Cisco 1721 router-how to configure IP to cross two router interfaces

[kuesteral]

Hi,

Long time since I have had to configure a router and have a small project I need help with.

Here is what I have in a simple diagram:

PC(static IP)--to--Switch--to--(Ethernet0 int, WIC0)1721 router(FastEthernet0 int, built-in)--to--Switch--to--PC(DHCP)

I need static IP PC to be able to ping the DHCP client. Static IP units 10.1.1.x/16, DHCP clients 10.1.3.x/16.

I have found these overlap and thus give errors and will even shutdown the interface if I enable certain things like routing or bridging.

So I gather I need to change one interface IP range, which would be the DHCP side, I need to stay 10.1.1.x/16 for static side.

Suggestions on how I get this done? NAT, bridging?

Thanks,

Al

[Reza Sharifi]

Al,

Make one interface (Ethernet0)10.1.1.x/24 and the other one (FastEthernet0 )10.1.3.x/24

thes subnets each will serve 254 hosts.

HTH

[kuesteral]

Thanks but they want to stick with a class B subnet.

There is a DHCP server setup on the router handing out addresses on the 10.1.3.x side. I only need about 30 addresses available form DHCP.

Any other ideas on how I can get the two itnterfaces to allow the units on each side to see each other?

[Richard Burts]

Al

You will not be able to get 2 layer 3 router interfaces to accept addresses that logically are in the same subnet. And it does not sound like a good idea to me to have 2 networks with overlapping address ranges. But there may be some aspects of the local situation that dictate this.

There are a couple of options that could work for this router:

- turn off ip routing and configure bridging on both of the router interfaces. This is a bit drastic but is the most simple solution to the requirement to have separate networks with overlapping addresses.

- you could configure the router with IRB (Integrated routing and bridging) which would allow the 2 router interfaces to bridge traffic between themselves (making the overlapping addresses work) and would still allow routing to be configured on the router.

- you could replace the router with a switch (either layer 2 or layer 3 switch depending on what else you need it to do.

I do not claim that either of these are particularly attractive solutions. But the attractive solution is really to change the requirement that 2 router interfaces connect to networks with overlapping addresses.

HTH

Rick

[kuesteral]

Richard,

Thanks for the help, your suggestions are moving me in the right direction. It is mostly working now with one exception.

When I enable bridging the router will no longer give out DHCP addresses to clients on the 10.1.3.x side. What am I missing?

Al

[Richard Burts · Accepted solution]

Al

I have not tested this but it appears that the router needs the layer 3 logic used in ip routing to be able to hand out DHCP addresses.

If that is the case then turning off ip routing and configuring bridging creates problems. So perhaps the next best alternative is to configure IRB. With IRB routing is still turned on so the router should be able to give out DHCP addresses. With IRB you configure a BVI interface and this virtual interface gets the IP address and the layer 3 logic. The physical interfaces do not get any IP address and will configure bridging so they can handle the overlapping address ranges. The BVI will just see a single /16 subnet and will be able to get to both parts through the physical interfaces.

HTH

Rick

[kuesteral]

Rick,

You are the man! BVI int solved the problem.Thanks!

Al

[Richard Burts]

Al

I am glad that my suggestions did lead you to a solution for your problem. Thank you for using the rating system to mark the question as answered (and thanks for the points). It makes the forum more useful when people can read about an issue and can know that a solution was found. Your marking has contributed to this process.

HTH

Rick