No -- it only has to be enabled on routers with smaller outbound MTUs
than inbound. A router for which all links have a 1500-byte MTU
doesn't need path MTU discovery; it will never need to fragment
anything.
--Steve Bellovin, http://www.research.att.com/~smb
A subtle correction...
A router where all MTUs are the same will never have to fragement
anything. A router where all MTUs are >=1500 will probably not
need to fragment anything. However, it is possible to attach
a host via GIG-E or other media which supports jumbo frames
(Frame relay, for example) and need to fragment to support a
1500 octet MTU. Currently, this would be a rare occurrence, but,
it is possible in some circumstances. Eventually, if this assumption
were to circulate widely, it could have similar consequences to many
other errant assumptions on the internet.
Owen