Commentary

The VLAN configuration follows a straightforward and familiar pattern. In this case, however, the configuration in the suggested answers happens to omit any vlan vlan-id global commands. In each switch, the first time the switchport access vlan vlan-id global command identifies a new VLAN not formerly known by the switch, the switch automatically adds the matching vlan vlan-id global command.

A VLAN trunk forwards traffic from multiple VLANs at the same time. Most switches created since the mid-2000’s use IEEE 802.1Q for the trunking protocol, with most switches today no longer even supporting the older ISL trunking protocol. With 802.1Q, with a default native VLAN setting, Ethernet frames from all VLANs except for VLAN 1 (the default native VLAN) will have an additional tag added to the frame while being forwarded over the VLAN trunk. This tag is essentially a label that marks traffic with its respective VLAN. Once the traffic reaches the second device, that device can strip the tag off and use the information in it to properly forward the traffic.

Cisco Catalyst switches default their administrative trunking setting to a mode that tells the switch to use the Dynamic Trunking Protocol (DTP) to negotiate whether to operate as a trunk or not. The instructions told us statically set trunking rather than dynamically negotiate trunking.  To do that, a simple config is needed on both switches: the switchport mode trunk command is used on both switches. Both happen to connect to each other with their G1/0/3 interfaces.

Trunks support all defined VLANs by default. To achieve that final requirement of disallowing any new VLANs, at least until additional configuration is added, you could remove all the other VLANs from the trunk besides VLANs 100, 200, and the (default) native VLAN 1. The sample answers use the switchport trunk allowed vlan remove 2-99,101-199,200-4094 interface subcommand to do so, which removes all VLANs except 1, 100, and 200 from the list of VLANs from 1 through 4094. Alternately, you could directly define the VLANs using the switchport trunk allowed vlan 1,100,200 interface subcommand.

Finally, depending on how you read the requirements, you might have added the switchport nonegotiate command to the access ports (G1/0/1 and G1/0/2 on each switch) in addition to the trunk ports (G1/0/3 on each switch.) Switches disable DTP if you configure the switchport mode access command, but not on ports with the switchport mode trunk command configured. To be doubly sure, you could just use the switchport nonegotiate command on each port.

Known Issues in this Lab

This section of each Config Lab Answers post hopes to help with those issues by listing any known issues with Packet Tracer related to this lab. In this case, the issues are:

Why Would Cisco Packet Tracer Have Issues?

(Note: The below text is the same in every Config Lab.)

Cisco Packet Tracer (CPT) simulates Cisco routers and switches. However, CPT does not run the same software as real Cisco routers and switches. Instead, developers wrote CPT to predict the output a real router or switch would display given the same topology and configuration – but without performing all the same tasks, an actual device has to do. On a positive note, CPT requires far less CPU and RAM than a lab full of devices so that you can run CPT on your computer as an app. In addition, simulators like CPT help you learn about the Cisco router/switch user interface – the Command Line Interface (CLI) – without owning real devices.

CPT can have issues compared to real devices because CPT does not run the same software as Cisco devices. CPT does not support all commands or parameters of a command. CPT may supply output from a command that differs in some ways from what an actual device would give. Those differences can be a problem for anyone learning networking technology because you may not have experience with that technology on real gear – so you may not notice the differences. So this section lists differences and issues that we have seen when using CPT to do this lab.

Beyond comparing your answers to this lab’s Answers post, you can test in Cisco Packet Tracer (CPT) or Cisco Modeling Labs (CML). In fact, you can and should explore the lab once configured. For this lab, once you have completed the configuration, try these verification steps. 

1. Test with IP pings once you complete the PC configurations as follows:
   1. Create configuration on PC1 and PC3 to place them in the same subnet.
   2. Ping PC3’s address from PC1, and vice versa. If the ping works, the traffic must have crossed the trunk in VLAN 100.
   3. Likewise, configure PC2 and PC4 with IP addresses in a second subnet.
   4. Ping PC4’s address from PC2, and vice versa. If the ping works, the traffic must have crossed the trunk in VLAN 200.
2. Note that because this lab uses no routers or layer 3 switches, once working, the PCs in the same VLAN should be able to ping each other, but they should not be able to ping PCs in other VLANs.
3. To test the restriction to support only VLANs 100 and 200:
   1. Reconfigure the switches to put the PC1 and PC3 ports into a new VLAN (300).
   2. Again ping PC3 from PC1; the ping should now fail because the trunk does not forward VLAN 300 traffic.
   3. Reconfigure the trunk to allow VLAN 300.
   4. Repeat the ping test, which should now work.