In an FCoE environment, the PAUSE mechanism is used to provide lossless behavior for Fibre Channel traffic over Ethernet. The PAUSE mechanism allows a receiver to send a PAUSE frame to a sender when its ingress buffer is full or close to full, instructing the sender to stop transmitting for a specified period of time.
This prevents packet loss due to buffer overflow at the receiver. However, the PAUSE mechanism also introduces latency and jitter, which can affect the performance of FCoE applications. Therefore, the receiver must provision sufficient ingress buffer to accommodate the following two sets of data12:
* Frames that were processed and transmitted by the transmitter before the PAUSE frame left the sender.
These frames are already on the wire and cannot be stopped by the PAUSE frame. They will arrive at the receiver and consume ingress buffer space.
* Frames that were sent on the link but not yet received. These frames are also on the wire and will arrive at the receiver after the PAUSE frame. They will also consume ingress buffer space.
The amount of ingress buffer required depends on the link speed, the link distance, and the PAUSE duration. The receiver must ensure that the ingress buffer can hold at least twice the link distance worth of data, plus some margin for safety3. For example, if the link speed is 10 Gbps, the link distance is 10 km, and the PAUSE duration is 100 microseconds, the receiver must have at least 25 MB of ingress buffer for each FCoE virtual lane4.
References := 1: Nexus 7000 F2/F2e Ingress Buffer modifications for FCoE MultiHop over long distance - Cisco 2: Cisco Nexus 7000 Series FCoE Configuration Guide 3: In an FCoE environment, for which two sets of data must an interface that implements the PAUSE mechanism always provision sufficient ingress buffer?
(Choose two) 4: Fibre Channel over Ethernet (FCoE) - Cisco