National Australia Bank has included a US$202 million tranche in an issue of residential mortgage-backed securities it launched yesterday.The bank is seeking to raise A$750 million through the issue of National RMBS Trust 2011-2 notes. This follows a $1 billion NAB RMBS issue in July.The issue is backed by 2597 fully amortising residential mortgages, with a weighted average loan-to-valuation ratio of 58.5 per cent. The average seasoning is 18.4 months.The US$202 million of class A1 notes are floating-rate soft bullet securities. No principal will be paid to the A1 note-holders prior to the scheduled maturity date, which is on the second anniversary of the issue. Any principal that would have been paid to A1 note-holders will be directed to the A2 note-holders.On the scheduled maturity date of the A1 notes, the manager will attempt to market floating rate pass-through Australian dollar A1-R notes to redeem the A1 notes.In the event the A1 notes are not fully redeemed, the remaining notes will convert to pass-through notes with an increased margin.NAB will retain the $37.5 million of B notes and the $15 million of C notes.