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We currently have 11 Dell PowerEdge servers running CentOS 5.6 in a cabinet at a colo facility. Our colo provider recently informed us that they want to move us to a new VLAN, and give us all new IPs on a new subnet in a contiguous block. I want to be able to migrate the servers to the new IPs with no (or at least minimum) downtime.

Each of the Dell servers has at least two NICs, so let's say existing NIC A of each server is plugged into existing switch A, which connects to our provider's existing subnet A.

My plan is to: 1) turn the DNS TTLs down for our domain names, 2) connect a second switch (switch B) to our new subnet (subnet B), 3) configure each server's second NIC (NIC B) with the IP, netmask, and gateway information for subnet B, 4) plug each server's NIC B into switch B.

But after that, I'm lost (I'm a routing n00b). I don't know how to configure the routing so that all servers will be reachable on BOTH IPs while we transfer DNS. Then, after the DNS is moved over, how do I modify the routing again so that only the new IPs and subnet are used?

Thanks in advance!

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up vote 1 down vote accepted

You don't have to do anything, usually. The tcp stack will only use routing if the destination is outside of the scope of known ip addresses. Given that your traffic will be coming in on unique ip addresses on different adapters, it will be able to send the traffic out without issue.

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I was hoping that was the case, but wanted to make 100% sure before trying anything on live servers. :) – SteveJ Aug 7 '11 at 3:53

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