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Flow control is a congestion-control mechanism that prevents data loss at congested ports.
Flow control prevents packet loss by controlling the flow of data from the transmitting device
to ensure that the receiving device can handle all of the incoming data.
A managed switch typically provides two types of flow control methods for minimizing packet
loss: back-pressure flow control and IEEE 802.3x flow control.
Back-pressure flow control is a collision-based mechanism used with ports operating in
half-duplex mode. If the receiving port becomes congested, it fakes a collision with the
transmitting port by sending a collision packet to that port. The collision triggers the back-
off algorithm defined in the Ethernet specification, causing the sending port to temporarily
stop transmitting. This gives the receiving port time to handle the data it has received before
processing any more.
IEEE 802.3 flow control is used with ports operating in full-duplex mode. If the receiving
device becomes congested, it sends a pause frame to the transmitting device. The pause
frame instructs the transmitting device to stop sending packets for a specific period of time.
The transmitting device waits the requested time before sending more data.
Advanced Features of a Managed Switch
The advanced features of a managed switch are designed to optimize total network
performance, design, and security. The following sections describe these advanced features:
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Port Trunking
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VLAN support
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Traffic Prioritization and Class of Service (CoS)
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Spanning Tree Support
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Simple Network Management Protocol (SNMP) Management
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Remote Monitoring (RMON)
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Multicasting and IGMP
Port Trunking
Port trunking (or “link aggregation") provides a cost-effective way for small businesses to meet
their network capacity and availability needs. It also provides a cost-effective way to increase the
bandwidth between switches, or between servers and switches, as your network requirements
grow.
With port trunking, multiple switch ports are combined (or aggregated) to form a single high-
speed connection. Figure 1 shows an example of port trunking.
With port trunking, bandwidth increases by the number of links combined. For example, using
three 100 Mbps connections in a port-trunking configuration results in a 300 Mbps capability.
Port trunking also provides redundancy. If one of the combined links fails, the remaining
aggregated link(s) continue to function, sharing the traffic for the down link.
In addition, all higher-level network functions
including spanning tree protocol (STP) and
virtual LANs (VLANs)
do not distinguish a trunk from any other network port. To these
higher-level functions, a trunk is viewed as one big logical link into the network. In this way,
these functions treat a trunk as a single entity, rather than as separate links. If you define a
