TTL question?


I'm not a network administrator so please bare with me. Because our current ISP is not very stable one, our company is thinking to add in another internet line for times when the primary line is down. But the problem is with IP address. If for instance primary line goes down and all the traffic is routed over the backup line the IP of the server changes. Because of that the resolver can't resolve

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to servers IP address. I was just wondering if we set TTL to some low value (5) will that help to solve our problem? Thank you,

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No, just changing the TTL will not fix the problem, not by itself.

If you know the alternate IP address in advance, then your DNS servers can advertise -both- IP addresses -- just have two "A" records for the same hostname. If the backup link is not up, then the hosts which happen to try that backup IP will time out the initial connection and will then try the other (usual) IP. If the regular IP becomes available, then the next time the clients attempt to make a connection, the connection will fail and the client will proceed to try the other (backup) IP.

With such a setup, if your DNS server is using "round-robin" as to which IP it puts first (fairly common) then this could lead to roughly half of all clients having an initial delay, even when the main link is up. Turning off round-robin at your end will not necessarily help, as caching DNS servers further along might choose to round-robin.

But there are other ways of doing failover, most of which are more complex (and more expensive) but some of which have important advantages.

You would probably -prefer- if existing connections were not disrupted when you fell-over to the backup ISP. I would suggest you have a look at the white papers on Vincent C. Jones' web site.

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Walter Roberson


Do you have a link to those white papers? Thanks a lot.

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