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We have currently installed new DNS servers in our environment and instead of having to physically re-configure all servers and static devices, I was thinking of adding a static route to forward all L3 traffic from old to new. Is this possible using a route entry or does this need to be done via a firewall filter?
If I understood you correctly, you are looking to keep all DNS queries going to the old server while configuring the old server to forward these requests to the new DNS. if this is the case, then it is really a pure routing issue between the ip addresses of the old and new DNS servers which totally depends on your particular network setup.
To clarify a bit, I actually want to decommission the old DNS server, and have the juniper switch forward all DNS requests from 192.168.2.2 to 192.168.2.3 as an example. Basically I am looking for a way to easily have DNS working without having to change the DHCP dns server entry and re-configuring all servers. I did find the forwarding-options helpers dns server interesting, but it would require all devices to have the DNS server set to the switch, which in essence I am trying to avoid.
I found that adding a static route does not work.
in this case, since you are going for replacing the old DNS with the new one, it is much better to swap ip addresses ( give the new server same IP address of the old one and change the old server to another address). despite the service outage this might cause, you will gurantee no changes in DHCP or clients. then you just have to make sure that the arp entries relevant to the old server IP address are all updated after the change.