Transcript CCNA 1 Chapter 6 Ethernet Technologies and Ethernet Switching

Introduction to Ethernet
• In 1985, the Institute of Electrical and Electronics Engineers
(IEEE) published standards for LANs. These standards start
with the number 802. The standard for Ethernet is 802.3.
• Essentially, Ethernet and IEEE 802.3 are the same standards.
• In 1995, IEEE announced a standard for a 100-Mbps Ethernet.
This was followed by standards for gigabit per second
(Gbps, 1 billion bits per second) Ethernet in 1998 and 1999.
• In 2002 IEEE announced the standards for 10 Gigabit
Ethernet
• All the standards are essentially compatible with the original
Ethernet standard. An Ethernet frame could leave an older
coax 10-Mbps NIC in a PC, be placed onto a 10-Gbps
Ethernet fiber link, and end up at a 100-Mbps NIC.
Ethernet and the OSI Model
Not Included
Ethernet and the OSI Model
MAC Rules and Collision Detection/Backoff
Ethernet
10BaseT Half-Duplex Operation
and Collision Domains
• Stations separated by hubs are within the same
collision domain
• Let’s look at packet tracer to see how hubs work
Naming
• MAC addresses are sometimes referred to as burned-in
addresses (BIA) because they are burned into read-only
memory (ROM) and are copied into random-access
memory (RAM) when the NIC initializes.
Naming
Ethernet Encapsulation
RFC 894
Ethernet Encapsulation
Destination
Address
Source
Address
6 Bytes
6 Bytes
Type
Data
CRC
2
46-1500 Bytes
4 Bytes
Type
IP Datagram
0800
2B
Type
0806
2B
46-1500 Bytes
ARP Request/Reply
PAD
28 Bytes
18 Bytes
Ethernet Encapsulation
• 48-bit (6bytes) source and destination addresses. These are
what we call hardware addresses.
• The Ethernet type field identifies the type of data that
follows.
• The data field is the actual payload and must be at least 46
bytes
• If needed (frames with less than 46 bytes of payload), Pad
bytes are inserted to assure that the frame is long enough.
• The CRC field is a cyclic Redundancy check (a checksum)
that detects errors in the frame. (This is also called FCS or
Frame Check Sequence)
Address Resolution Protocol: ARP
• Is needed in order to for the source to Learn the MAC address for
the destination
• The IP address of the destination is used to learn the MAC address
of the destination
• Two-step process: ARP Request and ARP Reply
• ARP Requests are sent to the broadcast address of
FFFF.FFFF.FFFF and heard by all nodes in the same broadcast
doamin (coming later)
• Only the node with the matching destination IP address replies
• AN ARP table is saved in RAM and can be viewed by using the
command arp –a from DOS
Ethernet Encapsulation
Destination
Address
Source
Address
6 Bytes
6 Bytes
Type
Data
CRC
2
46-1500 Bytes
4 Bytes
Type
IP Datagram
0800
2B
Type
0806
46-1500 Bytes
ARP Request/Reply
PAD
28 Bytes
18 Bytes
2B
© 2003, Cisco Systems, Inc. All rights reserved.
13
Ethernet and MTU
• As you can see there is a limit on the size
of an Ethernet frame.
• This limits the number of bytes of data to
1500 bytes.
• This characteristics of the data link layer
is called MTU, or Maximum Transmission
Unite
Typical MTUs
Network
MTU (bytes)
16 Mbits /sec token ring
(IBM)
17914
FDDI
4352
Ethernet
1500
Frame Relay
1500
Ethernet and MTU
• When two hosts on the same network are
communicating with each other, it is the MTU of
the network that is important.
• But, when two hosts are communicating across
multiple networks, each link can have a different
MTU. The important numbers are the MTUs of
the two networks to which the two hosts
connect, but rather the smallest MTU of any data
link that packets traverse between the two hosts.
Ethereal Exercise
• Do the following using Ethereal:
Capture packets
Filter IP packets. What is the value of the
Ethernet type field and what does it mean?
Clear the previous filter
Filter ARP packets. What is the value of the
Ethernet type field and what does it mean?
Ethernet and the OSI Model
Not Included
© 2003, Cisco Systems, Inc. All rights reserved.
18
IEEE 802.3 Frame