I currently took over a network that uses layer 3 switch for most of their routing. It currently setup with inter-vlans, uing ip-helper address to find dhcp on the network. How can each subnet communicate with each other if they are on different vlans? Please help.....
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Have a look at the OSI model here: https://en.wikipedia.org/wiki/OSI_model

Layer 3 is where a router operates. Different networks, or in your case VLAN, are just different Layer 3 areas. When computer1 from VLAN 1 wants to talk to computer2 on VLAN 2 it first talks to its router (Default Gateway) to forward the message on to computer2. You need a Layer 3 device to talk to different networks.

Just like how your computer at home wants to talk to a website on the internet it asks your router to pass on the data.

In general, VLANs are used to separate traffic by using things like firewall rules so that you can create a network that can be talked to by another but need to use the same internet IP. Such as a guest network. 


The DHCP helper is so that you can run 1 DHCP server for all your networks. One spot to manage all our settings that way.
Answered 01/05/2016 by: smalls
Senior Yellow Belt

  • Thanks for answering my question. How would these vlans communicate with each other? This would be in the router settings right?
    • Depending on what equipment you are using the VLANs could be set so that they talk by default or if they need rules setup sot that they can talk. What gear are you using?
      • Thanks again. Cisco 3750. None of the access rules are stating to allow traffic to other networks. I only see an access rule that allow traffic from with in go to the main router/firewall. That's why I'm guessing the traffic is routed through the firewall, but how would you do this?
    • Is "ip routing" turned on in the Cisco 3750? Might just be a normal switch with VLANs and using the main router for all the interVLAN work as well.
      • I was looking through the configuration and it doesn't seem to be. Thanks again. It's been awhile since I touch an cisco environment. I tried using packet tracer to simulate the environment but currently stuck on the routing part.
    • Always best to start with a network map to figure things out when you take over a network.

      You should be able to copy the config from the router/switching and dump it into packet tracer. It is a handy tool.

      Not sure if I will be much more help unless you have a network map so i can wrap my head round how things are setup.
      • I understand. I was able to figure out how this was working. Apparently the previous engineer trunk allowed all vlans for each trunking port. Thanks again
    • Glad to help in any way I can.
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