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Layer 3 Switch - When does it become a Layer 3 device?!

UselessUser
Frequent Advisor

Layer 3 Switch - When does it become a Layer 3 device?!

Hi,

This might be a stupid question but hey if you don't ask you might never know!

Basically going back to old school talk, a layer 3 device seems to be the separator/end point for any layer 2 stuff. In particular I mean things like VLAN's and broadcast domains, STP/RSTP etc...

With a router due to its limited interfaces this seems to work kind of how you would expect, but with a layer 3 switch (thinking 5400/5600 series)... how does this actually manifest itself??

For example if I plugin a load of computers to this switch with a fresh config, it is essentially a layer 2 device, and broadcasts sent from one machine will be seen by another... what makes this change? Is it the assignment of VLAN's to ports?

I mean with a dedicated router device if I plug in a computer to each "side" they will not be able to see each other at the layer 2 level...

Just trying to work out when the layer 3 stuff takes effect?

Also when assigning VLAN tags, are these maintained on the packets after they have passed through a layer 3 device?

2 REPLIES
Richard Brodie_1
Honored Contributor

Re: Layer 3 Switch - When does it become a Layer 3 device?!

If you imagine putting every port on a separate VLAN, a layer 2 switch wouldn't forward anything. A layer 3 would be able to route packets between the separate VLANs.

VLAN tags will be added/removed as necessary. Even if the packet is forwarded at layer 2, it may be necessary to add/remove the VLAN tag if moving from a tagged to an untagged port.
Pieter 't Hart
Honored Contributor

Re: Layer 3 Switch - When does it become a Layer 3 device?!

it's simple,
with a single vlan you basically just work layer-2.

with two vlan's you got two separate layer-2 networks.
At this point no layer-3 function is used.

when activating the layer-3 functions of the switch (enable ip routing) you can basically ADD a router between (v)lan's.

Depending on the design of this layer-3 switch some of the routing may actually be done by forwarding tables in ASIC's that control the port, hereby forwarding between ports directly, instead of forwarding by a central device.