A synchronized communication system, comprising: a plurality of network communication devices , including one network communication device that is designated as a root device and one or more others of the network communication devices that are designated as slave devices , each network communication device comprising: one or more ports , which are configured to transmit and receive respective communication signals over respective network links; and communications circuitry , which is configured to process the communication signals received by the one or more ports so as to recover a respective remote clock from each of the communications signals; a synchronization circuit , which is integrated in the root device and is configured to provide a root clock signal to the communications circuitry in the root device; clock links coupled to convey the root clock signal from the root device to the slave devices so as to serve as a local clock signal for the communications circuitry in the slave devices; and a host processor , which is coupled by control links to the network communication devices and is driven by software to select one of the ports of one of the network communication devices to serve as a master port, to find a clock differential between the root clock signal and the respective remote clock recovered from the master port, and to output, responsively to the clock differential, a control signal causing the synchronization circuit to adjust the root clock signal. a plurality of network communication devices , including one network communication device that is designated as a root device and one or more others of the network communication devices that are designated as slave devices , each network communication device comprising: one or more ports , which are configured to transmit and receive respective communication signals over respective network links; and communications circuitry , which is configured to process the communication signals received by the one or more ports so as to recover a respective remote clock from each of the communications signals; one or more ports , which are configured to transmit and receive respective communication signals over respective network links; and communications circuitry , which is configured to process the communication signals received by the one or more ports so as to recover a respective remote clock from each of the communications signals; a synchronization circuit , which is integrated in the root device and is configured to provide a root clock signal to the communications circuitry in the root device; clock links coupled to convey the root clock signal from the root device to the slave devices so as to serve as a local clock signal for the communications circuitry in the slave devices; and a host processor , which is coupled by control links to the network communication devices and is driven by software to select one of the ports of one of the network communication devices to serve as a master port, to find a clock differential between the root clock signal and the respective remote clock recovered from the master port, and to output, responsively to the clock differential, a control signal causing the synchronization circuit to adjust the root clock signal. <EOS>