Best Practices for Authoritative Servers

[Jim McAtee]

From: "Kevin Darcy"

> But, we have a few zones here and there that we slave from
> business partners, and we've occasionally run into problems with the
> master becoming unavailable and the cross-connected slaves continuing
> to serve up a stale version. I'll have to put more thought into how
> to prevent that from happening.

What's the reason to have slaves cross-connected like this?  Isn't that 
the whole purpose of the expiry in the SOA?  Is there anything 
(performance?) to be gained by treating other slaves as masters?