Generally there should be no difference
between the performance of the 2500 and
2600 routers. However, with X.21 there is
only has one clock, RXD. There is no
terminal timing as with the other standards,
such that X.21 really should never be
used above 64K because of this. At any
rate, I would treat this like a normal link
issue and first try loopback tests -
although I'm not sure that there is a loopback
definition for X.21 DTE devices - but you
can check with the carrier about that.