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New Member

DHCP clarifications

Hi,

While going through one of our old routers configurations I came across a different type of DHCP configurations ..

ip dhcp pool Floor3

import all

network 192.168.2.0 255.255.255.0

dns-server 10.0.0.138

default-router 10.0.0.138

domain-name mycompany.com

In the above configuration the network and the default-router are in different subnets. How is it possible to point 10.0.0.138 address as the default router. 10.0.0.138 is ip address of a DMZ server. With this configuration, all the machines are working fine.

Gone through Cisco materials to make sure how its possible. But, I couldn’t find much about. So, is it a good way of assigning like this ??

Thanks

Rajesh

2 ACCEPTED SOLUTIONS

Accepted Solutions
Purple

Re: DHCP clarifications

Hi Rajesh,

The method you have described above works only because the router interface on that LAN segment has proxy-ARP enabled. The devices would ARP for 10.0.0.138 and since the router has a route to it, it will respond with its own MAC address. While it works, this is bad practice in general and I would not recommend it. You should configure the default gateway to be in the same subnet as the host addresses.

Hope that helps - pls rate the post if it does.

Paresh

Purple

Re: DHCP clarifications

Yes, they just used that sort of config in order to route through that particular server.

As I said, this not good practice although it does work.

Hope that helps - pls rate the post if it does.

Paresh

6 REPLIES
Purple

Re: DHCP clarifications

Hi Rajesh,

The method you have described above works only because the router interface on that LAN segment has proxy-ARP enabled. The devices would ARP for 10.0.0.138 and since the router has a route to it, it will respond with its own MAC address. While it works, this is bad practice in general and I would not recommend it. You should configure the default gateway to be in the same subnet as the host addresses.

Hope that helps - pls rate the post if it does.

Paresh

New Member

Re: DHCP clarifications

Thanks Paresh..

But in this router i cannot see any command like ip proxy-arp inside any of the local interfaces.

I thought of digging out line by line. If i come across any line that helps for DHCP, will let you know.

Thanks

Rajesh

Purple

Re: DHCP clarifications

Hi Rajesh,

Proxy-ARP is enabled by default on Cisco routers. If you issue the following command, it will indicate whether or not proxy-ARP is enabled for that interface:

show ip interface

Hope that helps - pls rate the post if it does.

Paresh

New Member

Re: DHCP clarifications

Yes, I can see that Proxy-Arp is enabled in that interface.

So, it seems they used this type of configuration to push all the clients traffic to a particular server. Right ??

Purple

Re: DHCP clarifications

Yes, they just used that sort of config in order to route through that particular server.

As I said, this not good practice although it does work.

Hope that helps - pls rate the post if it does.

Paresh

New Member

Re: DHCP clarifications

Okk... got it.

Thanks Paresh

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