DHCP over different vlans with same name

[Azlord_Cisco]

Hi all,

I am setting up a lab and need some help. 

My environment consists of two sites. Site A and B. Each site has vlans 10, 20, 30, 40, 50 and so forth, but each respective vlan on each site has a unique subnet. That is vlan10 on site A has a subnet of 192.168.1.0 and vlan10 at site B has subnet 192.168.2.0. Each LAN is connected to an 1841 router: RA and RC which are subsequently connected via another router RB

I understand setting up dhcp pools on RA and dhcp helper-address on RB and RC, but what I would like to do is set RA to serve dhcp addresses for both sites, but have RA serve both subnets of vlan10 at each respective site. Is this possible to use same vlan names on each sites to serve different subnets or would I need to for example assign unique  vlans for both sites. I.e. vlan10 on site A subnet and vlan 20 to subnet found at site B? 

Cheers

Az

[Spooster IT Services]

Hi,

You can use same Vlan, because Vlan are unique per site or router. You can use Vlan10 on both sites ie site A and Site B to serve different subnets.

[Reza Sharifi]

Hi,

I would recommend you assign a unique vlan per subnet and not multiple subnets per vlan. Things can get ugly quickly and out of control and the next thing you know you spending a lot of time troubleshooting unknown issues that can arise. Design it right to begin with, so you don't have to worry about it later. 

HTH

[Richard Burts]

I am not sure where my colleague Reza is getting multiple subnets per vlan. I am assuming that the topology is using routed links between router A and router B and between router B and router C. Perhaps Reza is thinking that the topology is using L2TPv3 which could extend the vlans from site A to site B. And that would create problems. If an IP packet originates from vlan 10 at site A and is forwarded to router B over a routed link then there is no information about the source vlan that goes with the packet. So when that packet gets to site B on router C it does not matter at all whether the destination is in vlan 10 of site B. It only matters that the subnet of site A and the subnet of site B are unique. As I understand the original post the subnets at each site are unique. So it does not matter whether each site has a vlan identified as vlan 10.

HTH

Rick

[Azlord_Cisco]

Hi all,

Thank you to everyone who has replied.

Okay, good to know I can use the same vlan name for different sites. I tried doing this on PT but clients at both sites weren't getting DHCP addresses. I'm not sure if it's a limitation of PT or I have incorrectly set it up?

I have uploaded the topology along with the devices configurations. Wondering if someone could please have a look at them and provide feedback on the DHCP configuration.

Cheers

[Richard Burts · Accepted solution]

Az

Thanks for posting the additional information. While we can not rule out issues with PT as the cause of the problem I believe that there are several issues in the config that should be resolved before we conclude that it is a PT issue.

I have looked at your diagram and have looked at parts of the configs that you posted. First let me say that I believe that the config of DHCP is ok. The scopes configured on site A router match up correctly with the configured vlans, the default router configured with each scope does match up with the router address, and each vlan interface on site B does have a helper address pointing to the router at site A. So if the DHCP config looks ok we need to look elsewhere for the problem. I believe there is a problem impacting DHCP at site A and a different problem impacting site B.

On site A when I look at the config of switch 1 and looking at the config of the Gig interface which is the connection to the router I do not see the switchport mode trunk command. You might use the show interface trunk command on switch 1 to verify this.

My guess is that the problem with DHCP at site 2 is that there is a problem with communication between the sites and that the DHCP request is not getting to site 1. As a first test of that I suggest that you do an extended ping from the router at site 2. In the extended ping the destination would be the address specified in the helper address command of 200.9.70.254 and the source address should be specified as one of the vlan addresses. If that is not successful we might start troubleshooting by looking at the IPsec security associations on the router at site 2.

HTH

Rick

[Azlord_Cisco]

Hi Rick,

Thanks for having a look at the configurations. I checked the switch at Site-A and you were correct in that g0/1 wasn't in trunk mode. This prevented necessary communication to Site-A LAN. So for helping me discover that I have marked your reply as the solution. As for Site-B, I am not sure what the issue was, however I have since managed to source a configuration that meet my requirements.

I have also managed to use the same VLAN names for both sites using different subnets.

Thank you Rick and to everyone else who helped me with this.

Cheers
Az

[Richard Burts]

Az

You are welcome. I am glad that you have found solutions to the problems and configs that do accomplish what you need. I am happy that my suggestions were helpful. Thank you for marking this question as solved. This will help other readers in the forum to identify discussions with helpful information. These forums are excellent places to ask questions and to learn about networking. I hope to see you continue to be active in the forums.

HTH

Rick

[Dennis Mink]

Your vlans do not have to be unique, but your subnets have to. so if you have 10 remote sites all with vlan 10, but different subnets, than you can assign ip addresses using 1 DHCP server (through a single ip helper at each site). most definitely.