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In-Depth Analysis Of VTP

Posted in Virtual Trunk Protocol (Cisco VTP)

The previous article introduced the VTP protocol and we saw how it can be used within a network, to help manage your VLANs and ease the administrative overhead providing a stress-free VLAN environment, automatically updating all the network switches with the latest VLAN information.

This article extends on the above by delving into the VTP protocol itself and analysing it's structure and format in order to gain a better understanding and enhance those troubleshooting skills.

VTP Introduction & Modes

Posted in Virtual Trunk Protocol (Cisco VTP)



The invention of VLANs was very much welcomed by all engineers and administrators, allowing them to extend, redesign and segment their existing network with minimal costs, while at the same time making it more secure, faster and reliable!

If you're responsible for a network of up to 4-6 switches that include a few VLANs, then you'll surely agree that it's usually a low overhead to administer them and periodically make changes - most engineers can live with that:)

Ask now an engineer who's in charge of a medium to a large scale network and you will definately not receive the same answer, simply because these small changes can quickly become a nightmare and if you add the possibility of human error, then the result could be network outages and possibly downtime.


Welcome To Virtual Trunk Protocol (VTP)

VTP, a Cisco proprietary protocol, was designed by Cisco with the network engineer and administrator in mind, reducing the administration overhead and the possibility of error as described above in any switched network environment.

When a new VLAN is created and configured on a switch without the VTP protocol enabled, this must be manually replicated to all switches on the network so they are all aware of the newly created VLAN. This means that the administrator must configure each switch separately, a task that requires a lot of time and adds a considerable amount of overhead depending on the size of the network.

InterVLAN Routing - Routing between VLAN Networks

Posted in VLAN Networks

This article deals with the popular topic of InterVLAN routing, which is used to allow routing & communication between VLAN networks. Our article analyses InterVLAN routing and provides 4 different methods of InterVLAN routing to help understand the concept
VLAN Configuration and InterVLAN routing for Cisco Layer 3 switches (3550, 3560 series, 3750 series, 4500 series and 6500 series switches) is covered extensively at the following article: Basic & Advanced Catalyst Layer 3 Switch Configuration: Creating VLANs, InterVLAN Routing (SVI), VLAN Security – VLAN Hopping, VTP Configuration, Trunk Links, NTP. IOS License Requirements for SVI Routing.


The Need For Routing

Each network has it's own needs, though whether it's a large or small network, internal routing, in most cases, is essential - if not critical. The ability to segment your network by creating VLANs, thus reducing network broadcasts and increasing your security, is a tactic used by most engineers. Popular setups include a separate broadcast domain for critical services such as File Servers, Print servers, Domain Controllers e.t.c, serving your users non-stop.

The issue here is how can users from one VLAN (broadcast domain), use services offered by another VLAN?

Thankfully there's an answer to every problem and in this case, its VLAN routing:


The above diagram is a very simple but effective example to help you get the idea. Two VLANs consisting of two servers and workstations of which one workstation has been placed along with the servers in VLAN 1, while the second workstation is placed in VLAN 2.

In this scenario, both workstations require access to the File and Print servers, making it a very simple task for the workstation residing in VLAN 1, but obviously not for our workstation in VLAN 2.

As you might have already guessed, we need to somehow route packets between the two VLANs and the good news is that there is more than one way to achieve this and that's what we'll be covering on this page.


VLAN Routing Solutions

While the two 2924 Catalyst switches are connected via a trunk link, they are unable to route packets from one VLAN to another. If we wanted the switch to support routing, we would require it to be a layer 3 switch with routing capabilities, a service offered by the popular Catalyst 3550 series and above.

Since there are quite a few ways to enable the communcation between VLANs (InterVLAN Routing being the most popular) there is a good chance that we are able to view all possible solutions. This follows our standard method of presenting all possible solutions, giving you an in-depth view on how VLAN routing can be setup, even if you do not have a layer 3 switch.

Note: The term 'InterVLAN Routing' refers to a specific routing method which we will cover as a last scenario, however it is advised that you read through all given solutions to ensure you have a solid understanding on the VLAN routing topic.

VLANs - IEEE 802.1q Trunk Link Protocol Analysis

Posted in VLAN Networks


Our VLAN Tagging page briefly covered the IEEE 802.1q protocol and we are about to continue its analysis here. As mentioned previously, the IEEE 802.1q tagging method is the most popular as it allows the seemless integration of VLAN capable devices from all vendors who support the protocol.

So, without any more delay, let's get right into the protocol.


IEEE 802.1q Analysis

The IEEE 802.1q tagging mechanism seems quite simple and efficient thanks to its 4-byte overhead squeezed between the Source Address and Type/Length field of our Ethernet II frame:


VLAN InterSwitch Link (ISL) Protocol Analysis

Posted in VLAN Networks


Deciding whether to use ISL or IEEE 802.1q to power your trunk links can be quite confusing if you cannot identify the advantages and disadvantages of each protocol within your network.

This page will cover the ISL protocol in great detail, providing an insight to its secrets and capabilities which you probably were unaware of. In turn, this will also help you understand the existence of certain limitations the protocol has, but most importantly allow you to decide if ISL is the tagging process you require within your network.


InterSwitch Link (ISL)

ISL is Cisco's propriety tagging method and supported only on Cisco's equipment through Fast & Gigabit Ethernet links. The size of an ISL frame can be expected to start from 94 bytes and increase up to 1548 bytes due to the overhead (additional fields) the protocol places within the frame it is tagging.

These fields and their length are also shown on the diagram below:


We will be focusing on the two purple coloured 3D blocks, the ISL header and ISL Frame Check Sequence (FCS) respectively. The rest of the Ethernet frame shown is a standard Ethernet II frame as we know it. If you need more information, visit our Ethernet II page.


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