Understanding the OSI Model - CompTIA Network+ N10-007 Prof Messer

steve

Youtube Video

One of the most important things that someone will learn in their early networking days.

Part of the Seventh Generation Prof Messer CompTIA Network+ N10-007 Certification Training Series

melvinsilva

OSI Model, always was a pain for me, excellent video.

scottalanmiller

@melvinsilva said in Understanding the OSI Model - CompTIA Network+ N10-007 Prof Messer:

OSI Model, always was a pain for me, excellent video.

I always hated it when I was a student. Now I think it's the most useful thing that I ever learned.

WrCombs

Just watched this one ; Prof. Messer does a great job of explaining this; very understandable.

One question I do have:
What would make a switch a 'layer 3 switch' ? I see this a lot on ML specifically.

would it just be a switch that also does some kind of routing ?

Dashrender

@WrCombs said in Understanding the OSI Model - CompTIA Network+ N10-007 Prof Messer:

Just watched this one ; Prof. Messer does a great job of explaining this; very understandable.

One question I do have:
What would make a switch a 'layer 3 switch' ? I see this a lot on ML specifically.

would it just be a switch that also does some kind of routing ?

Layer 3 means the switch can do at least rudimentary routing, and typically also understands VLANs.

Pure layer 2 switch doesn't do any routing at all, though might at least understanding VLAN tagging

There's a weird middle layer called smart switches which definitely understand VLANs but don't do routing.

scottalanmiller

@WrCombs said in Understanding the OSI Model - CompTIA Network+ N10-007 Prof Messer:

What would make a switch a 'layer 3 switch' ? I see this a lot on ML specifically.

I consider it a misnomer. It's a marketing name, but it's used for a reason.

In REAL terms, you have three core devices for moving packets...

Layer 1: Bus
Layer 2: Bridge
Layer 3: Router

A multiport bus is called a Hub. A multiport bridge is called a switch. A multiport router is supposed to still be called a router, because there's no two port limitation on routing in any inherent way. But because people wanted everyone to buy them as they cost more, they marketed them as Layer 3 Switches (which makes no sense as switch means layer 2... see the conflict?)

What people NOW argue, decades later to rationalize a non-rational decision, is that switching is done in hardware not in software (this is a false statement, but it's the logical base used) and so when you have multiple ports with hardware handling of the traffic and it goes to layer 3, it gets called an L3 switch.

Of course, if you built and sold a L3 switch done in software, people wouldn't know the difference and later they'd come up with a new excuse. End of the day... it's a made up marketing term.

But in real world usage, it's a switch-like device intended for switching, but does routing too so that you don't need a separate router.

WrCombs

@Dashrender said in Understanding the OSI Model - CompTIA Network+ N10-007 Prof Messer:

@WrCombs said in Understanding the OSI Model - CompTIA Network+ N10-007 Prof Messer:

Just watched this one ; Prof. Messer does a great job of explaining this; very understandable.

One question I do have:
What would make a switch a 'layer 3 switch' ? I see this a lot on ML specifically.

would it just be a switch that also does some kind of routing ?

Layer 3 means the switch can do at least rudimentary routing, and typically also understands VLANs.

Pure layer 2 switch doesn't do any routing at all, though might at least understanding VLAN tagging

There's a weird middle layer called smart switches which definitely understand VLANs but don't do routing.

Making it a "Layer 2.5" switch?
That's similar to what I thought.

scottalanmiller

@Dashrender said in Understanding the OSI Model - CompTIA Network+ N10-007 Prof Messer:

Layer 3 means the switch can do at least rudimentary routing, and typically also understands VLANs.

VLANs are at Layer 2. You can be an L3 switch and not know VLANs. It's silly, but they are unrelated.

scottalanmiller

@WrCombs said in Understanding the OSI Model - CompTIA Network+ N10-007 Prof Messer:

@Dashrender said in Understanding the OSI Model - CompTIA Network+ N10-007 Prof Messer:

@WrCombs said in Understanding the OSI Model - CompTIA Network+ N10-007 Prof Messer:

Just watched this one ; Prof. Messer does a great job of explaining this; very understandable.

One question I do have:
What would make a switch a 'layer 3 switch' ? I see this a lot on ML specifically.

would it just be a switch that also does some kind of routing ?

Layer 3 means the switch can do at least rudimentary routing, and typically also understands VLANs.

Pure layer 2 switch doesn't do any routing at all, though might at least understanding VLAN tagging

There's a weird middle layer called smart switches which definitely understand VLANs but don't do routing.

Making it a "Layer 2.5" switch?
That's similar to what I thought.

No, VLAN is Layer 2. There's nothing in that that is in any way suggestive of being "more than" an L2 Switch (aka a Switch.)

WrCombs

@scottalanmiller said in Understanding the OSI Model - CompTIA Network+ N10-007 Prof Messer:

@WrCombs said in Understanding the OSI Model - CompTIA Network+ N10-007 Prof Messer:

What would make a switch a 'layer 3 switch' ? I see this a lot on ML specifically.

I consider it a misnomer. It's a marketing name, but it's used for a reason.

In REAL terms, you have three core devices for moving packets...

Layer 1: Bus
Layer 2: Bridge
Layer 3: Router

A multiport bus is called a Hub. A multiport bridge is called a switch. A multiport router is supposed to still be called a router, because there's no two port limitation on routing in any inherent way. But because people wanted everyone to buy them as they cost more, they marketed them as Layer 3 Switches (which makes no sense as switch means layer 2... see the conflict?)

What people NOW argue, decades later to rationalize a non-rational decision, is that switching is done in hardware not in software (this is a false statement, but it's the logical base used) and so when you have multiple ports with hardware handling of the traffic and it goes to layer 3, it gets called an L3 switch.

Of course, if you built and sold a L3 switch done in software, people wouldn't know the difference and later they'd come up with a new excuse. End of the day... it's a made up marketing term.

But in real world usage, it's a switch-like device intended for switching, but does routing too so that you don't need a separate router.

Ah, Makes sense. Anyway to make money.

Would I be better off with a switch and router separate ?

scottalanmiller

@WrCombs said in Understanding the OSI Model - CompTIA Network+ N10-007 Prof Messer:

Would I be better off with a switch and router separate ?

Totally depends on the use case. Multiport routing is a very important function... when you need it. If you don't need it, you are wasting money on a slower device. L3 devices are slower than L2 (with the same level of hardware.) So that's why there is so much marketing... to sell high cost L3 switches where L2 is better. But when you need an L3, an L2 can't do the job. So it's all about need.

Dashrender

@WrCombs said in Understanding the OSI Model - CompTIA Network+ N10-007 Prof Messer:

Would I be better off with a switch and router separate ?

that depends, do you need ACLs? and other considered higher end routing functions? if so, less expensive L3 switches can't do that. And the cost of a L3 that can is likely higher than a L2 + the cost of a router (say a Edge Router 4). Plus it separates the functions.

scottalanmiller

@Dashrender said in Understanding the OSI Model - CompTIA Network+ N10-007 Prof Messer:

@WrCombs said in Understanding the OSI Model - CompTIA Network+ N10-007 Prof Messer:

Would I be better off with a switch and router separate ?

that depends, do you need ACLs? and other considered higher end routing functions? if so, less expensive L3 switches can't do that. And the cost of a L3 that can is likely higher than a L2 + the cost of a router (say a Edge Router 4). Plus it separates the functions.

Now you are getting distracted by features and not discussing L2 vs L3 switches.

1337

I'd like to think of "L3 switch" as being short for switch with layer 3 functions.
Often they are used for routing traffic between VLANs. A router can do that as well but a L3 switch can usually do faster and on more ports at the same time.

Typical is to have a bunch of L2 switches where the end points connect and then the L2 switches are uplinked to one L3 switch that takes care connecting the L2 switches together and do inter-VLAN routing.

There are also L3 Lite switches (aka L2+) which are L2 switches with a limited L3 functionality like for instance static routing.

L3 switches are usually the ones with the most features in all categories so sometimes you need to buy a L3 switch because you are after some other feature, like redundant power or what not.

In small networks L3 switches are usually not needed.

As an example Dell's PowerSwitches for instance are managed switches for building out the network to end points (campus).

They are like this:

N1100 series - L2 switch
N1500 series - L3 Lite switch
N2000, N3000 series - L3 switch

They have the same OS and work the same but each step up in the series gives you more features and more speed and higher cost.