If you hold bonds in a taxable account, consider the tax - exempt funds instead of the total bond market index funds. (noroboinvestor.com)
Unfortunately, most bond market index funds and ETFs attempt to match duration to a bond index. (indexologyblog.com)
Note also that some very low cost no load bond market index funds may have longer - term back end redemption fees that are paid to remaining shareholders and only expire after multiple years. (bondmarketindexfund.com)