Questions about DHCP routing on an MS125 switch

[David666]

When setting up DHCP forwarders, does each switch need its own IP per VLAN as the "forwarding" IP to that DHCP server, or can you set up the "From" IP per VLAN on one switch, then re-use the same IPs for the other switch's forwarders?

[ww]

If the vlan is just layer2 on the other switches, and you trunk that vlan between the L2 switch and the Switch that has the  L3interface+forwarder then you dont need to do anything on the other switches

[KH]

Hey @David666 

If the switch is purely operating at layer 2, you don't need to set up any kind of DHCP relay, those DHCP discovers would be broadcast and go out any port with the VLAN in question. Otherwise, you will need to configure an SVI for each VLAN in question if DHCP Relay is needed, although that interface IP can be the exact same.

[richatfl]

Well, that is kind of the point.  Trying to keep the network clean from broadcasts.  If you have a DHCP helper, the DHCP discover packet coming in would  - only - go out the port that has the mac address that matches the IP address of the dhcp helper ip address (instead of all 48, and all 48 of the next switch and so on).

[GIdenJoe]

Let me kindly add to your logic here.

- If you would have a feature like DHCP discover or request suppression which most likely would be an option you could add into the DHCP snooping featureset.  This would in fact NOT be a feature you would do in the helper address since your topology could change during operation (links going up and down, STP reconvergences) making it undefined if you would have a mapping to the current DHCP server.  The fact a MAC address times out after 5 minutes especially if the DHCP server is quiet otherwise makes it's port towards that server uncertain.  So that will have to make flooding again.

It would make sense to add a feature like this to the DHCP snooping featureset.  However on Meraki they chose to implement DHCP snooping through using MAC addresses.  On Cisco software DHCP snooping is done through trusted and untrusted ports which would be a good candidate to have only trusted ports forward any DHCP discover and request messages.

The method proposed of the message starter has following caveats.
To have DHCP relay on each switch would mean that each switch would have to have an SVI in each VLAN.
The only good thing you are achieving here is limiting broadcasts to other switches in the network but each port on the local stack will still receive the discovers and offers.

[GIdenJoe]

An MS125 is an entry level L2 switch.  So it cannot have any SVI's.
Another upstream device like a L3 switch or router or firewall can be the gateway for each VLAN.

So if you are centralizing DHCP onto a server then you will need to configure DHCP relay on that upstream device but not on a MS125 since you simple can't.

[David666]

I don’t understand.  You can configure DHCP forwarding on MS125 switches.  The option is there under DHCP and Routing.  All I am asking is can I set the same IP to be the sender IP in the DHCP forwarder on all the switches, instead of using a different IP for each.

This article specifically mentions setting up a DHCP relay on MX/MS switches:  https://documentation.meraki.com/General_Administration/Cross-Platform_Content/Configuring_DHCP_Serv... so I know it is a feature.  I am just trying to fins out does the helper for each VLAN need to be a unique IP address, or can I just use the same IP for a specific VLAN on all the switches where helpers are configured.

[KH]

Hey @David666 

I answered that in my previous response, the interface IP can be the exact same for multiple VLANs for DHCP relay.

[GIdenJoe]

You should not be able to create an SVI on an MS125.  It has no SVI capabilities, it's a pure L2 switch.  Since that screen is general for your network, are you sure you didn't create an SVI for another switch?

Since you can't make an SVI you also can't do any DHCP relaying.

[KH]

Hey @GIdenJoe ,

Those models have the option to create an interface for IGMP and DHCP-Relay purposes:

This was done on a layer2 switch

[GIdenJoe]

I get your screenshot.  But can you provide a link to the documentation that states this feature.  I can't find anything on the IGMP querier page or the DHCP for MS page that states these switches have that feature.  The multicast page explicitly shows support starting from the MS250 family and does not specify any MS families for the IGMP querier function.  It would be a serious omission if there is no documentation on this.

The specifications and overview document should also mention specific support on said product families that they support DHCP relay and IGMP querier functionality.

It's kind of hard to provide meaningful insights if the documentation omits crucial information like this.

[KH]

Hey @GIdenJoe 

The MS family datasheet specifies the different "routing capabilities" that each model has and mentions the DHCP relay for said models. Other models add static routes, DHCP server capabilities, dynamic routing, or VRRP.
https://meraki.cisco.com/product-collateral/ms-family-datasheet/?file

[David666]

If you say so.  All I know is that I have the option to configure a DHCP relay on a Network that only has MS120 & MS125 switches in it.  If it can't do that, I am curious as to why the option is there.