CMBS vs MBB

CMBSCiShares CMBS ETF

MBBAiShares MBS ETF

In short

CMBS The index route into commercial mortgage bonds: debt on offices, hotels, malls and apartment blocks rather than home loans. Running since 2012, screened to investment-grade, top-of-the-waterfall slices.

MBB Launched in 2007, MBB is the plain-vanilla way to own U.S. agency mortgage bonds: one of the biggest and busiest funds in a small niche, tracking the Bloomberg U.S. MBS Index for four basis points.

Stated mandate

CMBS The fund seeks to track an index composed of U.S. investment-grade commercial mortgage-backed securities.

MBB The fund seeks to track an index composed of U.S. agency mortgage-backed securities.

Why people hold it

• CMBS The index takes only ERISA-eligible tranches with first priority of principal repayment, rated in one of the three highest categories, plus minimum deal and tranche sizes.sec.gov
• 0.25% a year sits under the 0.30% median fee for mortgage-bond ETFs, notable for a corner of the market this specialized.
• Roughly 500 bond positions, monthly distributions, and a track record going back to its 2012 launch.
• Single mandate, clearly stated: track an index of U.S. investment-grade commercial mortgage-backed securities. No credit tourism, no off-benchmark detours.ishares.com

• MBB Four basis points to rent the entire agency mortgage market, against a category median near 0.35%. In low-coupon bond land, that gap compounds quietly.
• Scale is the real feature: one of the largest and most actively traded funds in the mortgage-bond category, which generally shows up as tighter spreads.
• Pays monthly, and the mandate stays narrow: an index of U.S. agency mortgage-backed securities, nothing exotic bolted on.
• Among the stronger builds in a thin peer group of mortgage-bond ETFs, on the combination of cost, trading friction and issuer staying power.

Worth knowing

• CMBS Thinly traded next to the agency-MBS giants in its category, so the spread you pay and the order type you use matter more here.
• Cheap for a niche, still not cheap outright: agency-mortgage peers VMBS and SMBS run 0.03%, so broad mortgage exposure costs a fraction of this.
• BlackRock flags prepayment and extension risk on these bonds, and notes small interest-rate moves can sharply cut the value of certain mortgage securities.ishares.com

• MBB This is one slice of the bond market. Agency mortgage paper only, with no Treasuries or corporates alongside it.
• Mortgage bonds cut both ways with rates: borrowers refinance when yields fall and sit tight when they rise, so the portfolio's duration shifts underneath you.
• On price it's a photo finish: SPMB tracks the same index at the same 0.04%, and VMBS runs a basis point cheaper.

Full CMBS profile⁠

Full MBB profile⁠