What is a VLAN, and how do they help your network?
Relevant CCNA Topics Covered: Interconnecting Cisco Networking Devices Part 1 (100-105) Section 2.0
VLAN stands for Virtual Local Area Network. It is a term and technology that you will deal with quite often as a network engineer. A VLAN and its application within Cisco gear allows you to segment broadcast domains, and compartmentalize network traffic as required by your network design. The creation of VLANs occur within LAN switches and are created to logically define your layer 2 network. When I mention “layer 2” i’m referring to the second layer or Data Link layer within the OSI model. As you should recall from your CCENT reading, the PDU for the Data Link layer is the Frame.
By default on Cisco switches, all switchports are in VLAN 1 which also serves as the “Native VLAN”. In this configuration all ports are within the same broadcast domain. The layer 3 (Network Layer) broadcast address is 255.255.255.255 and signifies all hosts on a network. The specific broadcast address for your network may look a bit different. Say your network is 192.168.1.0/24. The layer 3 broadcast address for this network can be found by setting all of the host bits of the network to binary 1’s, and the result of this effort will reveal the broadcast address for your network. In this case it would be 192.168.1.255. The layer 2 all hosts broadcast address is ffff.ffff.ffff in hexidecimal. If there is an application that needs to reach all hosts on a network it will use the broadcast address for this communication. An example of such an application would be DHCP. When an NIC enabled with dhcp comes up for the first time it broadcasts its DHCPDISCOVER messages in an attempt to find the closest dhcp server. That communication will be replicated to every port, in every switch within the vlan for that network. If you have a vlan, but it is present on only one switch in the network, then the ports assigned to said vlan within the single switch is the extent of the broadcast domain. This broadcast domain is the logical boundary in which broadcasts can be forwarded. If you have many hosts on a network, spanning many switches, using applications that utilize the broadcast address to communicate to all hosts, you can imagine situations where the network may be over-saturated with traffic! The amount of broadcast traffic on any given network scales with the number of devices assigned to the vlan.
In our example network above we have the possibility of 253 hosts on the network with our /24 network address. This size network is manageable within the confines of a single Vlan. An entire /8 network would not be. Good network design practice limits one subnet per vlan, and that vlan can span multiple switches. Some Cisco design models recommend one subnet per switch, but that discussion is out of the scope of this blog post. I will discuss some design topics in future posts. Layer 3 devices such as routers do not forward broadcasts, and are required in topologies with multiple vlans in order to route traffic between them. Layer 3 switches such as the Cisco catalyst 3750 can perform these routing functions as well, if configured for IP routing.
Normal range vlans 1 – 1005, can be configured in both the global configuration and vlan database configuration modes. All vlan changes in either configuration mode modify the Vlan.dat file directly. Pay close attention to the last statement because if you make a Vlan change that terminates your access to the switch for some reason, a reboot of the switch will not revert your configuration since the instant modification of the vlan.dat file is not reversible by restoration of the startup configuration. Vlan 1 is the default ethernet vlan and cannot be deleted, pruned by VTP or manually pruned. Vlans 2 – 1001 can be used at your discretion to build multiple broadcast domains for your LAN.
I think that’s enough information on vlans for now, thank you for reading! I will expand on the topic further in the future. please respond with any feedback or questions you may have. If you find yourself in need of the ICND1 100-105 here’s may affiliate link to amazon where you can purchase: CCENT/CCNA ICND 100-105 Official Cert Guide by Wendell Odom (2016-05-17)