In case of congestion, the long distance ingress ports sends a pause frame to its peer to pause all data transmission on the link for a specified period of time in order to absorb the in-flight packets. When a pause frame is generated from a switch, it takes time to reach the peer switch. Until the packet reaches to peers all the in-flight packets need to be buffered and should not be dropped. Reference: https://www.cisco.com/c/en/us/td/docs/switches/datacenter/mds9000/sw/6_2/configuration/guides /qos/nx-os/qos_bk/qos_pfc.pdf