Friday, February 25, 2011


(Note that this is not an official Basic Draft article. It does not meet Basic Draft standards and will not be listed. If you've arrived through a search engine, please use the search function (top-left) to find a more suitable article).
A trunk is a physical link that assumes the role of multiple logical links. In a VLAN switching environment, a trunk is a single point-to-point link that carries multiple VLANs. The purpose of a VLAN is to save ports between switches implementing VLANs.

Tagging protocols, such as 802.1Q and ISL, form an agreement to transmit different VLANs across a trunk link according to the available VLAN ports on both ends. A trunk link does not belong to a specific VLAN, though the native VLAN of a trunk determines where untagged traffic belong.

ISL is Cisco proprietary. 802.1Q is an IEEE standard that tags each frame with a VLAN ID as it crosses the trunk link. The tag is removed on the receiving end and forwarded accordingly.

End-user stations do not see tags as it is removed before they are forwarded out Access ports.

Trunk configuration must be done on both sides. The first step is to change it to a trunking mode. We would then have to choose a trunk encapsulation.

VTP is a Cisco proprietary protocol that is used to solve potential operational problems (due to human errors or otherwise). It ensures consistency of VLAN information across all switches and simplifies the complexity of managing and monitoring VLAN networks. (In other words, simplifies administration).

VTP is a messaging protocol (?) that makes use of trunk links for propagating addition, deletion and renaming of VLANs. VTP centralizes the administration of the VLAN. VTP messages are encapsulated in ISL or Dot1Q messages before it crosses the trunk link.

According to the seminar, VTP allows trunking of VLANs over mixed media (such as through FDDI). (But I believe this is not a VTP function, but rather a standard). VTP tracks, monitors and reports VLAN changes across the network. We would also have plug-and-play configuration when adding new VLANs, or adding new switches.

All switches must belong to the same VTP domain before they will exchange VTP information. A domain is made up of one or more interconnected switches that share the same domain name. Each switch can only be configured to be in one VTP domain. The domain name is stored in the VTP message as it is sent out of the trunk.

VTP operates in these three modes:

VTP servers can create, modify and delete VLANs. They are allowed to save the VLAN information. VTP servers are also able to propagate updates sent by other servers.

VTP clients cannot create, modify or delete VLAN information. VTP clients are not allowed to save VLAN information if they've made any changes (because they are not allowed to modify). VTP clients can receive, process and propagate incoming VTP messages.

VTP transparent switches do not synchronize its VLAN table with other switches. Transparent switches only forward VTP information to other trunk ports. It does not process the information. It maintains its own VLAN table and will not send out updates about its own table.

VTP advertisements are sent out via a multicast address so that all VTP capable switches receive it.

For some reason, the seminar notes go back to Inter-VLAN routing. So here goes:
InterVLAN routing traditionally require one physical link for each VLAN. So a switch with 4 VLANs would require 4 physical links, which is a wastage. Network designers begin to use trunking protocols such as ISL and Dot1Q to interconnect switches and routers. A subinterface is created on the link for each VLAN required. Each subinterface can be configured an IP address and a VLAN it represents.

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