Methods and apparatus for load balancing in a network switching system

A first network switching device, comprising:

• a plurality of network interfaces configured to communicatively couple to i) one or more external network links, and ii) one or more other network devices within a network switching system;
• a packet processor coupled to the plurality of network interfaces, the packet processor configured to determine, based on a first set of packets received from one or more network devices in the network switching system that contain markings indicating congestion at one or more network devices in the network switching system, one or more respective first congestion levels of one or more first network paths through the network switching system;
• wherein the packet processor comprises:
• a forwarding engine configured to forward received packets to appropriate network interfaces for transmission, and
• path selection circuitry configured to:
• receive messages from other network devices in the network switching system that indicate respective second congestion levels of a plurality of second network paths within the network switching system, including receiving a subset of messages from a second network device, among the other network devices, that indicate respective second congestion levels of a set of second network paths between the first network device and the second network device, the respective second congestion levels of the set of second network paths having been determined by the second network device based on packets that i) were forwarded along the set of second network paths, ii) have been received at the second network device, and iii) were marked to indicate congestion, and
• select, using the respective second congestion levels of the plurality of second network paths indicated by the messages, ones of the second network paths via which a second set of received packets are to be forwarded through the network switching system for load balancing;
• wherein the packet processor further comprises:
• header alteration circuitry configured to alter information in one or more header fields of respective headers of the second set of received packets that are to be forwarded the information in the one or more header fields being altered to be suitable for use by other network devices in the network switching system to make network path selections for load balancing at the other network devices in the network switching system so that the second set of received packets are subsequently forwarded, by the other network devices in the network switching system, responsively to the information in the one or more header fields altered at the first network device in the network switching system, along the ones of the second the network paths, within the network switching system, the ones of the second network paths being selected by the path selection circuitry based on the respective second congestion levels of the network paths as determined by the packet processor of the first network switching device in the network switching system.