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Need configuration assistance (same subnet across two routers)

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

1 Accepted Solution

Accepted Solutions

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

View solution in original post

9 Replies 9

Martin L
VIP
VIP

/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 **

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.

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?

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.

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.

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

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

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

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

HTH
Rasika

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