R&M Data Center Handbook

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Switches
A “standard” layer 2 switch divides a network up into physical
subnets and thus increases network bandwidth (from shared
to switched). This means that each individual subscriber can
now use the bandwidth commonly available to hubs, if the
subscriber alone is connected to a switch port (segment).
Segmenting is achieved by the switch remembering what destination MAC address can be reached at what port.
To do this, the switch creates a “Source Address Table” (SAT) for itself and remembers the physical port (switch
connection) to which information is sent and received, that corresponds to the NIC address of the terminal device.
If the destination address that was received is not yet known, or in other words not yet available in the SAT table,
the switch forwards this frame to all ports, an operation known as broadcasting. When response frames come
back from recipients, the switch then makes a note of their MAC addresses and the associated port (entry in the
table) and then sends the data only there.
Switches therefore learn the MAC addresses of connected devices automatically, which is why they do not have
to be configured unless additional specific functions are required, of which there can be several.
A switch operates on the Link Layer (layer 2, MAC layer) of the OSI model (see section 3.8.1) and works like a
bridge. Therefore, manufacturers also use terms like bridging switch or switching bridge. Bridges were the actual
forerunners to switches and generally have only two ports available for sharing one LAN. A switch in this context
is a multi-port bridge, and because of the interconnection of the corresponding ports, this device could also be
called a matrix switch.
Different switches can be distinguished based on their performance ability and other factors, by using the following
features:
• Number of MAC addresses that can be stored (SAT table size)
• Method by which a received data packet is forwarded (switching method)
• Latency (delay) of the data packets that are forwarded
Switching method Description Advantages Disadvantages
Cut-Through
The switch forwards the frame
immediately after it reads the
destination address.
Latency, or delay, between
receiving and forwarding is
extremely small.
Defective data packets are
not identified and forwarded
to the recipient anyway.
Store-and-Forward
The switch receives the entire
frame and saves it in a buffer. The
packet is then checked and
processed there using different
filters. Only after that is the packet
forwarded to the destination port.
Defective data packets can
therefore be sorted out
beforehand.
Storing and checking data
packets causes a delay that
depends on the size of the
frame.
Combination of Cut-
Through and Storeand-Forward
Fragment-Free
Many switches operate using both methods. Cut-through is used as long as only a few defective
frames come up. If faults become more frequent, the switch switches over to Store-and-Forward.
The switch receives the first 64 bytes of the Ethernet frame. The data are forwarded if this portion
has no errors. The reason behind this process is that most errors and collisions occur in the first 64
bytes. In spite of its effectiveness, this method is seldom used.
The expression layer 3 switch is somewhat misleading, since it describes a multi-functional device which is a
combination of router and switch. Brouter was once also a term for this. In routing, the forwarding decision is
made on the basis of OSI layer 3 information, i.e. an IP address. A layer 3 switch can therefore both assign
different domains (IP subnets) to individual ports and also operate as a switch within these domains. However it
also controls the routing between these domains.
A wide variety of switch designs are available, from the smallest device with 5 ports up to a modular backbone
switch which can provide hundreds of high-speed ports.
There are also a multitude of extra functions available which could be listed here. The different switch types in the
data center (Access, Aggregation, Core) and their primary functions were already described in section 3.3. The
functions / protocols required for redundancy in the data center are listed in section 3.8.6.
R&M Data Center Handbook V2.0 © 08/2011 Reichle & De-Massari AG Page 85 of 156