How to configure two routers attached to same VLAN with two different subnets

Answered Question
Feb 10th, 2010

Hi

I have a Cisco switched network with one router (non-Cisco). This router acts as a router on a stick for the VLANs in the network.

I have now obtained a Cisco 2821 router which will eventually replace the non-Cisco router and I am using new subnets totally different from the existing subnets.

In order to minimise downtime, I am thinking of coexisting each new subnet with the existing one in the SAME VLAN.

In this way I can gradually move existing clients from the current network number to the new one simply by changing the IP address of each device from the current IP to the new one. Of course the new IP address scheme means going through the new router whereas the old IP scheme will go through the old router.

In addition, on the new router, on each subinterface, I have included a secondary address which is on the existing network. This should allow communication between the clients on the old and new network on the same vlan to communicate with each other.

E.g. for VLAN 10, the existing network is 192.168.100.0/24 with the gateway (non-Cisco router) being 192.168.100.1 and the new network is 10.10.100.0/24 with new gateway 10.10.100.1. The secondary address is 192.168.100.2 See attached diagram.


So my question is:

1. Is this a good idea and will this work?

2. What else do I need to do to get this working e.g. what do I need to configure on the non-Cisco router?

Thanks.

- Andre

I have this problem too.
0 votes
Correct Answer by normanjeter about 4 years 2 months ago

There is also the possibility of putting the new router in between the old one and the network, configuring a separate unused vlan with the new addressing scheme, and the current vlan with the current addressing scheme, and configure routing between the two. Start moving users from one scheme to the next and pushing them onto the new vlan, and once everyone is moved, you can make a quick change in the new router's routing tables and set it as the new default gateway for the network and your done. Just a thought.

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Richard Burts Wed, 02/10/2010 - 20:33

Andre

I believe that your approach is basically sound and should work. The issue is that is a host using the old addressing scheme attempts to communicate with a host using the new addressing scheme, then the originating host will send to its default gateway (the non Cisco router) the packet to be forwarded. So you would need to configure on the old/non Cisco router a route for the new subnet with the Cisco router secondary address as its next hop address.

HTH

Rick

Correct Answer
normanjeter Thu, 02/11/2010 - 06:39

There is also the possibility of putting the new router in between the old one and the network, configuring a separate unused vlan with the new addressing scheme, and the current vlan with the current addressing scheme, and configure routing between the two. Start moving users from one scheme to the next and pushing them onto the new vlan, and once everyone is moved, you can make a quick change in the new router's routing tables and set it as the new default gateway for the network and your done. Just a thought.

andrecsthompson Thu, 02/11/2010 - 16:19

Thanks to both Rick and Norman for your contributions.

Both are good advice.

However, since I do not know much about configuring the non-Cisco router, I will have to go with Norman's advice and create the new subnets in new VLANs and configure the Cisco router for access.

Thanks.

- Andre

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Posted February 10, 2010 at 7:53 PM
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