Understanding how VLAN Trunks work is essential to you passing the CCNA Exam. In this post I’m going to talk about VLAN Encapsulation with 802.1Q, and all the details about the VLAN Trunking Protocol VTP.
Trunks are needed in switching networks with VLANs in order for hosts within different VLANs to still communicate with each other. When you create a VLAN you are supposed to maintain a 1 to 1 ratio with the number of subnets you have. Which means if you have 4 VLANs you should also have 4 subnets. Since each VLAN is in a different subnet they are in different networks and for devices in different networks to communicate with one another a Layer 3 device such as a router is needed. Interfaces on switches by default can only be assigned to one VLAN at a time. So, in order for you to send data about 4 different VLANs configured on a switch to a router you need to configure an interface to be a trunk link. Trunk links are able to send data for many different VLANs at one time. In order to keep them from getting mixed up they tag the switch frame.
Trunk ports need to have at least 100 Mbps bandwidth. 10 Mbps just won’t cut it because it is sending data for multiple connections at one time. Also VLAN trunk ports will experience a little bit more lag than just a regular interface because it tags the frame which cause the Frame Check Sequence (FCS) to be recomputed again.
Trunking Encapsulation Types
There are two main trunking encapsulations that can be used, ISL and 802.1Q. ISL stands for Inter-Switched Link and is Cisco’s proprietary version. However ISL is older and not really used any more. 802.1Q is the standard trunking encapsulation made by the IEEE and is what the CCNA Exam focuses on.
The 802.1Q Tag
The 802.1Q tag is inserted right into the middle of the Ethernet frame between the source field and the length field. There are 4 different fields inside of the 802.1Q tag which is 32 bits long (4 bytes).
(1) The first field is the Ethernet type field or TPID which stands for Tag protocol IDentifier. The TPID basically just says hey this is an 802.1Q tag because always displays the value: 0×8100.
(2) The second field displays the priority and is known as the PCP field or Priority Code Point. This field is part of 802.1p and is only 3 bits in length. For example voice data could be set to have a higher priority so that it doesn’t lag because it is time sensitive compared to some other frames that might be traversing the trunk.
(3) The third field is only one bit long and is known as the CFI field or Canonical Format Indicator. The CFI is used to tell whether Ethernet or Token Ring is being used. A ’0′ bit means that you are using Ethernet and a ’1′ bit means that you are using Token Ring.
(4) The remaining 12 bits are for the VLAN ID (VID). Which is just to keep track of the correct VLAN the frame belongs to.
The VLAN Trunking Protocol (VTP) is used to greatly simplify the overhead of the network administrator by automatically propagating changes to VLAN information such as adding, deleting, or renaming VLANs to every switch inside of the VTP domain. Without VTP all these changes would have to made manually by hand. When you modify many different VLANs on a lot of different switches your chances of making a mistake misspelling a VLAN name or any other silly mistake like that are pretty likely, but if you use VTP it will minimize such errors greatly.
Switches running VTP are placed into 3 different modes, the server mode, client mode, or transparent mode.
Server Mode – Switches in server mode are able to add, delete, and change VLANs which will then be propagated to all other switches in the VTP domain.
Client Mode – Switches in client mode are only able to receive VTP advertisements and then apply those advertisements to their VLAN configurations if they have a higher revision number than what they currently have stored.
Transparent Mode – Switches in transparent mode relay VTP advertisemnts that it receives but does not apply them to their own VLAN configurations. You can add, delete, and change VLAN names to switches that are in transparent mode and those changes will not be sent out to any other switch in the VTP domain. Switches in transparent mode will always have a revision number of ’0′.
So how often are VTP Advertisements sent out? Every 5 minutes or immediately after there is a change.
When there is a change the revision number is increased by ’1′. Which means that whichever advertisement has the highest revision number means that it is the most current advertisement and the switch should apply it to its VLAN configuration. Also if a switch receives and advertisement it also must have the correct management domain name and password before it can be applied.
Some switches don’t have any ports in some VLANs that can be found in the VTP domain so it is pointless to send them traffic for that VLAN. VTP pruning is able to save that bandwidth by blocking all traffic across the trunk for VLANs that aren’t configured on switches on the other side of the trunk.
Resetting the Revision Number
In some cases you will need to reset the revision number. Such as when you are installing a new switch into your network that was used previously in another network. This is important to do because if the new switch has a higher revision number it could be applied to all other switches in your VTP domain and mess everything up because its VLAN configuration was for a different network. So, make sure whenever you add a new switch to the network you clear its VTP revision number.
The interesting thing though is that there is no “clear the revision number” command. Instead you will have to do one of the following things in order to set the revision number to ’0′:
(1) Set the VTP Client or Server to Transparent Mode and then set it back to either Client or Server Mode. This works because any switch in Transparent Mode will always have a revision number of ’0′ and then when you change it back to Client or Server mode it has no way of getting the old revision number back.
(2) Change the VTP Domain name to something different and then change it back to what you previously had it set to.
Remember that VLANs trunks need be be on 100Mbps ports or 1Gbps ports.
Regular switching ports can only be configured for 1 vlan.
VTP greatly minimizes errors that can occur through manual configuration.
VTP Transparent mode will always have a revision number of 0.
Advertisements with the highest revision number will be applied.
Reviewing regularly will also ensure that you will ready for the CCNA Exam. So, I decided to make you a 1 page review sheet about VLAN Trunks that you can save and print out if you like so that you can review it on a regular bases and pass your CCNA Exam the first time around!
Also feel free to share it with other people who will also be taking the CCNA Exam soon. I plan on making many more 1 page review sheets for every topic on the CCNA Exam so check back often and let me know what you think. You can use this link to download the CCNA EXAM NOTES – VLAN TRUNKS PDF or you can click on the image.