Content Overview Shared Ethernet works
extremely well under ideal conditions. When the number of
devices trying to access the network is low, the number of
collisions stays well within acceptable limits. However, when
the number of users on the network increases, the increased
number of collisions can cause intolerably bad performance.
Bridging was developed to help ease performance problems that
arose from increased collisions. Switching evolved from
bridging to become the key technology in modern Ethernet LANs.
Collisions and broadcasts are expected events in modern
networking. They are, in fact, engineered into the design of
Ethernet and higher layer technologies. However, when
collisions and broadcasts occur in numbers that are above the
optimum, network performance suffers. The concept of collision
domains and broadcast domains is concerned with the ways that
networks can be designed to limit the negative effects of
collisions and broadcasts. This module explores the effects of
collisions and broadcasts on network traffic and then describes
how bridges and routers are used to segment networks for
improved performance. Students completing this module should be
able to: - Define bridging and switching.
- Define and describe the content-addressable memory (CAM)
table.
- Define latency.
- Describe store-and
forward and cut-through switching modes.
- Explain
Spanning-Tree Protocol (STP).
- Define collisions,
broadcasts, collision domains, and broadcast domains.
- Identify the Layer 1, 2, and 3 devices used to create
collision domains and broadcast domains.
- Discuss data
flow and problems with broadcasts.
- Explain network
segmentation and list the devices used to create
segments.
Content 8.1 Ethernet
Switching 8.1.1 Layer 2 bridging As more
nodes are added to an Ethernet physical segment, contention for
the media increases. Ethernet is a shared media, which means
only one node can transmit data at a time. The addition of more
nodes increases the demands on the available bandwidth and
places additional loads on the media. By increasing the number
of nodes on a single segment, the probability of collisions
increases, resulting in more retransmissions. A solution to the
problem is to break the large segment into parts and separate
it into isolated collision domains. To accomplish this a bridge
keeps a table of MAC addresses and the associated ports. The
bridge then forwards or discards frames based on the table
entries. The following steps illustrate the operation of a
bridge: - The bridge has just been started so the
bridge table is empty. The bridge just waits for traffic on the
segment. When traffic is detected, it is processed by the
bridge.
- Host A is pinging Host B. Since the data is
transmitted on the entire collision domain segment, both the
bridge and Host B process the packet.
- The bridge adds
the source address of the frame to its bridge table. Since the
address was in the source address field and the frame was
received on port 1, the frame must be associated with port 1 in
the table.
- The destination address of the frame is
checked against the bridge table. Since the address is not in
the table, even though it is on the same collision domain, the
frame is forwarded to the other segment. The address of Host B
has not been recorded yet as only the source address of a frame
is recorded.
- Host B processes the ping request and
transmits a ping reply back to Host A. The data is transmitted
over the whole collision domain. Both Host A and the bridge
receive the frame and process it.
- The bridge adds the
source address of the frame to its bridge table. Since the
source address was not in the bridge table and was received on
port 1, the source address of the frame must be associated with
port 1in the table. The destination address of the frame is
checked against the bridge table to see if its entry is there.
Since the address is in the table, the port assignment is
checked. The address of Host A is associated with the port the
frame came in on, so the frame is not forwarded.
- Host
A is now going to ping Host C. Since the data is transmitted on
the entire collision domain segment, both the bridge and Host B
process the frame. Host B discards the frame as it was not the
intended destination.
- The bridge adds the source
address of the frame to its bridge table. Since the address is
already entered into the bridge table the entry is just
renewed.
- The destination address of the frame is
checked against the bridge table to see if its entry is there.
Since the address is not in the table, the frame is forwarded
to the other segment. The address of Host C has not been
recorded yet as only the source address of a frame is
recorded.
- Host C processes the ping request and
transmits a ping reply back to Host A. The data is transmitted
over the whole collision domain. Both Host D and the bridge
receive the frame and process it. Host D discards the frame, as
it was not the intended destination.
- The bridge adds
the source address of the frame to its bridge table. Since the
address was in the source address field and the frame was
received on port 2, the frame must be associated with port 2 in
the table.
- The destination address of the frame is
checked against the bridge table to see if its entry is
present. The address is in the table but it is associated with
port 1, so the frame is forwarded to the other segment.
- When Host D transmits data, its MAC address will also be
recorded in the bridge table. This is how the bridge controls
traffic between to collision domains.
These are the
steps that a bridge uses to forward and discard frames that are
received on any of its ports.
Web Links Bridging
Basics http://www.cisco.com/univercd/cc/td/doc/cisintwk/
ito_doc/bridging.htm
Content 8.1 Ethernet
Switching 8.1.2 Layer 2 switching
Generally, a bridge has only two ports and divides a collision
domain into two parts. All decisions made by a bridge are based
on MAC or Layer 2 addressing and do not affect the logical or
Layer 3 addressing. Thus, a bridge will divide a collision
domain but has no effect on a logical or broadcast domain. No
matter how many bridges are in a network, unless there is a
device such as a router that works on Layer 3 addressing, the
entire network will share the same logical broadcast address
space. A bridge will create more collision domains but will not
add broadcast domains. A switch is essentially a fast,
multi-port bridge, which can contain dozens of ports. Rather
than creating two collision domains, each port creates its own
collision domain. In a network of twenty nodes, twenty
collision domains exist if each node is plugged into its own
switch port. If an uplink port is included, one switch creates
twenty-one single-node collision domains. A switch dynamically
builds and maintains a Content-Addressable Memory (CAM) table,
holding all of the necessary MAC information for each port.
Web Links Bridging and Switching Basics
http://www.ctr.columbia.edu/~dimitri/teaching/ E6761/Lecture7/
switching_bridging.pdf
Content 8.1 Ethernet
Switching 8.1.3 Switch operation A switch
is simply a bridge with many ports. When only one node is
connected to a switch port, the collision domain on the shared
media contains only two nodes. The two nodes in this small
segment, or collision domain, consist of the switch port and
the host connected to it. These small physical segments are
called microsegments. Another capability emerges when only two
nodes are connected. In a network that uses twisted-pair
cabling, one pair is used to carry the transmitted signal from
one node to the other node. A separate pair is used for the
return or received signal. It is possible for signals to pass
through both pairs simultaneously. The capability of
communication in both directions at once is known as full
duplex. Most switches are capable of supporting full duplex, as