BROWN HASH

Cisco VTP

As a CCNA candidate you are expected to understand the purpose of the Vlan Trunking Protocol (VTP) it’s configuration and how it works.

CCNA Vlan Trunking Protocol (VTP).

VTP is a Cisco proprietary protocol supported only on Cisco switches. The purpose of VTP is to advertise the presence of vlans across trunk links to other Cisco switches so that the network can maintain a consistent vlan database.

VTP is a layer 2 protocols that advertises vlans across trunk links between switches, these VTP adverts are only transported over trunk links. The protocol works on a periodic timer of 5 mins, therefore change or no change the protocol sends adverts when the 5 min timer expires. The VTP protocols also sends triggered updates when a change is made to the vlan database. The adverts are sent across the trunks as multicasts.

VTP has three modes of operation in IOS (4 in CatOS), the three modes are Server which happens to be the default, client and transparent.

In the default mode of server the VTP protocol will allow the administrator to add, modify and delete vlans from the vlan database. This mode will also allow VTP to generate and issue adverts over its trunk links, as well as forward received adverts over other trunks. This mode will also synchronise to received adverts from other VTP speakers.

The client mode will not permit the administrator to add, modify and delete vlans from the vlan database. This mode will allow VTP to forward adverts which is receives via its trunk links and synchronises to received adverts from other VTP speakers.

The Transparent mode will permit the administrator to add, modify and delete vlans from the vlan database. This mode will not allow VTP to generate adverts but will forward adverts which is receives via its trunk links. This mode will not synchronise to received adverts from other VTP speakers or in other words this mode allows the switch to manage it’s own vlan database independently of any other switch in the network.

VTP changes made on a server switch are propagated to other switches over the trunk link if the VTP setup has a domain name configured, in default state VTP domain name is NULL; the domain name is case sensitive in all versions, once set this domain name can be changed but never set back to NULL unless the vlan database is erased and the switch restarted.

A Server or client switch with no domain name will synchronise to the first domain name they read in a VTP advertisement. Once it has learnt a domain name it can only be changed manually.

Once a Server switch has a domain name it can advertise it’s vlan database over the trunk links, to keep track of which switch has the latest VTP information a Configuration revision number is given to every change. A switch advertising a VTP advert with a higher Configuration number will synchronise to the higher number, irrespective of the contents of the VTP advertisement.

VTP can be protected using passwords which have to be configured locally on each switch. Passwords are never exchanged or learnt, an MD5 hash of the password is sent over with the VTP updates.

A Server or client switch can never be in more than one VTP domain at any one time. VTP only recognises only Vlan numbers 1-1005, these are referred to as normal range vlans

VTP can be a dangerous protocol to have on your network since if you place a server switch on your network which has never had a domain name configured but has had lots of Vlan’s setup there is a risk that the new switch will synchronise to a new domain name at that point if the new switch has a high configuration revision number it will advertise over the trunks links this information, all other switches in the network will synchronise to he new vlan’s. The worse case scenario would be that all switches will have no vlan’s or the completely wrong set of vlans.

For example, Lets imagine a switch on your network that is set to the default of server mode with out a domain name but has been configured with vlans. This setup can leave the switch with no domain name to being over written by another switch that comes on line that has a domain name, the switch with the domain name will advertise the information which is has via it’s trunk links to the upstream/downstream switches. The switch with no domain name will receive the advert and synchronise to the new domain name and over write its own vlan database irrespective of the current configuration revision number.

About the Author

Joe Spoto is a senior lecturer at Commsupport networks http://www.commsupport.co.uk in the United Kingdom. Joe teaches Cisco CCNA, CCNP, CCVP courses when he is not out on the road fixing and building networks.

Commsupport Provide instructor led classroom based CCNA, CCNP, CCNA Voice, Security, Wireless training. We also provide free one day classroom based introduction to networking courses, these can are delivered either live in the classroom or live as on-line webinars find out more about our free intro to networking courses here Free introduction to networking courses

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