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Network:

  1. ISPs Actiontec router LAN subnetted to /25 -
    • cannot ping into Cisco subnet
  2. Cisco RV130 subnetted to /25
    • WAN port to Actiontec LAN port
    • set as Gateway, Router mode would not allow internet response.
    • ping OK out

I was hoping some combination of DHCP Relay, Router mode and enabling Dynamic Routing(RIP) would allow me communication from the 1st subnet into the second and retain internet access from inside the 2nd subnet.

I do plan on bridging this modem eventually but wanted to see what it is capable of in this capacity for future reference. None of the suggested Similar Questions presented are quite on point and VLANs are not part of the equation yet. That is why I originally got the router to work with a managed switch on my LAN.

The link is to the emulator for the interface with what I am working with, though mine is not wireless: http://www.cisco.com/assets/sol/sb/RV130W_Emulators/RV130W_Emulator_v1-0-1-3_20140807/default-asp.htm

I hope this is direct enough to discuss here.

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  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
    – Ron Maupin ♦
    Commented Aug 13, 2017 at 18:50

3 Answers 3

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Will DHCP Relay work across different subnets?

That is the point of DHCP relays, so yes. DHCP relay takes the broadcasted DHCP discovery and converts it into a unicast packet destined to the DHCP server.

set as Gateway, Router mode would not allow internet response

and

I was hoping some combination of DHCP Relay, Router mode and enabling Dynamic Routing(RIP) would allow me communication from the 1st subnet into the second and retain internet access from inside the 2nd subnet.

I think your question is not actually about DHCP relay. You are really asking why when your Cisco device is set as a router you don't have access to the Internet from the second subnet.

My question would be how does your Actiontec router know about the subnet behind the Cisco device? Does it support a dynamic routing protocol (and do you have it configured correctly on both devices)? Did you configure a static route?

cannot ping into Cisco subnet

Combining that statement with the piece of information that it works when the Cisco is operating as a gateway leads me to believe that the Actiontec device simply doesn't know where to send traffic destined to your second subnet.

When the Cisco device is a router, the Actiontec is only aware of the two directly connected subnets (WAN and LAN). Traffic for any other unknown subnet (like your second subnet) it sends to it's default route which would be upstream.

When the Cisco device is operating as a gateway, it is NATing traffic to an IP address on the first subnet. Since this would be local to the Actiontec, it can send the traffic along as expected.

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Will DHCP Relay work across different subnets?

That is the point of DHCP relay.

It sounds like you may either be using NAT on the Cisco router, or you have not configured a route in the ISP router to let it know that the network exists on the other side of the Cisco router. If either of those situations exist, then you will not be able to get from the ISP router to the network on the other side of the Cisco router.

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  • Yes in Gateway mode, as it is now, NAT is working which is how I got internet access from inside the Cisco router. Unfortunately the ISP router is locked down so unable to manually edit any routing table, which is why I got the Cisco router. I wanted some different perspectives before doing something that might break the router config and end up having to reset and start all over again.
    – Willie
    Commented May 15, 2017 at 17:23
  • Your NAT will prevent things from originating outside your router, and unless the ISP router knows to send traffic to the network inside the Cisco router, it will send it to its default route (the Internet). Routers learn routes three ways: directly connected networks, statically configured routes, or through a dynamic routing protocol. Your business just needs to get its own router instead of the ISP router.
    – Ron Maupin ♦
    Commented May 15, 2017 at 17:26
  • Does DHCP Relay need the DHCP server, my ISP router here, to be able to define multiple subnets, which I remember hearing about? eg. the cisco router set for relay on 2nd /25 LAN subnet asking for IP from Actiontec on 1st LAN subnet of /25. That doesn't seem logical to me. I have been trying to pick up an IP from another machine inside the Cisco router with no success. APIPA, it can't see any DHCP server.
    – Willie
    Commented May 15, 2017 at 17:51
  • Your DHCP server, which may be in a router that has a built-in DHCP server, must have a scope configured for any network for which it is expected to supply DHCP. Unfortunately, the Actiontec router is a consumer-grade device, which makes it off-topic here. You should really just replace it with your Cisco, or another business-grade, router. Also, double-NAT is very problematic, and you simply do not want to do that.
    – Ron Maupin ♦
    Commented May 18, 2017 at 5:06
  • Tx, yes I suppose so...I may replace the actiontec but then dhcp relay is moot as the ISP facing router. I wanted an educational experience before going that route as a network and security student, so would have to get another business grade router to do that. I'll mark it answered but everyone contriuted.
    – Willie
    Commented May 25, 2017 at 19:05
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After you have your subnets routed test them. Set your computer with a static IP and ping the dhcp server from both/all vlans. Make sure this is successful first.

Then, to get DHCP working to a subnet other than the one the DHCP server is on put ip helper 10.10.10.10 (this address is the dhcp server) onto the gateway in the Cisco router (on the interface or sub-interface) of the vlan that isn't the same as the dhcp server. Do this on each gateway that needs to forward dhcp request 'cross subnet'. This allows the dhcp (UDP) broadcast to get to the server even though the layer 3 boundry stops broadcasts normally.

Build your dhcp scopes for each subnet in your dhcp server and they will hand out addresses based on the subnet the request came from.

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