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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:

vlans-routing-1

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

Introduction

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:

vlans-8021q-analysis-1

VLAN InterSwitch Link (ISL) Protocol Analysis

Posted in VLAN Networks

Introduction

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:

vlans-isl-analysis-1

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.

VLAN Tagging - Understanding VLANs Ethernet Frames

Posted in VLAN Networks

We mentioned that Trunk Links are designed to pass frames (packets) from all VLANs, allowing us to connect multiple switches together and independently configure each port to a specific VLAN. However, we haven't explained how these packets run through the Trunk Links and network backbone, eventually finding their way to the destination port without getting mixed or lost with the rest of the packets flowing through the Trunk Links.

This is process belongs to the world of VLAN Tagging!

VLAN Tagging

VLAN Tagging, also known as Frame Tagging, is a method developed by Cisco to help identify packets travelling through trunk links. When an Ethernet frame traverses a trunk link, a special VLAN tag is added to the frame and sent across the trunk link.

As it arrives at the end of the trunk link the tag is removed and the frame is sent to the correct access link port according to the switch's table, so that the receiving end is unaware of any VLAN information.

The diagram below illustrates the process described above:

vlans-tagging-1

Here we see two 3500 series Catalyst switches and one Cisco 3745 router connected via the Trunk Links. The Trunk Links allow frames from all VLANs to travel throughout the network backbone and reach their destination regardless of the VLAN the frame belongs to. On the other side, the workstations are connected directly to Access Links (ports configured for one VLAN membership only), gaining access to the resources required by VLAN's members.

VLANs - Access & Trunk Links

Posted in VLAN Networks

If you've read our previous article The VLAN Concept - Introduction to VLANs  then you should feel comfortable with terms such as 'VLAN', 'Static & Dynamic VLANs', however this is just the beginning in this complex world. This article will start to slowly expand on these terms to help understand how VLANs are implemented inside an enterprise network.

To begin with, we will take a closer look at the port interfaces on these smart switches and then start moving towards the interfaces connecting to the network backbone where things become slightly more complicated, though do not be alarmed since our detailed and easy to read diagrams are here to ensure the learning process is as enjoyable as possible.

VLAN Links - Interfaces

When inside the world of VLANs there are two types of interfaces, or if you like, links. These links allow us to connect multiple switches together or just simple network devices e.g PC, that will access the VLAN network. Depending on their configuration, they are called Access Links, or Trunk Links.

VLAN Configuration, InterVLAN routing,Trunk Link configuration 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.

 

Access Links

Access Links are the most common type of links on any VLAN switch. All network hosts connect to the switch's Access Links in order to gain access to the local network. These links are your ordinary ports found on every switch, but configured in a special way, so you are able to plug a computer into them and access your network.

Here's a picture of a Cisco Catalyst 3550 series switch, with it's Access Links (ports) marked in the Green circle:

vlans-links-1

We must note that the 'Access Link' term describes a configured port - this means that the ports above can be configured as the second type of VLAN links - Trunk Links. What we are showing here is what's usually configured as an Access Link port in 95% of all switches. Depending on your needs, you might require to configure the first port (top left corner) as a Trunk Link, in which case, it is obviously not called a Access Link port anymore, but a Trunk Link!

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