I don't know if this addresses your concern but here's a good link from CCO:
http://www.cisco.com/en/US/products/hw/switches/ps1938/products_installation_guide_chapter09186a00800d8513.html
Hope this helps,
Harold Ritter
Sr Technical Leader
CCIE 4168 (R&S, SP)
harold@cisco.com
México móvil: +52 1 55 8312 4915
Cisco México
Paseo de la Reforma 222
Piso 19
Cuauhtémoc, Juárez
Ciudad de México, 06600
México
