Configuring allowed VLANs

By default, all VLANs are allowed across a trunk link on a Cisco switch. We can verify that using the show interfaces trunk command:SW1#show interfaces trunk Port Mode Encapsulation Status Native vlan Fa0/1 on 802.1q trunking 1 Port Vlans allowed on trunk Fa0/1 1-1005 Port Vlans allowed and active in management domain Fa0/1 1,5,10 Port […]

Configuring voice VLANs

Most corporate networks today use IP telephony. This means that the phones are connect to the same network and use the same cabling as other network devices, such as workstations or routers. Since offices usually have only a single UTP cable to each desk, most of the IP phones today include a small switch that […]

Inter-Switch Link (ISL)

Another VLAN tagging protocol is Inter-Switch Link (ISL). This protocol is Cisco proprietary, which means that, unlike 802.1Q, it can be used only between Cisco switches. It is considered to be deprecated, and newer Cisco switches don’t even support it.ISL works by encapsulating a frame in an ISL header and trailer. The encapsulated frame remains […]

IEEE 802.1Q

IEEE 802.1Q is one of the VLAN tagging protocols supported by Cisco switches. This standard was created by the Institute of Electrical and Electronics Engineers (IEEE), so it an open standard and can be used on non-Cisco switches.To identify to which VLAN a frame belongs to, a field is inserted into the frame’s header.Original frame: […]

Frame tagging

To identify the VLAN a packet is belonging to, switches use tagging to assign a numerical value to each frame in a network with multiple VLANs. This is done to ensure that switches know out which ports to forward frames.For example, consider the following network topology.There are two VLANs in the toplogy pictured above, namely […]

Configuring access & trunk ports

To configure an interface to be an access interface, the switchport mode acess interface command is used. This type of interface can be assigned only to a single VLAN.To configure a trunk interface, the switchport mode trunk interface command is used. This type of interface can carry traffic of multiple VLANs.An example will help you […]

Access and trunk ports

If you intend to use VLANs in your network, you will need to configure some ports on a switch as access ports and other as trunk ports. Here is a description each port type:access port – a port that can be assigned to a single VLAN. This type of interface is configured on switch ports […]

Configuring VLANs

By default, all ports on a switch are in the VLAN 1. We can verify that by typing the show vlan command from the IOS enable mode of a switch:In the picture above, you can see that all of the 24 ports of the switch are in the same VLAN, namely VLAN 1.Two steps are […]

What is a VLAN?

VLANs (Virtual LANs) are logical grouping of devices in the same broadcast domain. VLANs are usually configured on switches by placing some interfaces into one broadcast domain and some interfaces into another. Each VLAN acts as a subgroup of the switch ports in an Ethernet LAN.VLANs can spread across multiple switches, with each VLAN being […]