Routing through a switch

I know switches just pass data at a L2, unless configured for L3 (not in this case). Now, if you have two routed interfaces that are connected to a L2 switch, how is the switch passing L3 packets? I am sure this is an easy answer, but for some reason I cant get my head around it. If an interface is routed, then it is encapsulated within IP. But if a switch does not understand IP, how is it making it through the switch?

Apparently, someone at work was testing something and some of us were under the impression you will have issues if you connect two routed interfaces into a switch and assume they will communicate properly.

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Comments

AlanJames

Do you mean between vlans? you set up a "trunk" between the layer 2 device and the layer 3 device.

If you had a layer 3 switch, you just set the "ip routing" command on it to route between subnets/vlans.

What are the switchs you have at work? Do you know what he was trying to do?

Sanis4life

You know what, I just found out he used a L3 swith, which totally makes sense now. I thought the tech mentioned he was using a L2 switch, which is when the confusion started.

Thank you for your reply!!

Netwurk

The IP packets won't have any problem making it through a switch. You can even have two routers connected to a hub and they will be able to communicate just fine if they are on the same subnet.

Of course no routing will take place at the hub or L2 switch, but the packets will still get through.

You don't need a complicated piece of hardware to simply forward packets. The simplest piece of hardware you have is your CAT cables and packets make it through those just fine.

Sanis4life

Netwurk wrote:

You don't need a complicated piece of hardware to simply forward packets. The simplest piece of hardware you have is your CAT cables and packets make it through those just fine.

Excellent point!

networker050184

The ip packets are encapsulated into ethernet frames before they are sent out the ethernet interface regardless if it is a routed or switchport.