The switch ingress port's VL receive buffer [see “Virtual Lanes” on page 119 and “QoS within the Subnet: SL and VLs” on page 617] selected by the packet's LRH:SL would fill up very rapidly.

The ingress port's Link Layer would very quickly fill up the egress port's selected VL transmit buffer.

The speed at which the egress port's Link Layer can unload its VL transmit buffer is severely limited by its link speed (in the example, either 3x or 12x slower than the ingress port's link speed).

Upon achieving a buffer full condition, the Flow Control Packets (see “Link-Level Flow Control” on page 637) sent back to the corresponding VL transmit buffer in CA X's port Link Layer by the switch ingress port's VL receive buffer will indicate the switch's VL receive buffer is full (i.e., no credits available). The specification refers to this as “back pressure.”

CA X's VL transmit buffer is prevented from transmitting any additional packets from that VL transmit buffer due to the lack of buffer space in the switch ingress port VL's receive buffer.

CA X's VL transmit buffer quickly becomes full and stops accepting packets for transmission to any destination CA port from any QPs and/or EECs within CA X whose specified SL selects the same VL transmit buffer.