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5406zl routing same subnetz

Occasional Advisor

5406zl routing same subnetz

Hello all, i have to merge to networks. Company A and Company B. Company A is on the left streetside and Company B is on the right streetside. The companys have a 5406zl. The switches have a 10Gbit uplink. In Company A the 5406zl is configured as a routingswitch with vlans and the 5406zl in Company B is configured with vlans without routing. So far so good. Everything is working (routing and Vlans). But now in company A i have the Subnetz and in Company B i have the Subnetz Now the Prolem starts. Is there any way with the 5406zl switches to merge this to Networks?
I will say thanks to any help.
Regards Markus
Valued Contributor

Re: 5406zl routing same subnetz

Hello huether!

Are you sure that the Company B network is with mask (/21)?

If you look at the third octet in binary you will see this:

3rd mask octet = 248 = 11111000
3rd ip octet = 4 = 00000100

which means that the "4" is not in the network part, but instead is a host-address on the network

However, you will not be able to have both networks like this, because of several issues. One thing is that duplicate addresses will exist, that is - on network A a host could have ip address and the same address could legaly exist on the other network, but they can not communicate.

Other problem is that if a host on network A ( want to reach a host a network B with for example address - then the A host will know that it has to go through its default gateway to reach the B host, but the B host will (thanks to its larger network area) think that the A host exist in its own L2 broadcast/L3 ip domain and will not send the answer through default gateway = no contact.

André Beck
Honored Contributor

Re: 5406zl routing same subnetz

Hi Markus,

the problem described by RicN is of course best solved by a renumber, but if all else fails, a hack to make it still work is Proxy ARP. Like most hacks, it's kinda dangerous and it is no solution when the larger network has IPs in the range overlapped by the smaller network. But if is not used in the, proxy ARP might be a way to have it running for some weeks while planning and performing the actual renumber to something sane.