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Please consider the following scenario

  1. The router (Linksys E2000) has an INTERNET port where the main network is plugged in. The router has some ETHERNET/OUT ports where a switch, laptop or PC can plug in. The router has the address 192.168.X.Y

  2. A LAN wire from one of the ETHERNET ports of the router is plugged back into the main network port.

Will main network see the router's DHCP address 192.168.X.Y?

Please let me know if I should clarify further.

Ron Maupin
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Hado99
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  • What do you mean by an OUT LAN cable or an IN port? Network interfaces are bidirectional, and traffic flows both directions through the interfaces. Maybe you should tell us the network device models, how they are are connected (a diagram would be good), and the configurations. – Ron Maupin Feb 21 '17 at 20:09
  • Sorry for the lack of clarity. – Hado99 Feb 22 '17 at 19:29
  • Edited. Please let me know if I should add more information. – Hado99 Feb 22 '17 at 19:53
  • Still isn't clear what you're asking. If you're asking if your router will interfere with DHCP on the main network, my advice is just don't do it. – Ron Trunk Feb 22 '17 at 20:06
  • Did you not get your question answered? – Ron Maupin Mar 24 '17 at 20:22

2 Answers2

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Switches are transparent devices. Switches, themselves, don't broadcast anything. A switch will flood unknown unicast frames, and broadcast or (possibly) multicast frames will be sent to all switch interfaces, but switches do not originate traffic, nor do they care anything about layer-3 protocols, e.g. IP, so a switch doesn't know about IP addresses, much less broadcast them.

Each device on a network has its own addresses, and each knows what the local network is based on its configured network mask.

Based on your edit:

The Linksys router, and all consumer-grade devices, are explicitly off-topic here.

DHCP requests are broadcast from the hosts to the network, so DHCP requests will go to all switch interfaces, including the switch interface of your DHCP server (possibly running in your router chassis). The DHCP server will respond to the requesting host with the necessary information (IP address, mask, gateway address, etc.).

Ron Maupin
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  • If another device plugs into the main network, is it possible that, due to this configuration, the IP address lease request could be intercepted by router and the device could be assigned an IP address from router's subnet pool (192.168.x.y) rather than from the main networks pool (10.178.A.B) – Hado99 Feb 22 '17 at 20:30
  • A host may receive multiple DHCP offers from different DHCP servers, and it will pick one. Being broadcast, DHCP requests don't travel through a router. Broadcasts are limited to the LAN. LAN traffic stays on the LAN, and it doesn't get sent through a router unless it is destined for a different LAN, in which case the frames use the gateway address as the destination address, not the broadcast address. – Ron Maupin Feb 22 '17 at 20:59
  • @RonMaupin `but switches do not originate traffic, nor do they care anything about layer-3 protocols` That is not entirely true. We don't know exactly what switch the user is using. Some switches and configurations send BPDU packets in addition to providing layer three routing services. – SuperAdmin Mar 25 '17 at 00:51
  • We do know the switch the user has. It is a simple layer-2 switch module in a Linksys router. Also, a layer-3 switch is first and foremost a layer-2 switch. It has a router built into it so that some interfaces (logical and/or physical) can be layer-3 interfaces. BPDUs are only relevant to other switches, and layer-2 switches are transparent devices to the hosts on the LAN. They do not originate traffic to the hosts, not do they modify the host traffic (they do on a trunk, but that is only between switches, and it doesn't change any user data).. – Ron Maupin Mar 25 '17 at 01:00
  • @RonMaupin Ahhh, I seemed to have missed the `can` in `can plug in.` I thought OP was saying another switch was plugged in. So if I understand this correctly now, OP has the LAN and WAN plugged into the upstream network? This would essentially create a rouge DHCP server correct? – SuperAdmin Mar 25 '17 at 02:29
  • Its a totally messed up up topology. Both the WAN and one of the LAN interfaces are connected to the main network. Because it is a consumer-grade device, it is off-topic here, and there is no telling how it is configured. – Ron Maupin Mar 25 '17 at 03:31
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Although your network topology is not quite clear to me (consider including diagrams in questions) there are a few concepts to keep in mind here. The first of which is a broadcast domain.

enter image description here

A broadcast domain is a logical division of a computer network, in which all nodes can reach each other by broadcast at the data link layer. A broadcast domain can be within the same LAN segment or it can be bridged to other LAN segments. - Wikipedia

This essentially means that all broadcasts including DHCP are not going to make it any further than your router. If a unicast or multicast packet destined for another network reaches your router, it will be forwarded.

Another concept to consider is layer 3 switching. It is unknown to me if this applies to your network, but it is plausible that you can have a switch that does same network or inter-VLAN routing to take load off of your router and minimize packet travel. However, a switch also has a Source Address Table (SAT) used to forward traffic without flooding all switch ports with the traffic. The initial DHCP request will reach the entire broadcast domain, but then further traffic of the DHCP process will be between the server and client exclusively, and will not be seen by other devices connected to the switch or router.

I hope this answers your question. If you provide an updated explanation of your topology or a network diagram I may be able to better answer your question.

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