What is a CMBS IO tranche and why does it trade like a corporate bond?

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AcadiFi · Accepted Answer

Class X in a CMBS is an interest-only tranche that receives a notional-based coupon from excess interest in the trust — but because CMBS loans have strong prepayment protection (yield maintenance, defeasance, lockouts), the IO's cash flow is far more predictable than an agency MBS IO.

Key structural features:

- The notional amount follows one or more reference classes (e.g., Class X-A follows the AAA super-senior balance).
- Class X receives the spread between the weighted-average pool coupon and the certificate coupon rate.
- Because CRE borrowers cannot refinance without stiff penalties, voluntary prepayment is essentially zero; the IO loses cash flow only from maturity defaults that reduce the balance early.

Because of prepayment lockouts, CMBS IOs trade more like spread products than option-adjusted products. The main risks are:

- Credit losses that reduce the notional balance earlier than expected.
- Maturity defaults from refi failure at balloon, extending risk for some classes but truncating IO cash flow for others.
- 2008-era 'IO strip risk': IOs attached to subordinate classes took heavy losses when those classes were written down.

Contrast with agency RMBS IO: residential borrowers freely refinance, so agency IOs have negative duration and are convexity trades. CMBS IOs have positive, bond-like duration and are primarily credit/spread trades.

For FRM: identify which reference class the IO tracks and what credit events would erode its notional — that's the core risk.

What is a CMBS IO tranche and why does it trade like a corporate bond?

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