Transcript Module 8 Review

Sem1 - Module 8
Ethernet Switching
Shared media environments
• Shared media environment:
– Occurs when multiple hosts have access to the same medium.
– For example, if several PCs are attached to the same physical
wire, optical fiber, or share the same airspace, they all share
the same media environment.
• Extended shared media environment:
– Is a special type of shared media environment in which
networking devices can extend the environment so that it can
accommodate multiple access or longer cable distances.
• Point-to-point network environment:
– Is widely used in dialup network connections and is the most
familiar to the home user.
– It is a shared networking environment in which one device is
connected to only one other device, such as connecting a
computer to an Internet service provider by modem and a
phone line.
Collision domains
• Collision domains are the connected physical network
segments where collisions can occur.
• Collisions cause the network to be inefficient.
• Every time a collision happens on a network, all
transmission stops for a period of time.
• Bridges/Switches (Layer 2) and Routers (Layer3)
devices breaking up, or increase the number of collision
domains - also known as segmentation.
• Layer 2 devices filter using MAC addresses; Layer 3
devices filter using IP addresses.
• Layer 1 devices, such as repeaters and hubs, serve the
primary function of extending the Ethernet cable
segments.
Ethernet Bridging
• 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.
• A router use the destination IP address to make a forwarding
decisions.
• 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.
Ethernet Switching
• 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 and Broadcast
storms:
– This causes slower data transmissions
• A solution to the problem is to break the large
segment into parts and separate it into isolated
collision domains.
Ethernet Switching
• 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.
• A switch dynamically builds and maintains a Content-Addressable
Memory (CAM) table, holding all of the necessary MAC information
for each port.
• Most switches are capable of supporting full duplex, as are most
network interface cards (NICs). In full duplex mode, there is no
contention for the media.
• Thus, a collision domain no longer exists. Theoretically, the
bandwidth is doubled when using full duplex.
Ethernet Switching
• 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 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.
Ethernet Switching
• Host A is now going to ping Host B, Host C & Host D.
• Host B, C & D processes the ping request and transmits a ping
reply back to Host A.
• When these hosts transmit data, their MAC addresses will also be
recorded in the bridge table.
• This is how the bridge controls traffic between to collision domains.
Latency
• Latency is the delay between the time a frame first
starts to leave the source device and the time the first
part of the frame reaches its destination.
• A wide variety of conditions can cause delays as a
frame travels from source to destination:
– Media delays caused by the finite speed that signals can travel
through the physical media
– Circuit delays caused by the electronics that process the signal
along the path.
– Software delays caused by the decisions that software must
make to implement switching and protocols.
– Delays caused by the content of the frame and where in the
frame switching decisions can be made.
– For example, a device cannot route a frame to a destination
until the destination MAC address has been read.
Switch Latency
• A switch adds 21 microseconds of latency.
• This can be reduced by using a different switching
method
• As opposed to store-and-forward, the switch can
use cut-through switching which switches the
packet as soon as the destination MAC is read.
Two Switching Methods
• Store-and-Forward
• The switch receives the entire
frame, calculating the CRC at
the end, before sending it to
the destination
• Cut-through (no error checking)
• Fast forward switching-only checks the destination
MAC before immediately
forwarding the frame
• Fragment Free--reads the
first 64 bytes to reduce errors
before forwarding the frame
Spanning-Tree Protocol
• When multiple switches are arranged in a simple tree,
switching loops are unlikely to occur.
• However, switched networks are often designed with
redundant paths to provide for reliability and fault tolerance.
• While redundant paths are desirable, they can have
undesirable side effects.
• Switching loops are one such side effect.
• To counteract the possibility of loops, switches are provided
with a standards-based protocol called the Spanning-Tree
Protocol (STP).
• Each switch in a LAN using STP sends special messages
called Bridge Protocol Data Units (BPDUs) out all its ports to
let other switches know of its existence and to elect a root
bridge for the network.
• The switches then use the Spanning-Tree Algorithm (STA) to
resolve and shut down the redundant paths.
Spanning-Tree Protocol
• Each port on a switch using Spanning-Tree Protocol exists in one of
the following five states:
–
–
–
–
–
Blocking (receives BPDUs only)
Listening (Building “active” topology)
Learning (Building Bridging/Switching table)
Forwarding (Sending and receiving user data)
Disabled (administratively down)
• A port moves through these five states as follows:
–
–
–
–
–
From initialization to blocking
From blocking to listening or to disabled
From listening to learning or to disabled
From learning to forwarding or to disabled
From forwarding to disabled
Chapter #8
Test!