Upgrading system software in a chassis without traffic loss

Rolling software upgrades may be employed for a network device in a modular chassis and/or virtual chassis. The network device may include memory devices to store a software upgrade package and a group of instructions, and a processor. The processors may install the software upgrade package on a backup routing engine; determine subsets of multiple line cards on which to perform a software upgrade, where ports in each of the multiple line cards are part of a link aggregation group (LAG); initiate a reboot process for each of the subsets of multiple line cards, in sequence, where the reboot process for each of the line cards results in a software upgrade without deactivating any LAG. The processors may also switch the backup routing engine and a master routing engine to create a new master routing engine and a new backup routing engine, and install the upgrade package on the new backup routing engine.