacing a router with a Layer 3 switch is going to pose a problem, because the Layer 3 switches do not have e.g. serial ports. But the latest generation of routers, e.g. the 2800 series, have switching modules that let you integrate the switching functionality in the router. If you put e.g. the NM-16ESW (see the link below) into a 2800 router, you will have a router with an integrated 16-port switch:

Cisco EtherSwitch Network Modules


Have a look at the document below as well:

CCNP Practical Studies: Layer 3 Switching


Does that make sense ?



minimintu Sat, 11/19/2005 - 02:54
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that means the only difference is of serial port. is there any other difference apart from this? does high ens switches i.e 4500, 6000 series doesnt have SERIAL ports?

qstyk Sat, 11/19/2005 - 04:52
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6500's pretty much don't have any serial ports. I suppose you could buy them if you wanted to (haven't checked) but the standard is to use port adapters with built-in CSU/DSU's.

A layer three switch works like this:

With the 6500 SUP2 generation, you could purchase something with it called an MSFC. This was basically a daughter-board that snapped onto the SUP and is basically a router. It has it's own CPU and memory, etc. What this allows you to do is use something called an SVI (switched virtual interface.) These are typically used at the distribution layer and allow you to configure a virtual interface, using the VLAN entity as your interface.

You literally type in:

conf t

int vlan 777

ip address

no shut

etc. At that point, any device in that VLAN can access that gateway. It provides a LOT of flexibility and can make life much simpler. It's not cheap, though. Since the Cisco sales team couldn't keep their fingers out of the realm of sanity, the SUP3 is now called a SUP720. If you have a SUP720, the MSFC isn't optional and you will need to run in native mode, not hybrid.

glen.grant Sat, 11/19/2005 - 08:11
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6500's do have serial ports , you just need to buy a flexwan card and buy the corresponding serial port adaptor that you need to put into the flexwan card . As an old analogy they basically like the old VIP cards in the old 7500 series routers.

Richard Burts Sat, 11/19/2005 - 12:59
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Glen is quite correct that on the high end switches you can obtain serial connectivity through the flexwan.

I believe that there is a significant answer to the question and an important difference between layer 3 switch and router that goes far beyond the difference in what type of interface is supported.

In the aspect of running routing protocols and forwarding packets based on their IP addresses the layer 3 switches and routers are pretty much the same. But there are a lot of features that are included in typical "router" code that are not included in typical "switch" code. One that I ran into recently was Network Address Translation. It is typically supported in router code and was not supported on the layer 3 switch that I was looking at recently.

So the question of whether a router can be replaced by a layer 3 switch does not have a simple answer. You need to evaluate where the router is placed and what functions it is providing. In many instances yes the router could be replaced by a layer 3 switch. But in other instances the router may be providing services that are not available in layer 3 switches.



minimintu Sun, 11/20/2005 - 20:18
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so if 6500's series have serial ports then why not replace the router with these switches?...i am unable to understand what is the difference between these switch and router?...from commercial point of view also, switches cost less so why not replace it with routers?

krishna.kg Sat, 11/19/2005 - 15:58
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Hi mini. the difference between a router and L3 switch is that the L3 gives more flexibility in packet forwarding using special hardware and using Layer3 or Layer4 based information. Your requirement is not clear. In case of WAN a router is required because it's spcialised in meeting WAN requirements. Yea high ends like Cat6500 supports termination of serial connectiions.


You can consult the above site to check which card supports your requirement.

martin_tsang Sun, 11/20/2005 - 20:55
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A 6500 series switch with a SUP card is pretty much a router. But you'll find that most of the lower end SUP cards dont really do traffic shaping and QoS like a 7200.

Although a 6500 can even run and hold full BGP tables just like a router, it doenst have as much grunt as a router.

I dont think you can run large networks using L3 switches, and their routing capabilities was probably designed for small networks or stub networks.

Try running NAT on a 3550 L3 switch, it wont even let you do that, let alone other capabilities a router provides. ( such as priority queuing, IPSec )

When you say you fail to see why ppl dont just all buy 6500s switches for everything, they are pretty expensive, and their cards aswell!!!


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