Networking 101: Understanding the Data Link Layer

Understanding how bridges, switches and VLANs relate will help you master the second layer in the OSI model.

By Charlie Schluting | Posted Jul 23, 2008
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What's more important than IP and routing? Well, Layer 2 is much more important when it's broken. Many people don't have the Spanning Tree Protocol (STP) (define) knowledge necessary to implement a layer 2 network that's resilient. A switch going down shouldn't prevent anyone from having connectivity, excluding the hosts that are directly attached to it. Before we can dive into Spanning Tree, you must understand the inner workings of layer 2.

Layer 2, the Data Link layer, is where Ethernet lives. We'll be talking about bridges, switching and VLANs with the goal of discovering how they interact in this part of Networking 101. You don't really need to study the internals of Ethernet to make a production network operate, so if you're inclined, do that on your own time.

Ethernet switches, as they're called now, began life as a "bridge." Traditional bridges would read all Ethernet frames, and then forward them out every port, except the ones they came in on. They had the ability to allow redundancy via STP, and they also began learning which MAC addresses were on which port. At this point, a bridge then became a learning device, which means they would store a table of all MAC addresses seen on a port. When a frame needed to be sent, the bridge could look up the destination MAC address in the bridge table, and know which port is should be sent out. The ability to send data to only the correct host was a huge advance in switching because collisions became much less likely. If the destination MAC address wasn't found in the bridge table, the switch would simply flood it out all ports. That's the only way to find where a host actually lives for the first time, so as you can see, flooding is an important concept in switching. It turns out to be quite necessary in routing too.

Important terminology in this layer includes:

Unicast segmentation: Bridges can limit which hosts hear unicast frames (frames sent to only one MAC address). Hubs would simply forward everything to everyone, so this alone is a huge bandwidth-saver.

Collision Domain : The segment over which collisions can occur. Collisions don't happen any more, since switches use cut-through forwarding and NICs are full-duplex. If you see collisions on a port, that means someone negotiated half-duplex accidentally, or something else is very wrong.

Broadcast Domain : The segment over which broadcast frames are sent and can be heard.

A few years later, the old store-and-forward method of bridge operation was modified. New switches started only looking at the destination MAC address of the frame, and then sending it instantly. Dubbed "cut-through forwarding," presumably because frames cut through the switch much more quickly and with less processing. This implies a few important things: A switch can't check the CRC to see if the packet was damaged, and that implies that collisions needed to be made impossible.

Now, to address broadcast segmentation, VLANs were introduced. If you can't send a broadcast frame to another machine, they're not on your local network, and you will instead send the entire packet to a router for forwarding. That's what a Virtual LAN (VLAN) does, in essence: It makes more networks.

On a switch, you can configure VLANs, and then assign a port to a VLAN. If host A is in VLAN 1, it can't talk to anyone in VLAN 2, just as if they lived on totally disconnected devices. Well, almost; if the bridge table is flooded and the switch is having trouble keeping up, all data will be flooded out every port. This has to happen in order for communication to continue in these situations. This needs to be pointed out because many people believe VLANs are a security mechanism. They are not even close. Anyone with half a clue about networks (or with the right cracking tool in their arsenal) can quickly overcome the VLAN broadcast segmentation. In fact, a switch will basically turn into a hub when it floods frames, spewing everyone's data to everyone else.

If you can't ARP for a machine, you have to use a router, as we already know. But does that mean you have to physically connect wires from a router into each VLAN? Not anymore, we have layer 3 switches now! Imagine for an instance, if you will, a switch that contains 48 ports. It also has VLAN 1 and VLAN 2, and ports 1-24 are in VLAN 1, while ports 25-48 are part of VLAN 2. To route between the two VLANs, you have basically three options. First, you can connect a port in each VLAN to a router, and assign the hosts the correct default route. In the new-fangled world of today, you can also simply bring up two virtual interfaces in each VLAN. In Cisco-land, the router interfaces would be called vlan1 and vlan2. They get IP addresses, and the hosts use the router interface as their router.

The third way brings us to the final topic of our layer 2 overview. If you have multiple switches that need to contain the same VLANs, you can connect them together so that VLAN 1 on switch A is the same as VLAN 1 on switch B. This is accomplished with 802.1q, which will tag the packets as they leave the first switch with a VLAN identifier. Cisco calls these links "trunk ports," and you can have as many VLANs on them as the switch allows (currently 4096 on most hardware). So, the third and final way to route between VLANs is to connect a trunk to a router, and bring up the appropriate interfaces for each VLAN. The hosts on VLAN 1, on both switch A and B will have access to the router interface (which happens to be on another device) since they are all "trunked" together and share a broadcast domain.

We've saved you from the standard "this is layer 2, memorize the Ethernet header" teaching method. To become a true guru you must know it, but to be a useful operator, (something the cert classes don't teach you) simply understand how it all works. Join us next time for an exploration of the most interesting protocol in the world, Spanning Tree.

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