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BP1028 EqualLogic iSCSI for Fibre Channel Professionals 5
4.2 Packet and frame size
A typical FC frame is around 2148 bytes (up to 2112 bytes in the data field). Ethernet packets are
typically around 1518 bytes; however, if “Jumbo Frames” are configured, that allows for a MTU
(Maximum Transmission Unit) of up to 9000 bytes. Depending on the device, the Jumbo Frame size
may be 9000 or slightly higher, usually depending on whether the packet header information is
included in that value. If Jumbo Frames are enabled, all devices in the path must support Jumbo
Frames, or the packets may be dropped or discarded. Check the specifications on each device for the
manufacturer’s recommendation.
EqualLogic PS Series array will always attempt to communicate using Jumbo Frames, however if the
attempt to create an iSCSI session fails, then it will revert to standard MTU and try the connection
again. Connections for replication sessions are independent of other SAN activity, so if the SAN is
capable of Jumbo Frames, but WAN connectivity for replication is not, the storage will automatically
use the appropriate frame size for each connection. Dell recommends the use of Jumbo Frames for
optimal SAN performance.
4.3 SAN topologies and interconnects
FC switches are typically arranged in dual-fabric designs, where a pair of switches is used to make two
independent storage area networks that provide redundant and usually load-balanced connections to
hosts and storage arrays. If switches are connected together (such as to expand the number of
available ports in a fabric), then an ISL (Inter-Switch Link) is created on Expansion-Ports (E-ports)
between the switches. Usually an ISL consists of two or more links to provide for redundancy and
added bandwidth. Fabric Shortest Path First (FSPF) accommodates multiple switch environments and
selects optimal paths based on bandwidth and utilization.
Figure 2 - Dual vs. Single Fabric Design
All network ports of the EqualLogic array must be able to communicate with every other array that is
part of the same group, therefore they must all reside on the same (Layer 2) network. The simplest
example of this is a single Ethernet switch, however to provide redundancy, a minimum of two
switches are usually recommended (Figure 2 – Single Fabric). However, when two or more switches
are used they still must operate as a single flat network and not as a dual-fabric design as found with
many FC SANs (Figure 2 – Dual Fabric).