Cisco Trunks

Nov 2017

Introduction

In the previous topic regarding Cisco VLANs and Trunks there are some videos outlining the theory of using VLANs and Trunks. Here we will look more at a worked example, and see some of the commands.

Overview

First of all, I would like to outline what I am trying to achieve. Look at the diagram below.
There are two sets of receivers (so consider two sets of multicast) coming from different head-ends. To keep the traffic separate, lets consider that the ports connected to the receivers on the left are all in VLAN A, and the receivers on the right are in VLAN B, as show below.
VLAN C is on another switch where our destination equipment is located. The link between the two Cisco switches is our Trunk.
The reason we are using a Trunk is that a single port (interface) can only carry traffic from a single VLAN, because an interface can only exist in one VLAN. So if you have multiple VLANs that you wish to carry from one switch to another, you need to use a Trunk. A Trunk can carry the traffic from mulitple VLANs.

Example

VLANs, while technically the same, can be broken down in to funciton. Some of these are:

Default VLAN
Native VLAN
Data VLAN
Management VLAN
Voice VLAN

Default VLAN

As the name suggests, the default VLAN is the VLAN that the interfaces are initially located in. On a Cisco this is VLAN 1. From the Cisco CLI you can use the command show vlan to list the currently configured VLANs.

VLAN Name Status Ports
---- -------------------------------- --------- -------------------------------
1 default active Fa0/1, Fa0/2, Fa0/3, Fa0/4
Fa0/5, Fa0/6, Fa0/7, Fa0/8
Fa0/9, Fa0/10, Fa0/11, Fa0/12
Fa0/13, Fa0/14, Fa0/15, Fa0/16
Fa0/17, Fa0/18, Fa0/19, Fa0/20
Fa0/21, Fa0/22, Fa0/23, Fa0/24
Gig0/1, Gig0/2
1002 fddi-default act/unsup
1003 token-ring-default act/unsup
1004 fddinet-default act/unsup
1005 trnet-default act/unsup

Above we can see that VLAN 1 is listed as the default VLAN, this VLAN contains all the ports on the switch, this is how the switch will be configured from new when powered on.

Native VLAN