16-10-2012, 12:19 PM
CCNA 640-801 Exam Notes Fundamentals of Switching
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LAN Segmentation
In a collision domain, a frame sent by a device can cause collision with a frame sent by
another device in the same collision domain. Moreover, a device can hear the frames
destined to any device in the same collision domain.
In a broadcast domain, a broadcast frame sent by a device can be received by all other
devices in the same broadcast domain.
A LAN segment or an Ethernet network segment consists of the devices connected with
a coaxial cable or a hub. The devices are in the same collision domain.
Ethernet congestion problem occurs when too many devices are connected to the same
Ethernet network segment, such that the high network bandwidth utilization increases the
possibility of collision, which causes degradation of network performance.
LAN segmentation solves the congestion problem by breaking the network into separate
segments or collision domains using bridges, switches or routers (but not hub s or
repeaters). LAN segmentation can reduce the number of collisions in the network and
increase the total bandwidth of the network (e.g. 10 Mbps for one segment, 20 Mbps for
two segments, 30 Mbps for three segments, and so on).
Spanning Tree
In a switched network with redundant paths (i.e. with loops), the following problems will
occur:
Broadcast Storm – A broadcast or multicast frame will be forwarded by a switch out
all its active ports except the source port. The resulted frames will then be
forwarded by the other switches in the network similarly. Some of the frames will
be forwarded around the network loop and back to the original switch. The process
then repeats. Therefore, the frames will loop indefinitely in the network and
eventually exhaust the processing power of the switches and the bandwidth of the
network.
Receiving multiple copies of a frame – When a switch receives an unicast frame to
a destination device that it does not have an entry in its bridge table, it will forward the frame out all its active ports except the source port. Therefore, the destination
device may receive multiple copies of the frame through the redundant links.
Bridge Table Thrashing – A switch may receive frames from a source device at
more than one ports if there are redundant links. It needs to update its bridge table
whenever a frame from the source device arrives at a port differs from the last time.
If the arrival frequency of such frames is high, the processing power of the switch
will be exhausted.
Virtual LAN (VLAN) and VLAN Trunking
Virtual LAN
A Virtual LAN (VLAN) is a broadcast domain created based on the functional,
security, or other requirements, instead of the physical locations of the devices, on a
switch or across switches. With VLANs, a switch can group different interfaces into
different broadcast domains. Without VLANs, all interfaces of a switch are in the same
broadcast domain; switches connected with each other are also in the same broadcast
domain, unless there is a router in between.
Different ports of a switch can be assigned to different VLANs. A VLAN can also span
multiple switches (i.e. have members on multiple switches).