Using DNS for 99,99% reliability

[Jakob Bak]

Hi,
Can anybody help me with the following questions which I can't seem to find
any information on:
1) Does (most) nameservers on the net respect a TTL of only 5 minutes when
caching our DNS records?
    (or do they often keep the information over longer periods e.g 1 hour to
avoid to much load)
2) How much traffic does a secondary DNS get when the primary is running?
   (nothing at all or 5% or up to 50% ?)

Background:
our system must now provide 99,99% uptime on our web site while our
host/provider "only" garanties 99,9% so we are considering a DNS solution
which might enable us to achieve this.
Essentially, we have been suggested to set up
- a primary DNS at the same location as our main cluster pointing our domain
at this cluster
- a secondary DNS at a different location(server park) pointing our domain
at a smaller cluster at the same location

The idea is then, that in case our primary host is down all traffic will go
to the second cluster as the secondary nameserver takes over for the
unavailable primary. This is of course pointless if our DNS information is
cached all over the internet for e.g. 24 hours, so it is essential that we
can set TTL to for example 5 minutes and that at least a majority of DNS
servers respect this TTL. Secondly, we will have to scale our secondary
cluster according to the load which it will receive (from the secondary DNS
server).