Understanding Inter VLAN Routing

Modern switches allow you to create network segmentations called Virtual LAN. How is the communication between VLANs? Learn more detail inter VLAN routing.

In this article:

  • Inter VLAN routing overiew
  • Routing using layer 3 switch
  • Routing using layer 4 switch
  • Diagram

Overview

Modern Switches allow you create Virtual LANs to divide the network into two or more segments virtually to limit the size of broadcast domain, to enforce better security, and to separate specialized traffic from mainstream traffic. But Switches do not forward frames between different VLANs because ordinary switches work in layer 2 of the OSI model.

So how does communication between VLAN? You need an Inter VLAN routing mechanism to enable communication between VLANs.




 

Inter VLANs diagram

For ease of explanation how the inter-VLAN communication occurs, the following diagram helps you better understand how communication takes place. A switch is enabled to 3 virtual segments i.e. VLAN1, VLAN2 and vlan3. A router with multiple Ethernet ports is used to bridge the inter-VLAN communication.

Since the Switches are layer 2 devices they cannot forward frames between VLANs, to allow communication between VLANs you need a Layer 3 device which is a router as described on the above diagram. Note that for home wireless routers (such as wireless AC1200 routers or any models of high speed wireless AC1900 routers including D-link DIR-880 and Asus AC68U)that usually feature multiple SSID, or commonly guest network is different with VLAN.

Notice in the above example diagram there are three VLANs each corresponding to a different subnet. For each network segment requires a router interface for each subnet in order to forward traffic between subnets to allow Inter VLAN communication.

 

inter vlan routing diagram

Fig 1 Inter VLAN routing diagram

 

One interface router for each VLAN

Each router interface connected to each switch interface corresponding to each VLAN1, VLAN2, and VLAN3. Each host in each VLAN that needs to communicate to another host on different VLAN, it must send the packets to the router, which then forwards it to another interface to another VLAN.

Inter VLAN communication using the above method where each subnet / VLAN needs an interface connected to each router interface is wasteful. Therefore you need a router with Fast Ethernet port that supports trunking and use a single physical connection from the router to the switch.

Using Layer 3 Switch

Layer 3 Switches have the capability of routing features. So you don’t need a router to allow routing between VLANs.

Router Vs layer 3 switch

The only difference between routing using a router and a Layer 3 switch lies in the internal processing. L3 switches use specific hardware to make the forwarding process run very fast. The actual receipt, changing of headers, and forwarding of the packets uses the same high-speed internal processing of the L2 switch. The L3 switch also includes the software used to run other processes, such as routing protocols.

Layer 3 switch devices

There are various models of layer 3 switches available in the market such as Cisco Catalyst 3750e-48td. This is a Layer3 Switch with 2 X X2 Uplink and 48 X 10/100/1000base-t LAN ports.

Another layer 3 device you may also consider is Dell Power Connect 6248 which includes 48-port Gigabit Ethernet and optional 10 Gigabit Ethernet uplinks and you can stack up to 12 systems.

Using Layer 4 Switching

Layer 4 Switching considers the information in the Layer 4 headers when forwarding the packet. The forwarding decisions in some cases are based upon information inside the Layer 4 headers, and the other cases based on layer 3 headers, but the switch does accounting based on the Layer 4 headers.

Include UDP and TCP

Inter VLAN Routing decision in Layer 4 Switching include the function of TCP and UDP port numbers.

L4 switching

Fig 2. L4 Switching

 

The application process of the sender and the receiver of a packet are identified by the port numbers. Decision to where to forward the packet based on the information in the TCP or UDP header, typically the port numbers. Alternately, L4 Switch can also simply keep track of the numbers of packets and bytes sent per TCP port number, while still performing Layer 3 forwarding.

Inter VLAN routing using Layer 4 Switching can be described as in the above diagram where L4 switch making its forwarding decisions based on the TCP port number.

The figure shows a server farm, with two servers that have replicated web content, meaning that either server can be used to serve any user. The third server processes all FTP traffic—so when a user of the web server clicks something to start an FTP download, the download comes from SVR-3.


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