VLANs (Virtual LANs)


VLAN’s improve on the old switched networks that had one huge broadcast domain.  They allow you to :



VLAN Trunk Protocol (VTP) automatically distributes the information to all the devices in the management domain. 



Static vs Dynamic VLAN’s  -  most VLAN’s are static (you must manually add hosts and client switches), but there are also Dynamic VLAN’s that take care of it for you – so switches can be moved and will be auto-re-configured.


Once a network is configured with routers, you can now connect the Ethernet switches that reside at each LAN.  These LAN’s can even be far apart from one another, but if the switches are connected via trunks, using VTP (VLAN Trunking protocol) commands - it all appears transparently as a single LAN.  This is called a VLAN.  You may have hundreds of locations and wish to logically create several large combined LAN’s, in which case you would have VLAN1, VLAN2, etc.


The Cisco Catalyst 1900 supports 64 VLAN’s, each with a separate STP


VLAN1 is a factory-default VLAN – the switch’s IP address is in the broadcast domain of the first VLAN  (VLAN1)


VTP (VLAN Trunking Protocol)


Set up server

vtp domain  domain
trunk on

vlan # name name


Set up client/s

vtp domain domain

            vtp client

            vlan membership static #


to remove:    delete vtp


*** enabled by default on all Cisco switches and routers

*** requires ISL so that switches can communicate with each other and VLAN’s can communicate with each other




Manages the additions/deletions/name changes of VLAN’s across networks.  All switches in a VTP domain must be configured with the same domain name.  There is one VTP server (by default, the first switch you add to the domain) and the rest are clients (use the vtp client command).  Trunking must be enabled between all switches in a VTP domain – will typically be ISL trunking. 


VLAN1 - VTP messages are only carried through VLAN 1, also called the “Management VLAN”. VLAN1 is automatically created whenever a VTP domain is created.


There are 3 modes that a VTP switch can be in :


1)      VTP Server maintains a full list of all VLANs within the VTP domain. Information is stored in NVRAM. The server can add, delete, and rename VLANs.


2)      VTP Client also maintains a full list of all VLANs. However, it will not store in NVRAM. The client can not add, delete or rename VLANs. Any changes made must be received from a VTP Server advertisement.


3)      VTP Transparent does not participate in VTP. However, it will pass on a VTP advertisement. VLAN, as defined, is only local to the switch and is stored in NVRAM.


VTP Pruning - Enhances network bandwidth use by reducing unnecessary flooded traffic, including broadcast, multicast, and flooded unicast packets. This feature restricts flooded traffic to only those trunk links that the traffic must use to access the appropriate network devices


VTP Passwords - must be the same for all switches in the VTP domain – the default is no password


Creating VLAN’s using VTP


1)  go to one switch and create a VTP domain  (a domain in which you can create numbered VLANs):
get to config mode, and:            vtp domain  domain_name

            for this example, vtp domain acme


2)  create Trunks – the switches have a “trunk on” command to allow a port to “trunk” with a VLAN.  The “trunk on” command means you have a direct line (crossover cable, or hub between 2 switches) to another switch.  You must first get to Interface Mode.  For example (the switch is named 1900a) :


> en

1900a# config t

1900a(config)# int f0/26

1900a(config-if)# trunk on


then exit to enabled mode and type:  sh trunk a            and       sh trunk b        etc, to see the status

(“sh trunk” alone does not work)


3)  create a VLAN.  By default, all ports of a switch are members of VLAN 1.  Therefore when you add another VLAN, always start with VLAN 2.

            vlan 2 name Sales      (by default, VLAN 1 is created when the VTP domain is created)

            then:  sh vlan  (to see all)  or  sh vlan 2  (to see only vlan 2)


4)  got to another switch, and set it up as a VTP client, to communicate with the first switch (VTP server):

config t

            int f0/26

            trunk on

            exit       (not CTRL-Z, since you want to stay in Config mode)

            vtp domain acme

            vtp client


Now – you have set up this switch on the same VTP domain (acme) as the first – so the VLAN info will automatically be sent, and if you type “sh vlan” you will see that the switch is on VLAN2, without having to type it in as you did with the first switch!!!


5)  Configure the ports for the hosts (workstations or servers, connected to the switch ports) with the “vlan-membership static” command.  In this example, the trunk between the switches was a Fast Ethernet (100 Mbps) port, f0/26.  Each of the two switches has a workstation connected to Ethernet (10 Mbps) port e0/1 :


config t

int e0/1

vlan-membership static 1

(repeat on 2nd switch, and then ping to test)