I'm assuming that the reason Tim's asking about
setting TTLs to numbers like 255 is because he wants to use the
BGP TTL Security Hack to protect against BGP DDOS attacks.
The hack works by setting TTL to a very high value,
and ACL-discarding any BGP packets that don't have TTLs >= ~254,
because it's very hard to fake TTLs, especially from far away.

http://www.nanog.org/mtg-0302/hack.html - Dave Meyer's Abstract
http://www.nanog.org/mtg-0302/ppt/meyer.pdf - Dave Meyer's Talk.
  (Hmmm.. Dave's abstract says he's at Sprint and U of Oregon
  Tim is at Oregon Health Sciences University.)

The internet-draft and Dave's talk both say that for multi-hop
you need to set the ACL thresholds a hop or two lower (obviously),
which expands the set of people who might be able to inject hostile packets,
but it's still pretty tightly contained.

    Bill Stewart