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ETHERNETChapter 5
Intro to Routing & Switching
OBJECTIVES Upon completion of this chapter, you should
be able to: Describe the operation of the Ethernet sublayers. Identify the major fields of the Ethernet frame. Describe the purpose and characteristics of the
Ethernet MAC address. Describe the purpose of ARP. Explain how ARP requests impact network and
host performance. Explain basic switching concepts. Compare fixed configuration and modular
switches. Configure a Layer 3 switch.
ETHERNET OPERATION
5.1.1
ETHERNET OPERATION Most widely used LAN technology
What 2 layers does it operate at?Data link & physical
What are the 2 sublayers of the data link?LLC & MAC
LLC IEEE 802.2 Helps communicate with the upper
network layer Adds control info implemented it in software
MAC SUBLAYER Encapsulates the data
Adds MAC addressesAdds error detection to frame
Media Access ControlPlacing the frames on the media
Ethernet is a logical bus; physical starSignal passes to allCan send wheneverCan be collisions
CSMA Ethernet is contention-based/non-deterministic
Data contends or shares for a spot on the media Doesn’t know when it’ll get access to it
Listens for signal on media No signal = transmit data Transmit at same time= collision
Devices do not keep track of whose turn it is
More collisions= less throughput SOLUTION: CSMA/CD & CSMA/CA
CSMA/CD ON ETHERNET NETWORK Listens for silence
Silence= transmit data If devices transmit at same time,
collisionBackoff random time, listen for silence,
retransmit
Collision detection not a problem much anymoreUsing switches & full-duplex, this is not a
problem anymore
CURRENT SWITCHED NETWORK
CSMA/CA Wireless
Listen for silence Sends Ready to Send message to AP Gets a Clear to Send message from AP Sends data
MAC ADDRESSES Used to identify frame Each host has a unique address
Burned into NIC hardware 48-bits / 12 hex digits / 6 bytes 1st 24 bits OUI; 2nd 24 bits serial #
FORWARDING FRAMES
ACTIVITY
REVIEW How many bits is a MAC address?
48 bits What is the OUI in this MAC?
A2:07:CC:F6:AD:32A2:07:CC
What does a wireless network use to avoid collisions?CSMA/CA
What sublayer places the frames on the media?MAC
End of Day One
REVIEW The OUI is how many bytes?
3 Ethernet shares access to the media. It
contends for the media and does not take turns transmitting. Not taking turns means the network is…Nondeterministic
Is the MAC address found in hardware or software?Software
ABOUT THE ETHERNET FRAMES
5.1.2
FRAME SIZE Min. frame size is 64 bytes; max is 1522 bytes
Data is 46-1500 bytes
64 bytes is considered a collision fragment & will be droppedPREAMBLE- used timing/synchronization of the frame between send & receiver. Basically saying., “Here comes a frame!”
ACTIVITY5.1.2.4
REVIEW What important addresses are
encapsulated into a frame?Source & destination MAC
What does the preamble used for?Timing/synchronization
What is the minimum frame size?64 bytes
What happens if a frame is less than 64 bytes?Considered a fragment & dropped
REVIEW How many bits in a MAC address?
48 bits How many hex digits?
12 The first 6 hex digits are what?
The OUI The OUI would then be the 1st ____ bytes.
3 What sublayer is used to communicate
with the upper layers?LLC
ETHERNET MAC
5.1.3
BINARY/DECIMAL CONVERSION IP Address: 32 bits, 4 octets
8 bits in each octet11111111.10101010.11001100.00100101
Written in decimal192.101.28.36
Value in each octet from 0-255That’s a total of 256 numbers.
BINARY/DECIMAL CONVERSIONS Add up the values of the binary 1’s
156
11100101229
128 64 32 16 8 4 2 1
1 0 0 1 1 1 0 0
MAC ADDRESSING Ipconfig /all Hexadecimal (Base 16) 0-9, A-F (10-15)
16 total #’s
BINARY/DEC/HEX CONVERSIONS
CONVERSION PRACTICE Handouts
HOW MAC ADDRESSES ARE SHOWN Begin with a 0x
0xA4
UNICAST MAC ADDRESS One to one communication
BROADCAST MAC ADDRESS One to all in a network Dest. MAC address will be all F’s DHCP & ARP use broadcasts
MULTICAST MAC ADDRESS One to a group in a network Remote gaming or video conference Dest. IP will be 224.0.0.0 -
239.255.255.255 Dest. MAC will begin with 01-00-5E
LAB 5.1.3.6
REVIEW What kind of message gets sent from
one PC to a group within a network?Multicast
Identify each as unicast, multicast, or broadcast:
MAC & IP
5.1.4
MAC & IP MAC address
Burned into NIC (DOES NOT CHANGE) Similar to the name of a person Physical address
IP address Similar to the address of a person Based on where the host is actually located Logical address
Both the physical MAC & logical IP addresses are required for a computer to communicate just like both the name and address of a person are required to send a letter
MAC & IP- TO OTHER NETWORKS Destination IP NEVER changes! Source & Destination MAC changes at
each router interface
LAB 5.1.4.3 Wireshark
REVIEW How do you change your MAC address?
Get a new NIC T or F. The destination IP address
changes during transmission.False
What happens to the source & destination MAC addresses as you go from router to router across the Internet?They change (router port substituted (mac))
REVIEW What is IEEE 802.2?
LLC sublayer What is IEEE 802.3?
Ethernet/MAC sublayer What is a layer 2 address?
MAC address What is a layer 3 address?
IP address
ARP
5.2.1
HOW ARP WORKS ARP table of IP/MACs
Added from communicationAdded from ARP requests
You have the dest. IP, not the MAC ARP request is all F’s where?
Destination MAC How is an ARP request sent?
Broadcast Who replies to the ARP request?
Only one with matching dest. IP
HOW ARP WORKS- LOCAL
HOW PROXY ARP WORKS- REMOTE
VIEWING THE ARP TABLE Windows-
Arp –a
Cisco routerRouter# show ip arp
REVIEW If you want to access a remote server,
and you don’t have the destination MAC, what gets substituted for it?The default gateway’s MAC address
A router has ports with MAC addresses. How do you view the router’s ARP table?Router#sh ip arp
What does ARP find? What do you know?Finds the dest. MAC; you know the dest. IP
ARP ISSUES
5.2.2
PROBLEMS WITH ARP Broadcasts
It’s a broadcast. If many devices started at same time, there’d be a flood of ARP requests which would cause a reduction in performance for a short period of time.
SecurityARP poisoning (or spoofing)Attacker forges MAC address to have frames
delivered to different computer
Solution: Use a switch
USING A SWITCH Segments network into smaller collision
domains Replies go to one device only Implement security too
SWITCHING
5.3.1
HOW A SWITCH WORKS
BUILDING A SWITCH TABLE
SWITCH DUPLEX SETTINGS Full duplex, Half duplex, Auto
Must match setting of deviceHalf duplex uses CSMA/CD to avoid
collisions
WHAT CABLE SHOULD IT BE?
MDIX AUTOWhen the auto-MDIX feature is enabled, the switch detects the required cable type for copper Ethernet connections and configures the interfaces accordingly
STORE-AND-FORWARD SWITCHING
when the switch receives the frame, it stores the data in buffers until the complete frame has been received.
During the storage process, the switch analyzes the frame for information about its destination.
In this process, the switch also performs an error check using the Cyclic Redundancy Check (CRC) trailer portion of the Ethernet frame.
CUT-THROUGH SWITCHING The switch buffers just enough of the
frame to read the destination MAC address so that it can determine to which port to forward the
The destination MAC address is located in the first 6 bytes of the frame following the preamble. The switch looks up the destination MAC address in its switching table, determines the outgoing interface port, and forwards
FORWARDING THE MESSAGE Cisco uses cut-through switching
As soon as destination MAC is read, it forwards the frame
Fast-forward Lowest latency; in and out
Fragment-free Store first 64 bytes before forwarding Most errors & collision happen there (runts) If it makes it through, should be error-free
ACTIVITY
ACTIVITY Complete 5.3.1.7
WHAT WILL HAPPEN?
REVIEW 2 switches connect to each other. One port
is 100Mbps and the other is 1000Mbps. What speed will that connection operate at? 100Mbps
What feature will allow you to use a straight-through cable to connect two switches together? Auto-MDIX
Your switch port is connected to a hub with 3 computers on it. How many MAC addresses will be in the table for that port? 3
ACTIVITY & LAB Switch it!
5.3.1.9Do it at least 4 timesDifferent scenario each time
Lab together
Draw network from MAC address table
TYPES OF CISCO SWITCHES
5.3.2
SWITCH FEATURES Fixed Configuration Switch
As is; no add-ons or changing A 24 port switch will always be 24 ports
Modular Add-in cards/ports
Stackable Connected by special cable to act as one switch Fixed may be stackable
PoE (Power over Ethernet) Delivers power to a device, like an IP phone We use this in school
More $$, faster forwarding rate
REVIEW Which type of switch can you add a card to
add more Ethernet ports or add fiber ports? Modular
You bought a 24 port switch and cannot add more ports to it. What kind of switch did you buy? Fixed
You have the switch above and need more ports. You then buy more 24 port switches and connect them with a special cable. What kind of switch do you now have? Fixed & stackable
LAYER 3 SWITCHING
5.3.3
LAYER 3 SWITCH Looks like a switch
Combined with router functions Adds in router functions
Knows which IP addresses are out each port too
Fast
ROUTED PORT Make an Ethernet port a routed port
To connect to the ISPConfigure a port
What have we configured with an IP on a switch? IP for remote managementThis is similar
CONFIGURE ROUTED PORT
F0/6
LAB 5.3.3.5 Configuring a Layer 3 Switch
REVIEW What address(es) does a layer 2 switch read?
MAC addresses What address(es) does a layer 3 switch read?
IP & MAC You want to connect your Layer 3 switch to
your ISP instead of using a regular router. What must you configure one of the ports as? Routed port
Which command enables the routing function on a switch port? No switchport
REVIEW & STUDY Complete the study guide handout
Take the quiz on netacad.com
Jeopardy review
SUMMARYIn this chapter, you learned: Ethernet is the most widely used LAN technology
used today. Ethernet standards define both the Layer 2
protocols and the Layer 1 technologies. The Ethernet frame structure adds headers and
trailers around the Layer 3 PDU to encapsulate the message being sent.
As an implementation of the IEEE 802.2/3 standards, the Ethernet frame provides MAC addressing and error checking.
Using switches in the local network has reduced the probability of frame collisions in half-duplex links.
SUMMARY The Layer 2 addressing provided by Ethernet
supports unicast, multicast, and broadcast communications.
Ethernet uses the ARP to determine the MAC addresses of destinations and map them against known IP addresses.
Each node on a network has both a MAC address and an IP address.
The ARP protocol resolves IPv4 addresses to MAC addresses and maintains a table of mappings.
A Layer 2 switch builds a MAC address table that it uses to make forwarding decisions.
SUMMARY Layer 3 switches are also capable of
performing Layer 3 routing functions, reducing the need for dedicated routers on a LAN.
Layer 3 switches have specialized switching hardware so they can typically route data as quickly as they can switch.
ETHERNETChapter 5
Intro to Routing & Switching