HP StorageWorks 2/24 FW 07.00.00/HAFM SW 08.06.00 McDATA Products in a SAN Env - Page 195

Intelligent Port Speed Implementing SAN Internetworking Solutions 4 • 10.2000 Gbps long link (mixed paddles) - A paddle pair with one 1.0625 or 2.1250 Gbps paddle and one 10.2000 Gbps paddle provides ten connections. 16 BB_Credits are assigned to the slowest eight short-link ports, and 96 BB_Credits are assigned to one 10.2000 Gbps short-link port (224 BB_Credits total). The remaining 1149 BB_Credits are assigned to the long-link port, supporting a repeated transmission distance of 190 km. When data ingress exceeds data egress for a network device, the device buffers fill, overflow, and drop data packets. Dropped packets are retransmitted. However, TCP flow control interprets these dropped packets as congestion and closes the TCP segment window. Cyclically closing and re-opening segment windows is inefficient and results in dramatically reduced link throughput. Figure 4-13 illustrates this phenomenon (data ingress exceeds egress). Figure 4-13 WAN Link Performance (No Rate Limiting) To prevent this problem, enable rate limiting to ensure the ingress data rate does not exceed the egress rate of the slowest link in the IP WAN path. Rate limiting reduces the probability a device buffer will overflow, cause packet retransmits, and invoke TCP flow control. This sustains and maximizes WAN link throughput. Figure 4-14 illustrates rate limiting. Implementing SAN Internetworking Solutions 4-51