Difference b/w a Router and Multilayer Switch, Their correct Usage and Installation

I placed few questions on Cisco forum to discuss the differences between the usage of a Router and Multilayer switch. There were my questions;Today’s Multilayer switches are versatile; they can do all the things a router can do;

  • Will Router be eliminated by Multilayer Switches?
  • Are there any limitations to a Multi-layer switch that it can not completely replace a router?
  • If both functions can be done by a multilayer switches then why a router is still produced?
  • Will in future be only one device in the market that will do Routing+switching, and will there be no separate Switch or Separate router?
These types of question are usually in the mind of Networking (Cisco) guys who are only studying and have no experience of networking hardware in any firm.

Here are the answers in the reply of above questions that will certainly answer to most of your questions.
Reply 1 
Generally router would have better processing power to perform routing more efficiently than layer 3 switch. Also most of the time it won’t have to route all of your devices on the network, so hopefully your routing table on a router will stay small, thus it will be more efficient.
Reply 2
How do you connect a switch to the Wide Area Network if it is not Ethernet.
Reply 3 
Multi-layer switches (hardware and software) are more expensive than a standalone router, and that is one of the many factors that drive purchasing decisions.
There are WAN modules for some of the multi-layer switches as well.
As time goes on, we have the ability to put more and more features in 1 physical chassis, and the decision on whether to use 1 box for everything will be based on details such as cost, features, fault-tolerance, network design, etc…
Reply 4
Well… That depends on the specifics that we get into!
In short, (IMHO) the answer is no. We’d have to go to an Ethernet-only world (not necessarily a bad thing) but even then, there’s different feature sets that people want for different things.
Routers cater to different needs than routing-enabled switches do.
If we are talking about 6500’s where there’s actually a separate routing blade within the chassis, and a wider variety of interface types supported, then I’d say it’s possible. However, those things aren’t cheap!
So since we don’t all live in the world of unlimited budgets, I’d hazard a guess that the answer is still no!
Reply 5
then again I don’t know any scenario where a 6500 was uses as a WAN edge device. they’re mainly as Distribution/Core layer devices, right? Not to get off on a design discussion……
It’s interesting, though, how WAN technologies is heading. Now days when want to redesign or increase your WAN connectivity, the Service provider is pushing multi-megabit Metro Ethernet. I’ve witnessed the demise of Frame Relay as many of us have over the past few years. It’s not uncommon to ask for a WAN connection and the SP wants to put you on a guaranteed minimum 3-5MB and its MPLS.
It used to be that with a 2600/2800 ISR as a WAN edge router that you’d have to purchase a WIC for the Serial Interface and the built-in was used to connect the LAN. Now the $800 T-1 cards are being ditched for the integrated Ethernet interfaces. With that said, I suppose you could technically interface 3550/3560/3750 catalyst switch with an MPLS metroEthernet but it seems like a waste for most environments outside a SP. I can see how you may want to use one of those due to the port density and the number of customes you’re interfacing with if you’re a SP but it still doesn’t seem practical, although technically possible.
By no means meant as a debate…..:)
Reply 6
I’ve seen a 6500 used as a wan device. Aside from the WAN modules that Scott mentioned, I have seen a 6500 used for metro Ethernet fiber wan links
Reply 7
6500’s can do quite well on the WAN edge. However, they are not the most economical devices to put there IMHO!
But, like anything, it depends on what features you are looking for and how integrated/redundant you want to make anything.
There are plenty of choices in life! But even from the concepts of the 6500, it doesn’t solve ALL problems in ALL situations! So from that, I’d say it will never mean the death of routers.
Besides, where do you draw the line about whether it’s a multilayer switch versus a multilayer router?
Again Some few more questions by me
thank u all of you for sharing your detailed thought on my questions.
plz correct me if I am still misunderstood but what I came to know with these answers is that there are various telcom technologies in the world, and all of those require different types of interfaces/services/bandwidth etc and for those different types of models of switches and routers with various types of ports are being produced.
It has almost answered to my question but created another questions in my mind…
but if we can even run BGP on 3550/3560 switches then why some common WIC slots are not being provided with these switch models? to combine both switch+router in one?
are there technical limitations? or it is just to create different models with different cost for earning money???
expanding my question further…
if there is a small organization with 3560/4500 at core and router 1800/2800 at Edge then is there any (low or moderate cost) single device which can serve function of both of them and replace both with one?
Reply 8
When I was preparing for CCNA your question came to my mind. But when I switched field into network engineering the choice of choosing a MLS or router becomes clearer.
In Singapore, there are organization that subscribed to Lease line circuit that requires a lease line modem (a G.703 compliant modem such as this brand Patton) to terminate the lease connection, now for example Patton G.703 modem requires a serial connection (x.21 female type or V.35 female type) a modular router which can support HWIC serial card or Serial module is an obvious choice, Cisco MLS (those 3xxx series and 4500 series) only have fixed switch ports and no module slots for you to mix and match with different interface card.
And if you need to do NAT, 3xxx series and 4500 series MLS series do not have NAT features at all, you want NAT and insist on using MLS? Your only bet will be an expensive all-in-one 6500 series chassis (the chassis itself is expensive not including those modules….)
Now what to choose is based on your customer’s requirement, you have to meet up with your customer and discuss what they want to do and how much money are they willing to pay for the solution…
For layer3 redundancy, MLS 3560/3750 only has HSRPv1/v2, no GLBP and no VRRP. But a 1800 series router with only an IPBase image has all three redundancy options (i.e. GLBP, VRRP and HSRPv1/v2).
If you customer wants wired speed switching and dun require to connect to a serial modem and do not require NAT, choose a MLS.
Btw your question about creating different models with different cost just for earning money, is partly correct If you are the vendor and want people to buy your product you should give consumer more options and pay according to what they need so that your market can expand. Same as cisco MLS, you choose and pay according to what you need.
Example the difference between 3560 and 3750?
Main difference is 3750 has stackwise technology, which can unify 3750 switches into one logical stacked switch (up to 8 stacks), this 3750 contains the same redundancy features as 3560, but you pay more for stackwise…;) Stack switch can be considered as a cheaper alternative for 4500 series switch, but 3750 only has up to 32Gbps backplane switch fabric.
Example difference between 4500 and 6500?
6500: Sup720 switch fabric is up to 720Gbps.
4500: Sup7 switch fabric is up to 848Gbps.
From superficial view it seems 4500 has a higher “up-to” switch fabric than 6500, but 4500 is a pure switch chassis, it does routing and switching only, period.
6500 on the other hand is an “all-in-one” integrated service solution. That is 6500 can support modules that are not necessary switchports, you can add Firewall module, Wireless module, WAAS module….blah blah blah…. sounds great? module and chassis are separate cost, and you may encounter the module is more expensive than the chassis itself…
Though all of the replies are clearing differences between Router and MLS Switches but Reply 8 was the clear Answer to my  Question.
Hope this helps you too…