Hi Sonal,

Trigger will fire. write the trigger for that on the parent object and update child values or write a trigger in child and parent also update. A roll-up summary field calculates values from related records, such as those in a related list. You can create a roll-up summary field to display a value in a master record based on the values of fields in a detailed record. The detail record must be related to the master through a master-detail relationship. For example, you want to display the sum of invoice amounts for all related invoice custom object records in an account’s Invoices related list. You can display this total in a custom account field called Total Invoice Amount.
Changes to the value of a roll-up summary field can trigger assignment rules to run. If a roll-up summary field is part of the criteria in an assignment rule, the field’s new value is used to evaluate whether to reassign the record.
A roll-up summary field can trigger workflow rules and field validations. 
However, workflow rules and field validations do not fire when the following changes cause a mass recalculation of roll-up summary values:
Changing the roll-up summary definition (such as the object, function, or field being aggregated).
Changing the expression of a formula field referenced in a roll-up summary field.
Replacing picklist values for picklist fields referenced in the roll-up summary filter.
Changing picklist record type definitions.
Changing currency conversion rates.
Changing price book entries.


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Thanks and Regards,
Deepali Kulshrestha
www.kdeepali.com