There are several relay services that are of interest to ISPs because they can set up tunnel relay servers to either provide better service to a mixed v4/v6 Internet, or to enable full reachability regardless of the end-user's version of IP. The following pages deal with the three main automatic tunnel/relay technologies:
- Cisco 6to4 Relay Service
- Juniper 6to4 Relay Service
- Linux or BSD 6to4 Relays
There is also useful information about these technologies on these pages:
The following conversation from the NANOG list explains how effective placement of 6to4 relays in content-provider networks can greatly reduce the likelihood of latency issues in a mixed v4/v6 Internet.
And here is a brief Q&A that makes it clear that installing proper relays gives you greater visibility of your IPv6 traffic.