Split routing for a particular subnet(s)

[smyap333]

Hi all

We currently have a network whereby every subnet is a separate VLAN and defined as a sub-interface on the router.

The IP on the sub-interface acts as the default gateway for the VLANs.

Due to the amount of traffic generated by one particular subnet in VLAN A (i.e. 192.168.100.0/24), we want to move the default gateway for that subnet to a layer 3 switch instead.

We also want to route traffic between that VLAN A (i.e. 192.168.100.0/24) and another VLAN B (i.e. 192.168.200.0/24) which is directly connected via a trunk on the layer 3 switch without bypassing the router.

The amount of traffic going to and from these two VLANs is causing the router to get saturated at times.

Obviously we also need to think about a physical upgrade but could someone please share some pointers on how I can go about configuring this just to alleviate the issue in the mean time?

Thank you

[Francesco Molino]

Hi

Just to make sure to give you an accurate solution, are you able to share a quick sketch of your network?

I'm a bit confused about vlan B. Is it sitting on the same switch where you want to move vlan A or another L3 switch?

[smyap333]

Hi Francesco

Thank you for your response.

Sorry let me try to rephrase.

So currently VLAN A users are on a stack of switches (switchA).

VLAN B is on another stack of switches (switchB) connected via a trunk to switch A.

In order for VLAN A to get to VLAN B, currently they need to go to the router.

We want to introduce switchC in between the router and both switchA and switchB so that if VLAN A wants to get to VLAN B, switchC will tell it how and no need to go to the router.

switchC will also have an uplink with higher throughput for traffic between VLAN A and VLAN B.

For all other traffic since we have VLAN C, D, E, F... it will go to the router.

I hope it makes a little more sense.

[Francesco Molino]

Ok doing so, it means you'll move both vlan A and B layer 3 from switch A and B to switch C good inter-vlan routing. Am I correct?

Then you'll need an interco subnet between switch C and router to allow communication between vlan A and B to others.
Yes that make sense. If you add a switch C like you described, this is the way to go.

Now, if we push a bit the reflection, why not moving all vlans to this L3 switch C and keep the router for external connectivity? What model of switch will it be?

What help do you need? Is it how to configure it?

[smyap333]

Hi Francesco

Yes, absolutely! Eventually we will move all vlans to this L3 switch :)

We will be replacing the router as well but currently have an urgency due to saturation between vlan A and vlan B.

The L3 switch is a C4500.

I was hoping on some pointers of how to do this with the lease impact.

On the router end, I believe all that is needed is to shutdown the subinterface for vlan A and B once I prepare the switch configs.

On switch C:

- add default route to router

- add interface vlan A and give it an IP

- add interface vlan B and give it an IP

- make sure both vlan A and vlan B are allowed through the trunks

Is there anything else I missed? Do I need to configure anything on the router end?

Thanks again and appreciate the help.

[Francesco Molino · Accepted solution]

You will move your vlan A and B IPs which means a small downtime here.
Default route to router yes but you need a specific subnet that interconnects your switch and router.
Don't forget the return traffic which means you need a route from your router to your switch to reach A and B.
That's it.

[paul driver]

Hello

you really don’t need to add anaother router or L3 switch for horizontal a-b traffic

you do have an option to make A-B vlans into one vlan by extending either one of the L3 subnets making a-b one larger broadcast domain  

[smyap333]

Hi Paul,

What did you mean by not needing another L3 device?

Please excuse but how would I achieve what I am trying to do in my scenario?

Unfortunately, extending the subnet is not an option in my case.