PPP over Sonet default MRU

While doing POS interoperability testing with Juniper and Cisco routers we found the following behavior; Even though Juniper POS interfaces have a default MTU of 4470, no MRU information is sent during LCP negotiations.

Even so Cisco routers that use the same default MTU, but DO send their MRU, still send packets of up to 4470 out. My question are the following:

1) Is 4470 the actual default MRU to be used for POS (and not 1500 as documented in rfc 1661) ?
2) If so, is this documented somewhere ?
3) If 1500 is in fact the default MRU, isn't there something "wrong" for lack of a better term with Juniper's behavior in not sending it's MRU and Cisco's in sending frames of more than 1500 bytes regardless.

The behavior observed might be specific to the actual routers and OS versions used, but I am still curious as to what the actual "standard" is.

Thanks for any clarifications.

Stephane St-Hilaire
ssthilaire@xxxxxxxxxxxxx
T:514-906-4915