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Jon Rhoades
Asked: 2010-03-18 22:26:15 +0800 CST

What's the difference between a Layer 2 & Layer 3 switch

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What's the difference between a Layer 2 & Layer 3 switch?

I've always wondered and never needed to know until now.

networking switch

5 Answers

  1. Best Answer

    I will complete Zoredache's answer.

    A L2 switch does switching only. This means that it uses MAC addresses to switch the packets from a port to the destination port (and only the destination port). It therefore maintains a MAC address table so that it can remember which ports have which MAC address associated.

    A L3 switch also does switching exactly like a L2 switch. The L3 means that it has an identity from the L3 layer. Practically this means that a L3 switch is capable of having IP addresses and doing routing. For intra-VLAN communication, it uses the MAC address table. For extra-VLAN communication, it uses the IP routing table.

    This is simple but you could say "Hey but my Cisco 2960 is a L2 switch and it has a VLAN interface with an IP !". You are perfectly right but that VLAN interface cannot be used for IP routing since the switch does not maintain an IP routing table.

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  2. The layer 3 vs 2 refers to the OSI model. A layer 3 switch supports routing. A layer 2 switch only knows ethernet, you may be able to setup VLANs.

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  3. Simply put, a layer 3 switch can forward packets between different networks like a router while layer 2 switches forward packets to different segments/or within a given network.

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  4. A switch can be thought of as a more powerful bridge and a less powerful router.

    If a switch is configured to work only as a bridge, it is called a layer 2 switch.

    If a switch is configured to work only as a router, it is called a layer 3 switch.

    More often, a switch is configured to perform both these functions(layer2 as well as layer3):

    1. Either together on the same ports(using Integrated Routing and Bridging, ie, IRB): If the DMAC in the incoming IP data packet is of the IRB interface, routing or layer 3 behavior is done. Otherwise, the packet is bridged(layer 2 behavior) on all the same vlan ports.

    2. Or, on separate sets of ports of the switch(some ports as L2 ports while some ports as L3 ports): A set of "x" ports on a switch may be configured as a bridge(and will bridge packets). While, another set of "y" ports may have IP addresses assigned to them and will act as router ports(routing received IP packets).

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  5. Layer 2 is generally hardware i.e. mac address "routing" or mac tables. Layer 3 has to do with ip's. Layer 3 devices are usually managed and they can create and route between vlans.

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