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Consider two scenarios:

  1. Scenario 1: Cisco SG200-26 -> 5-port Hub -> Test Machine
  2. Scenario 2: Cisco SG200-26 -> Test Machine

I have a device 192.168.2.1 connected directly to the Cisco SG200-26 acting as DHCP giving out IP addresses for his own subnet (192.168.2.XXX). I'm trying to set my Test Machine to a static IP on subnet 192.168.1.XXX.

In scenario 1, when I set my Test Machine to DHCP, it successfully receives a 192.168.2.XXX IP but cannot access 192.168.1.XXX subnet at all, and setting it with a static IP 192.168.1.XXX conversely doesn't allow it to route to the 192.168.2.XXX subnet at all.

In scenario 2 however, whether I set my Test machine to DHCP (192.168.2.XXX) or static (192.168.1.XXX), in both cases, both subnets are routable.

What am I missing in scenario 1 to have the static 192.168.1.XXX IP address route properly to the 192.168.2.XXX subnet as well ?

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  • You really need to provide more information. You left out the router. You should edit your question to include the router (which is what routes between the networks) and the configurations of the router and switches.
    – Ron Maupin ♦
    Jun 11, 2016 at 18:32
  • what is the default gateway on the test machine?
    – Ron Trunk
    Jun 11, 2016 at 18:47
  • You should provide a network diagram to get a better understanding
    – user23966
    Jun 11, 2016 at 19:31
  • It is a more general question than that... Just wondering if the DHCP traffic is supposed to work in Scenario 1 as it works in Scenario 2.
    – Wadih M.
    Aug 22, 2016 at 21:00
  • It depends on the information the DHCP server is sending. What is the default gateway for the Test machine?
    – Ron Trunk
    Aug 23, 2016 at 3:04

3 Answers 3

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Just wondering if the DHCP traffic is supposed to work in Scenario 1 as it works in Scenario 2

Yes, the hub should be transparent as far as DHCP is concerned so assuming the ports in both scenarios are in the same vlan, the behaviour should be exactly the same (assuming test machine is the only device connected to the hub).

So if you see different behaviour in scenario 1 and 2 and you want us to help to explain why, you will need to answer our questions for more information.

[edit: moved follow-up questions to comments]

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  • Hi! Thanks for your reply. I will have to dig in those details and get back to the question. Thanks!
    – Wadih M.
    Aug 24, 2016 at 15:36
  • Please do not use the Answer section to ask questions. If you need clarification, please use the comments.
    – Ron Trunk
    Aug 24, 2016 at 16:10
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+50

Posting your configuration would be quite helpful.

Most likely you need to enable inter-vlan routing and create an SVI for both VLANs.

Once they have a layer 3 interface to route through you should be able to talk between VLANs just fine.

I am not familiar with that particular switch, but for most IOS devices inter-vlan routing is enabled by default and all you will need to do is create the SVIs (for both 192.168.1.X and 192.168.2.X)

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  • I don't understand how this could be the correct answer; if there was no inter-vlan routing then how could scenario 2 have worked ok ?
    – hertitu
    Aug 30, 2016 at 8:52
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DHCP (Server) options typically include default gateway and DNS server at a minimum. The default gateway option should specify the router/gateway IP address for the subnet.

Typically, each VLAN carries traffic for a unique subnet and a layer3 device does inter-VLAN routing. To forward/direct DHCP request from a subnet to a DHCP server on a different subnet, you need to configure that to happen. In Cisco IOS, it is called an ip helper address and is applied to the layer3 interface for the VLAN(s) that the DHCP server(subnet) is not on.

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