VLAN Trunking

In VLAN Trunking, a tag will assign to the frame to indicate their #Virtual LAN (VLAN) belonging which is handled by either a switch or a router. After the frame passed through a trunk link, the tag will be removed from the packet when it reaches the destination node or non-trunk link, usually a switch.

It is advised to have access ports when only a single port is connected to the port, multiple devices that belongs to the same VLAN are connected to the port, or the link only carry a single VLAN when there is another switch connected to the interface. Trunk ports should be used when there is a need to carry multiple VLANs.

There are currently two standards for VLAN Trunking:

  • Inter-Switch Link (ISL), Cisco legacy proprietary method
  • IEEE 802.1Q#

Note: These two standards are incompatible to each other. If two Switch# support different standard and only that standard, then it is not possible to establish a trunk link between them.

Links to this page
  • Virtual LAN (VLAN)

    Since VLAN practically isolates devices in different VLANs, there is a need of routing in order for one device to communicate with other device reside in different VLAN. A naive approach would be to have separate links for each VLAN connected to a Router, and we can see that it will not scale well. Instead, we could utilise the capability of a modern router to divide a physical interface into multiple logical interfaces. After that, define the VLAN Trunking# standard used and its VLAN number using the command encapsulation {standard} {vlan-number}, and its subsequent subnet network address for the VLAN.

    We can apply access rules and provision services to the users of same or similar responsibilities without the need to change the topology of the network or changing the IP Address. Furthermore, VLAN can also create their own Broadcast Domain# without the help from Network Layer Network Device (modern #Switch and #Router). Basically, we could assign the hosts in a subnet# to be in the same VLAN. By using VLAN, we could have different broadcast domains even in a single switch so that broadcast flooding affects only those in the same grouping. If a link needs to handle more than one VLAN, #VLAN Trunking will be used.

  • TRS2251 Chapter 9: VLANs
  • IEEE 802.1Q

    IEEE 802.1Q is an efficient #VLAN Trunking technique introduced by Institute of Electrical and Electronic Engineers (IEEE)#. It only inserts 4 bytes to the Ethernet# frame.

  • Dynamic Trunking Protocol (DTP)

    DTP is a Cisco proprietary Point-to-Point Protocol# used between Switches# for automatic trunk# negotiation. Depending on modes set on both end, DTP can decide whether the link should be trunk or non-trunk. The following table shows the outcome for the link for different composition of opposite ports.

#networking