Flat layer 2 network

[mahesh18]

                   Hi Everyone,

Need to know the flat layer 2 network design.

If we have access and distribution layer switches acting as layer and only core has routing so is this example of flat layer 2 network

design?

Thanks

Mahesh

[Rick Morris · Accepted solution]

Here is a pretty good write up that discusses this question

http://etutorials.org/Networking/Lan+switching+first-step/Chapter+10.+LAN+Switched+Network+Design/Flat+Network+Topology/

[Abzal · Accepted solution]

Yes, when only core layer do routing. When you have campus wide VLAN. All access and distribution switches have L2 trunks to core.

This model as End to end VLAN.

The term end-to-end VLAN refers to a single VLAN that is associated with switch ports widely dispersed throughout an enterprise network on multiple switches. A Layer 2 switched campus network carries traffic for this VLAN throughout the network.

All devices in a VLAN are members of the same broadcast domain. If an end device transmits a Layer 2 broadcast, all other members of the VLAN receive the broadcast.

This solution not so scalable. I would not recommend it for large campus size network.

Hope it will help.

[mahesh18]

Hi Abzal,

Thanks for  reply.

Here we have layer 3 switches in access and distribution acting as layer 2 only.

So do you know how many switches we should have in end to end vlan?

Thanks

MAhesh

[Richard Burts · Accepted solution]

Mahesh

If it is really a flat layer 2 network then it is all within the same subnet and in this case there is not much routing to do. So what is the purpose of the core switches.

I would think that the number of switches needed would depend on how many devices you need to connect in this flat layer 2 network. Take the number of devices to be connected and divide by the number of ports per switch should give approximately the number of switches.

I would think that you would connect devices to access switches. And I would think that you would want one or 2 switches to provide uplink/downlink to the access switches. The alternative would be to daisy chain all the access switches and that would probably consume more bandwidth.

HTH

Rick

[mahesh18]

Hi Rick,

Currently access and distribution layer switches have same subnet and vlan number.

also they have same ip default gateway comamnd.so this confirms flat layer 2 network right?

Thanks

MAhesh

[Richard Burts · Accepted solution]

Mahesh

If all switches have the same subnet, have the same VLAN number, have the same default gateway, and have only one VLAN configured then yes you have a flat layer 2 network.

This would work ok for a fairly small network. As others have pointed out in this thread as the network gets larger (and if it extends to multiple buildings) it probably becomes better to make it into a layer 3 network. This would have advantages in terms of reducing the size of the broadcast domain and in the possibility of doing address summarization to reduce the size of the forwarding table.

HTH

Rick

[mahesh18]

Hi Rick,

Many thanks once again.

Regards

MAhesh

[Abzal · Accepted solution]

I don't know if there is such recomendaton. As I said before it's not scalable option.

If you have e.g. separate buildings and Layer 3 switches on each of them you can connect them as Layer 3 port to core.

And assign for each building different VLAN. It's much better because you are reducing broadcasts, each building separate VLAN. Second you reduce STP because it will run only on between switches in  the building. And STP changes in one building will not effect switches another buildings. Third dynamic routing protocol will give faster convergence in case failure less than one second.

But anyway it depends on your network design.

Hope it will help.

[mahesh18]

Hi Abzal,

Thanks for all your replies.

Regards

MAhesh