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c3750 Switch use helper adress without routing activated


I have a Cisco c3750 switch as a layer-2 device so ip routing is disabled. The router is still a Windows Server 2019 with an Intel network card an vlan activated. The windows server has ip and IP linked with the vlan-id.

That works so far very good because the clients in Vlan-10 on the switch had a static ip address configuration. Now, my new IT Leader decide to change it and will take advantages using a DHCP Server.

I figured out, that put the ip helper on vlan-10 is not enough because the switch has no routing functionality.

Is there a way to let the switch sill use as layer-2 and put on vlan-10 an static route without taking any use of ip routing?




interface Vlan1
 description "Management"
 ip address

interface Vlan10
 description "Windows Clients"
 ip address
 ip helper-address

switchport 1/0/1
 description "Uplink to Win Server Router"
 switch mode trunk

ip default-gateway




3 Replies 3

Joseph W. Doherty
Hall of Fame
Hall of Fame

A L3 switch is still a L2 switch.  I.e. whatever you did with in for L2, should still be capable.

If you're going to start pulling IPs via DHCP, you only need a helper if the DHCP server doesn't have a direct connect to the network you want to provide DHCP IPs for.  Further, I believe, in such a case, the helper will require routing to be active.

From what you've posted, and have described, though, what and where's the gateway IP for VLAN 10?  (Is it on the Windows Server?)

It will not work

Ip helper need ip routing to be enabled in SW to work.

Ip defulat-gateway not make ip helper work here.

One way to look at it is that ip helper-address instructs the switch to forward an IP packet to a destination in a remote network. And forwarding an IP packet to a remote network is the definition of routing. So ip helper-address does require that ip routing be enabled.

Changing layer 2 switches into layer 3 switches is a significant change. Will the switch run a dynamic routing protocol? Or will the switch use static routes? To forward from the switch interface that receives the DHCP request to the remote server will require that the switch have a routed interface connecting to the upstream device. The devices that have been doing the routing will have new downstream neighbors and will require config changes to accommodate the connection. Perhaps your new IT Leader might want to have second thoughts about the impact of using DHCP?


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