Need configuration assistance (same subnet across two routers)

[sysadmin@cwcmh.org]

I have a dhcp server that handles ip addressing for our sites. I made a 10.1.72.0/21 subnet. I want this subnet to house all the ips for devices on vlan 72. Locally I was able to achieve that. But I can't figure out how to achieve this from a different location. Location A is getting ip address from the subnet and i want location B to do the same. We have a hub and spoke network so all routers come back to location A. It's a layer 2 connection provided from the ISP. I found that I may be able to achieve what I want by using a BDI interface on the location B router, but it doesn't seem to allow bridging on the interfaces for some reason. 

I have this config so far

interface GigabitEthernet0/0/1.40

bridge-domain 72

interface BDI72
ip address 10.1.72.50 255.255.252.0
ip helper-address 10.1.5.44
encapsulation dot1Q 40

Location B is using vlan 40 for the 10.1.72.0/21 subnet

both routers are isr 4321. on 17.9.5 software

I can provide more info, Im just not sure whats relevant

[Martin L]

/21 subnet is large one;  perhaps u should split it into 2 equal parts and having 2 DhCP servers on both sides instead of 1 server serving both sides; 

Regards, ML
**Please Rate All Helpful Responses **

[sysadmin@cwcmh.org]

I could do that. But then it would defeat the purpose of one subnet for all the users spanning all my sites. I have 8 sites i manage. That means 8 subnets, which i want to avoid.

[Flavio Miranda]

sysadmin@cwcmh.org 

where is this config applied?

The ip helper-address should be applied on the router in the Site B facing your local network.

Now if the communication between site B and Site A happens with layer 2, you can extend the vlan 72 up to site B, right?

[sysadmin@cwcmh.org]

The config is on the router at location B. location A and B are connected by my ISP. they call it an ep lan connection. I would try extending vlan to location B but the existing config there is using vlan 10 20 etc. The config at location A using vlan 120 vlan 130 etc. So that tells me it won't extend the vlan.

[sysadmin@cwcmh.org]

Btw the config for interface GigabitEthernet0/0/1.40 is blank. I just wanted to illustrate that i was intending on using that sub interface for the bridging.

[Flavio Miranda · Accepted solution]

If you are sure you are not breaking anything. You mentioned that you have layer 2 connectivity, right? If does, you can just extend the vlan 72 to the far end.

 But, in order to make it work over layer 3, you need to add ip help-helper address on the far end layer 3 interface. When clients from client B broadcast asking for DHCP, the layer3 device will sent it out to the DHCP server via unicast

[sysadmin@cwcmh.org]

Thanks. I will do this. If it doesn't work then back to the drawing board. But otherwise i appreciate the help.

[Flavio Miranda]

Let us know about your progress.  Would be good to see some topology and device config if possible. It would make us help you better.

Good luck

[Rasika Nayanajith]

Unless you have a L2 connection between location A and location B, I do not think it is possible

HTH
Rasika