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What's the basic use case(s) for VLANs?

What are the basic design principles?

I'm looking for something like a two paragraph executive summary style answer so I can determine if I need to learn about VLANs to implement them.

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A VLAN (Virtual LAN) is a way of creating multiple virtual switches inside one physical switch. So for instance ports configured to use VLAN 10 act as if they're connected to the exact same switch. Ports in VLAN 20 can not directly talk to ports in VLAN 10. They must be routed between the two (or have a link that bridges the two VLANs).

There are a lot of reasons to implement VLANs. Typically the least of these reasons is the size of the network. I'll bullet list a few reasons and then break each one open.

  • Security
  • Link Utilization
  • Service Separation
  • Service Isolation
  • Subnet Size

Security: Security isn't itself achieved by creating a VLAN; however, how you connect that VLAN to other subnets could allow you to filter/block access to that subnet. For instance if you have an office building that has 50 computers and 5 servers you could create a VLAN for the server and a VLAN for the computers. For computers to communicate with the servers you could use a firewall to route and filter that traffic. This would then allow you to apply IPS/IDS,ACLs,Etc. to the connection between the servers and computers.

Link Utilization: (Edit)I can't believe I left this out the first time. Brain fart I guess. Link utilization is another big reason to use VLANs. Spanning tree by function builds a single path through your layer 2 network to prevent loops (Oh, my!). If you have multiple redundant links to your aggregating devices then some of these links will go unused. To get around this you can build multiple STP topology with different VLANs. This is accomplished with Cisco Proprietary PVST, RPVST, or standards based MST. This allows you to have multiple STP typologies you can play with to utilize your previously unused links. In example if I had 50 desktops I could place 25 of them in VLAN 10, and 25 of them in VLAN 20. I could then have VLAN 10 take the "left" side of the network and the remaining 25 in VLAN 20 would take the "right" side of the network.

Service Separation: This one is pretty straight forward. If you have IP security cameras, IP Phones, and Desktops all connecting into the same switch it might be easier to separate these services out into their own subnet. This would also allow you to apply QOS markings to these services based on VLAN instead of some higher layer service (Ex: NBAR). You can also apply ACLs on the device performing L3 routing to prevent communication between VLANs that might not be desired. For instance I can prevent the desktops from accessing the phones/security cameras directly.

Service Isolation: If you have a pair of TOR switches in a single rack that has a few VMWare hosts and a SAN you could create a iSCSI VLAN that remains unrouted. This would allow you to have an entirely isolated iSCSI network so that no other device could attempt to access the SAN or disrupt communication between the hosts and the SAN. This is simply one example of service isolation.

Subnet Size: As stated before if a single site becomes too large you can break that site down into different VLANs which will reduce the number of hosts that see need to process each broadcast.

There are certainly more ways VLANs are useful (I can think of several that I use specifically as an Internet Service Provider), but I feel these are the most common and should give you a good idea on how/why we use them. There are also Private VLANs that have specific use cases and are worth mentioning here.

7

As networks grow larger and larger, scalability becomes an issue. In order to communicate, every device needs to send broadcasts, which are sent to all devices in a broadcast domain. As more devices are added to the broadcast domain, more broadcasts start to saturate the network. At this point, multiple issues creep in, including bandwidth saturation with broadcast traffic, increased processing on each device (CPU usage), and even security issues. Splitting this large broadcast domain into smaller broadcast domains becomes increasingly necessary.

Enter VLANs.

A VLAN, or Virtual LAN, creates separate broadcast domains virtually, eliminating the need to create completely separate hardware LANs to overcome the large-broadcast-domain issue. Instead, a switch can contain many VLANs, each one acting as a separate, autonomous broadcast domain. In fact, two VLANs, can not communicate with each other without the intervention of a layer 3 device such as a router, which is what layer 3 switching is all about.

In summary, VLANs, at the most basic level, segment large broadcast domains into smaller, more manageable broadcast domains to increase scalability in your ever-expanding network.

5

VLANs are logical networks created within the physical network. Their primary use is to provide isolation, often as a means to decrease the size of the broadcast domain within a network, but they can be used for a number of other purposes.

They are a tool that any network engineer should be familiar with and like any tool, they can be used incorrectly and/or at the wrong times. No single tool is the correct one in all networks and all situations, so the more tools you can use, the better you are able to work in more environments. Knowing more about VLANs allows you to use them when you need them and to use them correctly when you do.

One example of how they can be used, I currently work in an environments where SCADA (supervisory control and data acquisition) devices are used widely. SCADA devices typically are fairly simple and have a long history of less than stellar software development, often providing major security vulnerabilities.

We have set the SCADA devices in their in a separate VLAN with no L3 gateway. The only access into their logical network is through the server they communicate with (which has two interfaces, one in the SCADA VLAN) which can be secured with it's own host based security, something not possible on the SCADA devices. The SCADA devices are isolated from the rest of the network, even while connected to the same physical devices, so any vulnerability is mitigated.

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In terms of design principles, the most common implementation is to align your VLANs with your organizational structure, ie Engineering folks in one VLAN, Marketing in another, IP phones in another, etc. Other designs include utilizing VLAN's as "transport" of separate network functions across one (or more) cores. Layer 3 termination of VLANs ('SVI' in Cisco parlance, 'VE' in Brocade, etc) is also possible on some devices, which eliminates the need of a separate piece of hardware to do inter-VLAN communication when applicable.

VLANs become cumbersome to manage and maintain at scale, as you've probably seen cases of already on NESE. In the service provider realm, there's PB (Provider Bridging - commonly known as "QinQ", double tagging, stacked tag, etc), PBB (Provider Backbone Bridging - "MAC-in-MAC") and PBB-TE, which have been designed to try to mitigate the limitation of the number of VLAN ID's that were available. PBB-TE more aims to eliminate the need for dynamic learning, flooding, and spanning tree. There's only 12 bits available for use as a VLAN ID in a C-TAG/S-TAG (0x000 and 0xFFF are reserved) which is where the 4,094 limitation comes from.

VPLS or PBB can be used to eliminate the traditional scaling ceilings involved with PB.

3

The basic use case for VLANs is almost exactly the same as the basic use case for segmentation of the network into multiple data link broadcast domains. The key difference is that with a physical LAN, you need at least one device (typically a switch) for each broadcast domain, whereas with a virtual LAN broadcast domain membership is determined on a port-by-port basis and is reconfigurable without adding or replacing hardware.

For basic applications, apply the same design principles to VLANs as you would for PLANs. The three concepts you need to know to do this are:

  1. Trunking - Any link that carries frames belonging to more than one VLAN is a trunk link. Typically switch-to-switch and switch-to-router links are configured to be trunk links.
  2. Tagging - When transmitting to a trunk link, the device must tag each frame with the numeric VLAN ID to which it belongs so that the receiving device can properly confine it to the correct broadcast domain. In general, host-facing ports are untagged, while switch-facing and router-facing ports are tagged. The tag is an additional part of the data link encapsulation.
  3. Virtual Interfaces - On a device with one or more trunk link interfaces, it is often necessary to attach, in the logical sense, the device as a link terminal to one or more of the individual VLANs that are present within the trunk. This is particularly true of routers. This logical link attachment is modeled as a virtual interface that acts as a port that is connected to the single broadcast domain associated with the designated VLAN.
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The original use of a vlan was to restrict the broadcast area in a network. Broadcasts are limited to their own vlan. Later additional funtionality was added. However, keep in mind that vlan's are layer 2 in for example cisco switches. You can add layer 2 by assigning an IP address to the port on the switch but this is not mandatory.

additional functionality:

  • trunking: use multiple vlan's through one physical connection (ex: connecting 2 switches, one physical link is good enough to have a connection for all vlan's, seperating the vlan's is done by tagging, see: dot1Q for cisco)
  • security
  • easier to manage (ex: shutdown on a vlan doesn't impact the other vlan's connectivity...)
  • ...
1

If I may offer one more piece of information, which might help.

To understand VLAN's, you must also understand two key concepts.

-Subnetting - Assuming you want the various devices to be able to talk to one another (servers and clients, for example) each VLAN must be assigned an IP subnet. This is the SVI mentioned above. That enables you to begin routing between the vlans.

-Routing - Once you have each VLAN created, a subnet assigned to the clients on each VLAN, and an SVI created for each VLAN, you will need to enable routing. Routing can be a very simple setup, with a static default route to the internet, and EIGRP or OSPF network statements for each of the subnets.

Once you see how it all comes together, it is actually quite elegant.

  • Thanks! subn/rtn I get, so now with all this VLAN info, yes, it makes perfect sense. I'm already thinking of building out "backside" VLANs and shifting things for systems that have a second, unused interfaces. – Craig Constantine May 21 '13 at 18:15
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    VLANs do not require L3 information, routing or SVIs. These are only needed if you want L3 (IP) or above functionality for hosts in that VLAN. – YLearn May 21 '13 at 18:15
  • just adding... one does not have to run IP over the VLAN. (see also: protocol based vlans -- vs. port based, which is what's used 99% of the time.) – Ricky Beam May 21 '13 at 20:12
  • I agree with both of your statements. L2 vlans have there uses for sure. However, if someone is thinking about adding vlans into their existing network, not explaining the L3 aspect is a serious omission. – Jonathan Davis May 22 '13 at 0:46

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