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Switch Concepts and Configuration
CSMA/CD
ÿ Shared medium
Physical shared
cable or hub.
ÿ Ethernet was
designed to work
with collisions.
ÿ Uses carrier sense multiple access collision detection.
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CSMA/CD
ÿ Device needs to transmit.
ÿ It “listens” for signals on the medium. ÿ If it
finds signals – it waits. If clear – it sends. ÿ Carry on
listening. If it receives while sending the first 64
bytes of the frame then collision. ÿ Stop
sending frame, send jam signal. ÿ Wait for
random time (backoff) ÿ Try again –
listen for signals etc
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No collisions
ÿ Fully switched network with full duplex operation = no
collisions
ÿ Higher bandwidth Ethernet does not define collisions –
must be fully switched.
ÿ Cable length limited if CSMA/CD needed.
ÿ Fiber optic – always fully switched, full duplex.
ÿ (Shared medium must use half duplex in order to detect
collisions.)
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Ethernet Communications
Unicast:
one-to-one
Broadcast:
one-to-all
Multicast:
one-to-many
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Ethernet Frame: Minimum 64 bytes, Maximum 1518 bytes
802.2 is data link layer LLC sublayer
ÿ Preamble: Synchronize to medium. ÿ
Destination Address: MAC Address of destination device. ÿ Source
Address: MAC address of source device. ÿ Length/Type:
Length of frame or protocol type code. ÿ Data: Encapsulated
data from OSI Layers 7 to 3. ÿ FCS: Frame Check Sequence.
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MAC address
ÿ 48-bits written as 12 hexadecimal digits. Format varies:
00-05-9A-3C-78-00, 00:05:9A:3C:78:00, or
0005.9A3C.7800.
ÿ MAC address can be permanently encoded into a ROM chip on a
NIC. ÿ Some
manufacturers allow the MAC address to be
modified locally.
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Collision Domains
ÿ Shared medium – same collision domain.
ÿ Collisions reduce throughput
ÿ The more devices – the more collisions
ÿ Hub – maybe 60% of available bandwidth
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How many collision domains
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11
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Broadcast domains
ÿ Layer 2 switches flood broadcasts. ÿ
Devices linked by switches are in the same broadcast
domain.
ÿ A layer 3 device (router) splits up broadcast domains, does
not forward broadcasts
ÿ Destination MAC address for broadcast is all 1s,
that is FF:FF:FF:FF:FF:FF
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How many broadcast domains
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2
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Switch Port Settings:
AUTO:
Auto-negotiation of duplex mode. The two ports communicate
to determine the best mode.
FULL:
Full-duplex mode.
HALF:
Half-duplex mode.
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Configure Duplex and Speed:
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ÿ If auto-negotiation fails because the attached device does not support
it, the Catalyst switch defaults the switch port to half-duplex
mode.
ÿ Half-duplex on one end and full-duplex on the other causes late
collision errors at the half-duplex end. ÿ To avoid this,
manually set the duplex parameters of the switch to match the attached
device.
ÿ A cross-over or a straight-through cable was required depending
on the type of device that was being connected to the
switch.
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Ethernet Switches and Bridges
ÿ Address learning ÿ
Forward/filter decision
ÿ Loop avoidance
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Switch MAC Address Table
ÿ A switch builds a MAC address table by learning the MAC addresses of
each device connected to each of its ports.
ÿ Once the MAC address has been added to the table, the
switch uses the table entry to forward traffic to that node.
ÿ If a destination address is not in the table, the switch
forwards the frame out all ports except the receiving port.
ÿ When the destination responds, the MAC address is added to the table.
ÿ If the port is connected to another switch or a hub, multiple
MAC addresses will be recorded in the table.
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MAC Address Table
Initial MAC address table is empty.
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Learning Addresses
ÿ Station A sends a frame to station C.
ÿ Switch caches the MAC address of station A to port E0 by learning
the source address of data frames.
ÿ The frame from station A to station C is flooded out to all ports
except port E0 (unknown unicasts are flooded).
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ÿ Station D sends a frame to station C.
ÿ Switch caches the MAC address of station D to port E3 by learning
the source address of data frames.
ÿ The frame from station D to station C is flooded out to all ports
except port E3 (unknown unicasts are flooded).
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Filtering Frames
ÿ Station A sends a frame to station C.
ÿ Destination is known; The frame is not flooded.
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Broadcast and Multicast Frames
ÿ Station D sends a broadcast or multicast frame.
ÿ Broadcast and multicast frames are flooded to all ports other than the
oriinatin ort.
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Switch Forwarding Methods
Cut-Through
Store and Forward
•Switch checks destination
Complete frame is received
address and immediately
and checked before forwarding.
begins forwarding frame.
Fragment-Free
•Switch checks the first 64 bytes,
then immediately begins
forwarding frame.
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Symmetric and Asymmetric Switching
ÿ Symmetric – all ports operate at
same bandwidth
ÿ Asymmetric – different bandwidths
used, eg server or uplink has greater bandwidth
ÿ Requires store and forward operation
with buffering.
ÿ Most switches now are asymmetric
to allow flexibility.
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Port Based Buffering
Each incoming port has its own queue.
Frames stay in buffer until outgoing port is free.
Each incoming port has a fixed and limited amount of memory.
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Shared Memory Buffering
ÿ All incoming frames go in a common buffer. ÿ Switch
maps frame to destination port and forwards it when port is free. ÿ Flexible
use of
memory allows larger frames.
ÿ Important for asymmetric switching where some ports work faster than
others.
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Layer 2 and Layer 3 Switching
ÿ Traditional Ethernet switches
work at layer 2.
ÿ They use MAC addresses to make
forwarding decisions.
ÿ They do not look at layer 3
information.
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ÿ Layer 3 switches can carry out the
same functions as layer 2 switches.
ÿ They can also use layer 3 IP
addresses to route between
networks.
ÿ The can control the spread of
broadcasts.
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Storage and start-up
ÿ ROM, Flash, NVRAM, RAM generally similar to router.
ÿ Boot loader, POST, load IOS from flash, load configuration
file.
ÿ Similar idea to router. Some difference in detail.
ÿ Boot loader lets you re-install IOS or recover from
password loss.
