Monday 12 december 2011 1 12 /12 /Dec /2011 07:28

Layer 3 switches were conceived as a technology to improve on the performance of routers used in large local area networks (LANs) like corporate intranets. The key difference between Layer 3 switches and routers lies in the hardware technology used to build the unit. The hardware inside a Layer 3 switch merges that of traditional switches and routers, replacing some of a router's software logic with hardware to offer better performance in some situations.


Layer 3 switches often cost less than traditional routers. Designed for use within local networks, a Layer 3 switch will typically not possess the WAN ports and wide area network features a traditional router will always have.

 

Discussion: Router vs. Layer 3 Switches   ---from Cisco learning home

Q: As we all know that Layer 3 switch can perform the routing tasks if routing is enabled. But I`ve some questions regarding this:

 1. What is the main difference between this two?

 2. What is the choosing criteria between this two i.e. when should I use which one? What’s about the cost effects?

 3. Why router is needed if there is existence of Layer 3 Switch?

 

Re1: L3 switches do not have WAN interfaces.

You can connect Ethernet circuits to a switch so you only need a router if you want to connect traditional circuits such as E1 E1 SDH or old technology such as X21 V35 or async circuits. As far as I know Call Manager Express does not run on a switch but does on a router. Switches support Wi-Fi controller, Firewall so are quite powerful. So you need to understand the business requirement before deciding router or switch. Also routers can include switch modules.

 

Re2: If it routes, it's a router.  说明: https://learningnetwork.cisco.com/4.5.3/images/emoticons/happy.gif

L3-switch is a marketing term.  It's a router with only Ethernet interfaces and lots of them.  It also has a switching function to it. Which makes it both a router and a switch? The differences will vary based on model.  It depends...  Cost varies as well, everywhere from inexpensive to very expensive! And truly there isn't a "need". You need an L3 device of some sort to exit your subnet. How you design that, or what specific piece you use is entirely up to you.

 

Re3: Traditionally, Routers were devices that connected the LAN to the WAN and switches were just LAN devices and you may add a layer 3 switch to the lan if you had some vlans and didn't want to use a router.

 

However, as technology changes, the tradition of the WAN and LAN are fading.  My "WAN" links are actually 1 gig single mode fiber circuits that terminate to an ethernet fiber interface on a Layer 3 switch, a 6500, 4500, 3750 or even a 3560.  Now some will say that I have a MAN with those kinds of links.  It seems that as Scott said, Cisco Marketing is still stuck on calling a router a device that terminates a traditional WAN link,  I do agree that if the device routes, it is a router.... to some degree.

 

One thing I did notice regarding routers and layer 3 switches, and I will admit that router model and IOS version may play very heavily into this, and that is Routers seem to support more traffic monitoring features, such as netflow and nbar where as Layer 3 switches don't seem to have that kind of support.... until you get to the 6500.

 

Re4: Technically, the differences are:

1- L3 Switch do switching at layer 3 by preserving the source and destination mac and preserving the TTL value of the IP header of the 1st routed packet, so the first packet is routed using normal routing lookup, but after that all packet are switched.

2- router do normal routing lookup, but by introducing fast switching and CEF, packets are also now switched on a router.

3- Switches doesnt support some QoS features.

4- Switches doesnt support NAT.

5- The forwarding on switches is done on ASIC (Application Specific Integrated Circuits) which is done in hardware rather than a software.

6- Forwarding on routers are done in a software.

7- router supports different WAN technologies (modules) unlike switches.

 

Re5: I was just thinking about this.  I didn't learn about Layer 3 switches until the bcmsn.  I know in CCNA they were still really pushing the router vs switch concept.  Talk about throwing a monkey wrench into things when you throw in the concept of Layer 3 switch.

 

So to review:

1. A pure router will do just that, typically no switch ports, in today’s cisco world I don't even know if they make one of these, wouldn't that be something like an ASA with 1 or (2) 100 mb or gig ports with a serial port or similiar?

2.  A switch will just allow connections to edge devices, a true layer 2 switch like a 2960, Int vlan's is what allows management of the switch at layer 3.  No routing between vlans, this is where router on a stick comes into play.

3.  A layer 3 switch integrates both abilities, but it depends on the model on how integrated and featurific it is.  Will it support netflow?  Will it route between vlans?  If you do a show ip route what will be displayed?  How does it implement vlans, is it traditional vlan.dat file or will it do the switching way with show vlan? -- The simplest true layer 3 switch will support all switching features, but have the ability to do routed ports and route between the vlans.  I have had a Integrated services router like a 1760 or 3725 or similiar where they had a small switch module, say 4-24 (100).

 

The definition of a layer 3 switch also may include the ability for a port to be either a routed port or a switched port, the commands switch port vs no switch port followed by having to assign it an ip address.

 

This is another point that also took some getting used to.  In a port that can be either layer 2 or layer 3, or strictly layer 3 or layer 2.    Example, a router can only do layer 3, so to do inter vlan routing  while connecting to another switch via  trunk port you have to give it sub interfaces to a physical switch port, give each one its own ip address and tag it with the encapsulation dot1q #.  Router on a stick, vs. switchport mode trunk command with layer 3 interfaces via the "int vlan 1" with an ip address assignment.

 

While I understand the ccna approach to teaching fundamentals and where thing started, it no doubts confuses someone especially when a question asks about the differences between a hub/switch/router.  In today’s world, hubs don't really exist, and in a large company odds are you’re going to be using a layer 3 switch.

 

Re6: Not sure that is accurate. I think most layer 3 switches can handle BGP, but to what extent? Full tables? Probably not. Dishing out money for 2 Cisco 2821's or Cisco 2921's is going to be way cheaper than purchasing another Cisco 6500 for our network....not to mention our Catalyst 6500 already does a lot of work...and now I am going to throw BGP at it....AH it would just shut off and give me the middle line card!

By Cisco & Cisco Router, Network Switch - Posted in: Cisco Switches, Cisco Firewall
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