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Virtual LANs

Broadcast sent by one host in a VLAN => received and processed by all hosts Devices in a VLAN need to be in the same subnet * at least one router needs to be involved * in real campus it’s usually a multilayer (L3) switch A switch interface can be: * access interface – sends/receives frames only in a single VLAN * trunking interface – sends/receives frames in multiple VLANs


* trunking – allows passing frames from multiple VLANs over a single physical connection * when there are multiple interconnected switches - VLAN trunking must be between switches * Cisco supported trunking protocols: ISL, IEEE 802.1Q


* proprietary * only between two Cisco switches that support ISL * encapsulates the original frame in another Ethernet header and trailer (adds VLAN number)

IEEE 802.1Q

* more popular * inserts an extra 4-byte VLAN header into the original frame’s Ethernet header * must recalculate FCS (frame check sequence) * defines one VLAN (VLAN 1 by default) as a native VLAN (no header is added to frames in this VLAN)

VLAN Trunking Protocol (VTP)

* Cisco proprietary * exchange of VLAN configuration info * when a switch changes its VLAN config => VTP causes that all switches synchronize VLAN IDs and names * three VTP modes: server, client, transparent Requirements for VTP to work between two switches: * the links must be operating as a VLAN trunk (ISL or 802.1Q) * case-sensitive domain names must match * if configured, case-sensitive VTP passwords must match

VTP Pruning

By default switch flood broadcasts to all active VLANs out all trunks. Usually VLANs don’t exist on all switches => VTP can determine which switches do not need a broadcast and then prune those VLANs from the trunks.


Stored in vlan.dat in flash memmory (good in case all switches lost power) Switches in transparent mode - in both the running-config file as well as the vland.dat file in flash To remove VLAN configuration: delete flash:vlan.dat Adding a new VLAN (by default there is VLAN 1, to which all interfaces are assigned): (config)#vlan 2 (config-vlan)#name My-vlan # defaults to VLAN0002 (config)#interface range fastethernet 0/13 - 14 (config-if)#switchport accesss vlan 2 (config-if)#switchport mode access # optional step to disable trunking #show vlan brief