HP Cisco MDS 9020 Cisco MDS 9000 Family Fabric Manager Configuration Guide, Re - Page 392

Buffer Credits Chapter 20 Configuring Interfaces Send documentation comments to [email protected] A cache contains entries for recently allocated ALPA values. These caches are maintained on various TL ports. If a device already has an ALPA, the Cisco SAN-OS software attempts to allocate the same ALPA to the device each time. The ALPA cache is maintained in persistent storage and saves information across switch reboots. The maximum cache size is 1000 entries. If the cache is full, and a new ALPA is allocated, the Cisco SAN-OS software discards an inactive cache entry (if available) to make space for the new entry. See the "TL Port" section on page 20-5 for more information on TL ports. Refer to the Cisco MDS 9000 Family CLI Configuration Guide to manage the TL Port ALPA cache. Buffer Credits Fibre Channel interfaces use buffer credits to ensure all packets are delivered to their destination. This section describes the different buffer credits available on the Cisco MDS Family switches and includes the following topics: • About Buffer-to-Buffer Credits, page 20-20 • Configuring Buffer-to-Buffer Credits, page 20-20 • About Performance Buffers, page 20-21 • Configuring Performance Buffers, page 20-21 • About Extended BB_credits, page 20-21 • Configuring Extended BB_credits, page 20-23 About Buffer-to-Buffer Credits Buffer-to-buffer credits (BB_credits) are a flow control mechanism to ensure that FC switches do not run out of buffers, because switches must not drop frames. BB_credits are negotiated on a per-hop basis. The receive BB_credit (fcrxbbcredit) value may be configured for each FC interface. In most cases, you do not need to modify the default configuration. Note The receive BB_credit values depend on the module type and the port mode, as follows: • For 16-port switching modules and full rate ports, the default value is 16 for Fx mode and 255 for E or TE modes. The maximum value is 255 in all modes. This value can be changed as required. • For 32-port switching modules and host-optimized ports, the default value is 12 for Fx, E, and TE modes. These values cannot be changed. • For Generation 2 switching modules, see the "Buffer Pools" section on page 22-8. Note In the Cisco MDS 9100 Series, the left most groups of ports outlined in white (4 ports in the 9120 switch and 8 ports in the 9140 switch) are full line rate like the 16-port switching module. The other ports (16 ports in the 9120 switch and 32 ports in the 9140 switch) are host-optimized like the 32-port switching module. Each group of 4 host-optimized ports have the same rules as for the 32-port switching module. Configuring Buffer-to-Buffer Credits To configure BB_credits for a Fibre Channel interface using Fabric Manager, follow these steps: 20-20 Cisco MDS 9000 Family CLI Configuration Guide OL-16184-01, Cisco MDS SAN-OS Release 3.x