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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 Vlans in spanning tree forwarding state and not pruned Fa0/1 1,5,10 In Read more […]

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 enable you to connect your PC to the phone sitting on the desk, and then connect the phone to the local network.Consider the backside of an IP telephone Yealink T21:As you can see from the Read more […]

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 unchanged. The VLAN ID is included in the ISL header.Original frame:ISL frame: Prerequisites for 200-301 200-301 is a single exam, consisting of about Read more […]

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: 802.1Q frame:An example will attempt to clarify the concept. Let’s say that we have a network of 2 switches and 4 hosts. Hosts A and host D are in VLAN 2, Read more […]

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 VLAN 3 and VLAN 4. Host A sends a broadcast packet to switch SW1. Switch SW1 receives the packet, tags the packet with the VLAN ID of 3 and sends it to SW2. SW2 receives Read more […]

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 understand the concept.Host A and host B are in different VLANs, VLAN 1 and VLAN 2. These ports need to be configured as access ports and assigned to their respective Read more […]

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 that are connected to end devices such as workstations, printers, or access points.trunk port – a port that is connected to another switch. This type of interface can carry traffic of multiple VLANs, thus Read more […]

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 required to create a VLAN and assign a switch port to the VLAN:create a vlan using the vlan NUMBER global mode commandassing a port to the VLAN by using two interface subcommands. The first command is the switchport mode access command. Read more […]

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 treated as its own subnet or broadcast domain. This means that frames broadcasted onto the network will be switched only between the ports within the same VLAN.A Read more […]