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Network new L3 switch into Legacy switch

Last Modified: 2019-11-18
Uplink/Integrate two seperate switch stacks

- We need connect a new Layer 3 switch with VLANS and Subnets into a dumb, legacy switch stack that has a flat network and no VLANS
- Dumb switch has all live devices plugged into it. L3 switch has test devices.
- We need all devices on both switch stacks to communicate with each other.


- Switch 1 (Meraki MS350) has 2 VLANS and 2 subnet
- VLAN 333 and subnet is, DFG
- VLAN 444 and subnet is, DFG
- Both VLANS have been configured to see each other
- Created a Trunk port that is a member of VLAN 333
- Looking to use Trunk port as an uplink port between the Layer 3 and Legacy switch stacks


- Dumb switch has no VLAN just 1 flat subnet of
- Flat network is, DFG (Issued by ISP on port uplink for DHCP requests/IP Helper)


Does the DFG of VLAN 333 need to match the DFG of the legacy switch before uplinking ?
Watch Question

nociSoftware Engineer
Distinguished Expert 2019

So you can create a port on the new switch where the VLAN 333 is connected untagged.
Then you can connect the dumb switch to that network.

For the OLD switch it needs to be untagged... so trunking as in allowing multiple VLAN's won't work.
atlas_shudderedSr. Network Engineer

Your design will fail as it is currently configured.  Your legacy switch does have a vlan configured - the default (VLAN1).

That vlan (VLAN1) is hosting the subnet range of

On your Meraki, you have the same subnet ( configured as hosted on VLAN333.  Additionally, you have to remember that the Meraki has vlan 1 built in by default, even if you aren't placing host subnets on it.

So, what you end up with is a vlan and subnet mismatch:


VLAN1:  no subnet configured

The first thing that you would need to do is resolve the mismatch issues.  (Don't make any of these changes yet)

You can do this in one of two ways:

1.  On Legacy switch, configure VLAN333 and then assign it to all ports as the access vlan in place of VLAN1.
2.  On the Meraki, blow out VLAN333 and move subnet to VLAN1

Once this is completed, you can start addressing the underlying routing issues, which there are most likely many.  I am going to bet that the DFG address being used by your existing devices on the Legacy switch is actually the IP address of your ISPs LAN interface correct?  If so, we are going to have to deal with the issue of getting traffic back and forth between VLAN444 and VLAN111.
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