1)
What are the implications of "be'Achad She'arecha"?
1.
Zevachim, 114b: It implies anywhere other than on the Mizbe'ach. Consequently, anyone who brings a Korban Pesach on a Bamah Ketanah transgresses a Lo Sa'aseh. 1
2)
Why would we have thought that it is permitted to bring the Korban Pesach anywhere?
1.
Seforno: Because Pesach Mitzayim was brought without a Beis-Mikdash and without a Mizbe'ach.
