Place chicken, marrow bones, carrots, parsnips, turnips, parsley roots, onion, and garlic in a 12-quart stockpot. Add water to cover bones and lightly simmer until foam starts to form on the top. Skim and discard the foam (schmutz) from the top.
Add parsley, peppercorns, and allspice (or cloves) and return simmer. Simmer, uncovered, over low heat for 1 hour. Cover and continue simmering stock for an additional 3 hours. Turn off heat and allow stock to steep.
Strain vegetables and bones and discard. Cool completely before storing, covered, in the refrigerator, for up to 3 days or freezing for 2 months.