What is the difference between router and L3 switching

Published on by dellpe

Question:

What is the difference between router and L3 switching apart from WAN interfaces?

Answer:

Routers were devices that connected the LAN to the WAN and switches were just LAN devices and you may add a layer 3 switch,for example WS-C3750X-48P-S 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 is 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.

1. A pure router will do just that, typically no switch ports, in todays 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 switchport vs no switchport 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 funamentals and where thing started, it no doubts confuses someone especially whena questino asks about the differences between a hub/switch/router. In todays world, hubs don't really exist, and in a large company odds are your going to be using a layer 3 switch.

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.

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

Published on cisco 2960 switch

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