Bring stock, coconut milk, mushrooms, shallots, lemongrass, ginger and red pepper to boil in large saucepan. Reduce heat to medium-low; simmer 5 minutes.
Stir in chicken; simmer 5 minutes longer or until cooked through. Stir in lime juice and fish sauce. Remove lemongrass pieces.
Ladle into soup bowls. Serve with cilantro.