Why do sites usually set a low TTL?

Hello!

I've checked about a dozen of web sites and I've noticed that most of them use a low TTL value in their SOA record.
Usually it's up to 1 hour. I set for my site 1 week (604800 sec.) .
So my question is... why for normal operation do sites set such low TTL values? What's the advantage? I can only see disadvantages here. More traffic, latency, more load on the servers. But maybe I'm wrong... Maybe there's something I don't know about the issue?
David1978Asked:
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andreasSystem AdminCommented:
Simple as it: Load balancing. And faster chance to change DNS records if necessary. e.g. if one IP gots DDoSed. Remove it and put another server on on another subnet/provider.
Mal OsborneAlpha GeekCommented:
Low TTL means changes propagate quicker, but more DNS traffic.  A week is fine if you are not planning to change anything, but if you are changing server IPs or adding a secondary MX or something, better to drop it to an hour.

Often ISPs ignore TTLs and have a minimum cache time anyway; even if you set it to an hour, they may keep returning stale data for 24.
David1978Author Commented:
Thank you both for your replies!

I didn't quite understand, so let me clarify couple of things here.

andreas, as per load balancing. Under normal operation, what does it mean exactly? Let's say we have a few PC's which share the traffic load. So how is it different vs. just one PC. Frankly, I don't see any difference in the context of TTL.

Malmensa, you mentioned that ISP's often simply ignore the low TTL values. They probably do that to save their resources. That's understandable. Most people use their ISP's DNS servers, not their owns. But:
a) that would mean DNS issues for their customers once in a while;
b) how technically it's possible, say on BIND9 (if you know)? I know that TTL is defined in a zone file which resides on authoritative DNS server, not the ISP's. What, there's a way to "overwrite" it somehow?
andreasSystem AdminCommented:
1st pc get ip number 1
2nd pc get ip number 2
3pc get ip number 3
4th pc get ip number 1
...
in this way you can distribute the load to individual web servers.

if now mor clients want to connect than servers can handle, you just update your DNS record and add another IP for your domain and you have 4 servers handeling the requests.

Or if one of the IPs of your domain gets DDosed you can ask your upstream provider to block out all the traffic to that specific IP so your ISP link does not get congested. But you still want to be reachable by your customers, so you just update your DNS record and publish other IPs for your servers.

If you have a long TTL it would take to long for the changes to take effect. As the various DNS servers throughout the net are caching your old entries too long.
andreasSystem AdminCommented:
regarding providers ignoring TTL values:

They do not overwrite the entries in the autoratives DNS-Servers. They just ignore them and use an own shorter time interval for the next recursive request to update the OWN cached hosts database. So even the owner of the domain says DNS entry can be cached one week, the ISP makes another recursive query to that domain after his shorter time period has passed. THis is done to catch up unexpected changes on DNS records to minimize customer complains about dysfunctional internet services because the DNS recods were changed but the providers DNS still deliver old information as the LOOOOONG TTL is not over yet.

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