What are Layer 3 Switches?

Dragon0x40
Dragon0x40 used Ask the Experts™
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I need help understanding what a layer 3 switch is and how to tell if I am working on one.

I have some Catalyst 6500 series switches that I believe are layer 2 only. They run CATOS. On these switches the command "show ip interfaces brief" will not run.

I have some CAt 6500 that run in Hybrid mode. The switch side runs CATOS and if I "session 15" to the MSFC running IOS and run "show ip interfaces brief" I don't see any physical interfaces just the vlans.

I have other CAT 6500 switches that run native IOS and if I run "show ip interfaces brief" then I see the vlan interfaces and the physical interfaces.

Is the Hybrid switch a layer 2 or layer 3 switch?

Does the hardware or IOS/CATOS determine if it is a layer 2 or layer 3 switch?

Can you change a Layer 2 to a Layer 3 by upgrading the IOS code?
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Commented:
There are many differences across brand and models. Lets focus on Cisco Catalyst 6500 fro the time being.

Catalyst 6500 gone through a long history. You can consider it as a platform which can do different things depends on what is added to it. The main processing unit is the supervisor engine. Some older engine do not natively support L3, and Cisco come up with the MSFC module dedicated for L3 switching. It add L3 switch feature to a L2 switch/supervisor.

Some newer supervisor module natively support L3 switching.

Typically in a L3 switch, you can route using VLAN interface, or in some newer verison of IOS, using physical interfaces. So it is perfectly normal that you only see VLAN interfaces in MSFC.

There is no direct relationship of IOS/CatOs vs L3 switching. There are CatOS switches that support L3. There are a lot of IOS switches that do not support L3.

To perform L3 feature, you need the necessary hardware (either L3 supervisor, MSFC, or natively supported).

Theoretically you can upgrade the supervisor to make a L2 switch a L3 switch. However there are many limitation of what combinations of chassis, backpane, supervisor and modules that it may not always compatible.



Author

Commented:
thanks GuruChiu,

If I understand you correctly the line cards will support Layer 3 if the supervisor will?
Commented:
For older 6500, the line cards are just L2 switches. What the L3 enabled supervisor or MSFC or other L3 module do is connect to the VLANs on the back plane and forward the packets across VLAN, just like a router will do. So the line cards do not need to support L3.
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Author

Commented:
So if I want to put an ip address on interface/port 3/1 then the line card 3 and the supervisor both must support L3?

If I don't need an ip address on port 3/1 then I just put port 3/1 in a vlan and use the layer 3 supervisor to route between vlans?
Commented:
If you need to connect port 3/1 to a device which want to communicate with a specific IP address. You create a VLAN interface on the L3 module. Assign that IP address to it. Then make port 3/1 to be on that VLAN.

Author

Commented:
thanks GuruChiu,

To use the "switchport" and "no switchport" commands to change from a layer 2 to a layer 3 port, then the line card must support layer 3 and the supervisor must also support layer 3?

How can I tell if layer 3 port capability is available?

Author

Commented:
CATOS does not support routed L3 interfaces So the switch must be running Cisco IOS not CATOS and have layer3 capable ports and code.

From the link you sent me.

Cisco Catalyst OS, which has all ports enabled, Layer 2 aware, and in VLAN 1 by default and does not support routed interfaces.

http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps708/prod_white_paper09186a00800c8441.html 


Commented:
The link explain very well what I said so far. Let me say it again:

For those L2 modules (all classic modules, and most modules available for 6500), you do not assign IP address to the port directly. You assign the IP address to a VLAN and put that port onto the VLAN.
Don JohnstonInstructor
Top Expert 2015
Commented:
A 6500 running CatOS is a layer 2 switch. All physical interfaces on this switch are layer 2 interfaces (there are some exceptions but we'll leave those out for now).

Installing an MSFC effectively puts a router inside of the switch chassis. The MSFC is a separate entity running IOS. The connection between the route processor and the switch is through a virtual (internal) trunk over the switches backplane.

Traffic to/from the MSFC is over the internal trunk to VLAN interfaces on the route processor.

Once a hybrid mode 6500 has an MSFC installed, it is now a Mulitlayer switch.

Author

Commented:
thanks donjohnston,

So if you run CATOS on a 6500 you have no physical interfaces on line cards to make L3 and put an ip address on?

If you run Native IOS and have L3 cards (not the norm) then you could use the "no switchport" command and put an ip address on that interface?

If the above statements are correct then the route to the new routed port would still have to go through an SVI? I am thinking this because the routed port would not be a member of a VLAN but you could turn on dot1q trunking?


Don JohnstonInstructor
Top Expert 2015

Commented:
You are correct.

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