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Broadcast Domains – Routing Between Segments

As your network begins to grow you start to see a need to logically separate different parts of your network into different broadcast domains. But what is a broadcast domains and how is it different from a collision domains? And how do I get two different broadcast domains to talk to each other? In this video blog we review what a collision domain is and understand what is needed to get two different layer 3 network segments to communicate.

Breaking up the Party with Broadcast Domains

Its not uncommon for many larger networks and even many smaller networks to have multiple layer 3 segments. These multiple networks are used to logically breakup the network to optimize performance, divide up administrative duties, or secure the networks from each other. But once you have segmented your network into multiple broadcast domains you need a router to allow traffic to get from one segment (broadcast domain) to another.

In the following video we introduce a router and discuss what changes are needed on your network to get traffic from one network segment to another. We connect a layer 2 switch to a layer 3 router and figure out what is need to make this communication happen.

Do you still have problems routing between segments? Why not let us know but leaving a comment below

About Joe

Senior Network Engineer, technology enthusiast, guitar and bass player. Joe Wilson is the creator of RouterFreak.com as well as other niche websites that can be found around on the Internets.

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  1. broadcast collision domain router ?

  2. Hi RF,

    Just got following associated queries:

    -What does this gig 0/0.1 and .2 means. Is that we are assigning the ip address to the same gig port of router. if so, then how much these sub addresses we can assign to the same gig port.

    -Please elaborate more on this trunk thing.

    -can we configure inter vlan communication on the switch itself .Is this router really required?


    • Hi Kunal,
      Great questions.

      When you see an interface such as Gig0/0.1 or FastEthernet 0/0.1, etc., this represents a sub-interfaces. These can be used when interfacing with a layer 2 switch.

      The maximum number of sub-interfaces you can configure depends on the maximum number if IDB (interface descriptor blocks) support by the router. This number depends on the version of IOS, model of routers and amount of memory available n the router. See this post on Cisco website to determine the maximum number of sub-interfaces per router platform

      On the switch side you would have a trunk with, for example, 3 vlans, vlan 1, vlan 2, vlan 3. On the router side, you would have this trunk connect to, for example port Gi0/0.
      Then, you configure sub-interfaces on that physical port of G0/0 into subinterfaces. You need to use the same subinterface number as my vlan number. So you would create sub-interface Gig0/0.1, 0/0.2, 0/0.3.

      The ip adresses would be configured on the subinterface with no ip address configure on the physical interface (gig0/0)

      All of this is really only needed if you have a layer 2 only switch connecting to a layer 3 only router and are limited on the number of physical ports you have at your disposal.

      If you have two multi-layers switches connected together or one multilayer switch connected to a layer 3 routers with plenty of interfaces, this is not needed.

      Realistically this scenario is rarely used any more as port costs have come way down as well as the cost of multi-layer switches.

      I hope this helps


      • Great Joe…I am preparing for CCNA….

        I'll keep bothering u my friend if needed..:)

        nice videos…

        n ofcourse suggestions are always welcomed…:)



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