Ethernet operates at datalink and physical layer
Ethernet most widely used LAN technology
defined by IEEE 802.2 and 802.3
speeds between 10 and 100,000 Mb/s
802.2 - LLC
802.3 - MAC and Physical layers
LLC sublayer
handles communication between upper and lower layers
between networking software and device hardware
implemented in software
considered driver software
MAC sublayer
lower sublayer of data link layer
2 responsiblities
data encapsulation
media access control
data encapsulation
frame assembly before trasnmission
frame dissassembly upon reception
adds header and trailer to network layer pdu
data encapsultion has 3 primary functions
frame delimiting
used to identify groups of bits that make up a frame
provides syncronization between transmitting and receiving
addressing
contains physical address
Error detection
contains a CRC value used to determine if the contents have changed
media access control
responsible for placement of frames on the media (wire)
removal of frames from the media
underlying logical topology is a multiaccess bus
all nodes in a network segment share the medium
uses CSMA
understanding CSMA
1) detect if there is a signal on the media
a) if busy, wait
2) if no signal, transmit
if there is a collision, all data is bad and must be resent
contention-based methods have less overhead than controlled access systems
do not scale well with heavy media use
CSMA/CD
widespread use of switched networks makes CSMA/CD largely unneeded
CSMA/CA in wireless still needed
Every device receives every frame
To prevent mass processing of frames not intended for that device, MAC was created.
MAC used to identify which device should process a frame
MAC address structure
1st 24 bits/ 6 hex digits - Organizationally Unique Identifer
Last 24 bits/ 6 hex digits - Vendor assigned
1) Source computer sends
2) each NIC in the network views the information
a) if the destination MAC matches, then the frame is processed further
b) if not, the frame is dropped
Ethernet Frame Attributes
created in 1973
2 primary versions
-802.3 Ethernet Standard
-DIX Ethernet, or Ethernet II Used in TCP/IP networks
framing style differences are minimal
both define minimum frame size as 64 bytes
max 1518
preamble/Start Frame Delimiter not included when describing frame size
any frame less that 64 bytes discarded as a "runt"
VLAN technology increased maximum frame size to 1522
frame too big or too small, frame is dropped
Ethernet Frame
-preamble and SFD - synchronization between sender and receiver
-Destination MAC -
-Source MAC
-Length Field - exact length of the frame's data field; can indicate which higher-layer protocol is used;
if size greater than 1536, frame is ethertype protocol indicated.
-if 1500 or less, then use of 802.3 frame format
-data field - contains encapsulated layer 3 pdu
-frame check sequence - used to detect errors
In Ethernet, different MAC addresses used for layer 2 unicast, broadcast, and multicast communications
Unicast -destination MAC of receiver
broacast - destination MAC of FFFF.FFFF.FFFF
Multicast - destination MAC is 01-00-5E
MAC and IP
2 primary addresses assigned to a host
Physical Address - MAC
Logical Address - IP
Source device sends based on IP address
DNS used to identify remote network
ARP protocol has 2 basic functions
-resolving IPv4 addresses to MAC addresses
-maintaining a table of mappings
Frame must have destination address to be placed on the media (wire)
refers to ARP table or ARP cache. Stored in RAM on all devices
Each entry binds a MAC with an IP
table maintained dynamically
-adds information by
-monitoring network
-ARP request - L2 broadcast to all devices on the LAN
-entries are time stamped; if the system doesn't get a message by the time stamp expires, the entry is removed
-also capable of static entries
ARP to remote network
-when host creates a packet for a destination, it compares destination IP address to its own IP address to determine if they are both on the same network.
-if not the same network, then ARP to router interface serving as the gateway
ARP issues
overhead on the media
-could initially flood the network
security - ARP spoofing or ARP poisoning
-broadcast adn security issues can be mitigated with modern switches
-switches segment LAN into independent collision domains
-switches send to only sender and receiver, reducing traffic flows
Switching
devices usually connected by a layer 2 switch
switch builds MAC table to make forwarding decisions
if destination MAC is not known, switch sends the frame out all port except receiving port
when destination responds, switch adds MAC to its MAC table
-capable of multiple MACs per port (switch connected to a switch)
switch MAC table sometimes referred to as CAM (content addressable memory)
-transparent to network protocols and user applications
-can operate in different modes that affect the end user
-duplex settings
-half duplex - one talks ata time
-full duplex - send and receive at the same time; CSMA/CD shut off in this case
-Cisco Catalyst supports 3 duplex settings
-half
-full
-auto
-proper cabling also required (or used to be)
-MDIX auto function
autmatic medium dependent interface crossover (auto-mdix)
switches use following forwarding methods
-store and forward
get the entire frame before forwarding
use CRC to determine if it needs to dump frame
analyzes destination for where to forward
required for QoS analysis
-cut-through
holds frame until it gets destination address, then forwards
no error checking
faster than store and forward, but can send broken frames
2 variations to cut-through
-fast-forwards
offers lowest level of latency
typical of cut-through switching
-fragment free
reads first 64 bytes before forwarding (most errors occur in first 64 bytes)
switches use buffering techniques
port-based memory buffering
frames are stored in queues that are linked to specific incoming and outgoing ports
delay occurs if there is nothing transmitting on other ports and buffer is full
shared memory buffer
frame stores in common buffer used for all ports
useful for asymmetric switching
Fixed or Modular switches
PoE allows power to be sent to devices
forwarding rate defines how many frames the switch can process in a second
stackable vs non-stackable
modular configurations - add more cards
Small form-factor pluggable (ports that can be purchased and easily changed out on each switch)
Layer 3 switch
capable of performing layer 3 routing as opposed to the standard switch that just handles layer 2
support Cisco Express Forwarding (CEF)
complex, but based on
Forwarding Information Base
conceptionally similiar to routing table
Adjacency table
maintains layer 2 next-hop addresses for all FIB entries
separation provides benefits
-adjacency table can be built separately from the FIB table
-MAC header rewrite used to forward a packet is not stored in cache entries
layer 3 switch variants
Switch virtual interface (SVI)
logical interface on a switch associated with a VLAN
Routed port
physical port on an L3 switch configured to act as a router port
allow L3 switch to act as a router
not associated with a particular vlan
L3 interface only and does not support L2 protocol
no switchport interface configuration command
Layer 3 EtherChannel
logical itnerface on a Cisco device associated with a bundle of routed ports
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