What is a light layer 3 switch?

In the course of learning buzzwords and perusing marketing literature we IT professionals can be aware of a technology without truly understanding its function. I’ve done an informal poll and decided to address one of those technologies that few seem to really grasp. If you aren’t familiar with the OSI model or the basics of switch technology then you should research those topics independently of this article due to the article’s focus on functional differences and VLANs, not foundational mechanics.

What is a layer 2 Switch?
What is a layer 3 Switch?
What is a light layer 3 switch?

Obviously these terms reference the layers of the OSI model but it doesn’t fully explain what the functional differences.

What is a layer 2 switch?

A layer 2 switch is a switch that forwards packets based on MAC addresses. This is often called a dumb switch as it maintains little intelligence. It simple keeps track of which MAC addresses are on which ports and forwards the packets appropriately. It has never heard of an IP address.

What is a layer 3 switch?

A layer 3 switch is a switch that makes routing decisions based on L3 addresses, also known as IP addresses, and simple routing protocols like RIP and IGRP. This switch is the uberswitch that can create VLANs and route VLANs as well as changing routing paths based on protocol metrics like cost (distance) and load (traffic). Note L3 switches have all the functionality of L2 switches with the addition of full L3.

What is a light layer 3 switch?

Light Layer 3 (LL3) switches also make decisions based on IP addressing and static policies. The difference lies in the fact that LL3 does not support routing protocols. The switch allows VLAN creation, VLAN routing, and IP routing based off static routes but it cannot dynamically route packets based on dynamic metrics like load and cost.

Simply put, L2’s don’t do IPs, LL3’s don’t do routing protocols.

Now for the Bonus

What is the difference between a layer 3 routing switch and a router?

L3 switches don’t support Border Gateway Protocol (BGP). Internal networks use protocols like RIP and OSPF where Routers use BGP. The functional difference is found the in the stability and robustness of the BGP protocol.

This concludes today’s lesson. Let me know what you think.

3 responses to “What is a light layer 3 switch?”

  1. Ken Stewart says:

    In our environment we have a little bit of an eclectic mix.

    I was interested in the differences between L3 and LL3 swithces. We have implemented VoIP using Dell PowerConnect switches.

    Our core and edge switches are GB switches that support VLAN tagging.

    What is the most efficient way for me to confirm I have a LL3 or L3 switch?

  2. Tsu Doh Nimh says:

    This will seem a rudimentary method but I’d search the PDF manual or website specs for RIP or routing protocol and see if you get a hit.

    From Dell’s site http://tinyurl.com/5rnug3 it appears that the full L3 line begins in the 6200 series and the 3000 line is LL3

  3. Ken Stewart says:

    I figured as much… I used the 5000 line and 3000 lines, and Dell calls them L3, but didn’t get a sense they were.

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