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.
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
IEEE 802.2 Helps communicate with network layer Adds control info Performs it in software
Encapsulates the data Adds MAC addresses Adds error detection to frame Media Access Control Placing the frames on the media Ethernet is a logical bus; physical star Signal passes to all Can send whenever Can be collisions
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
Listens for silence Silence= transmit data If devices transmit at same time, collision Backoff random time, listen for silence, retransmit Collision detection not a problem much anymore Using switches & full-duplex, this is not a problem anymore
How many bits is a MAC address? 48 bits What is the OUI in this MAC? A2:07:CC:F6:AD:32 A2:07:CC What does a wireless network use to avoid collisions? CSMA/CA What sublayer places the frames on the media? MAC
The OUI is how many bytes? 33 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 End of Day One
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
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 1 st ____ bytes. 33 What sublayer is used to communicate with the upper layers? LLC
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
Destination IP NEVER changes! Source & Destination MAC changes at each router interface
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)
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 table of IP/MACs Added from communication Added 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
Windows- Arp –a Cisco router Router# show ip arp
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
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. Security ARP poisoning (or spoofing) Attacker forges MAC address to have frames delivered to different computer Solution: Use a switch
Segments network into smaller collision domains Replies go to one device only Implement security too
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
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? 33
Switch it! 188.8.131.52 Do it at least 4 times Different scenario each time Lab together Draw network from MAC address table
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
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
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
Complete the study guide handout Take the quiz on netacad.com Jeopardy review
In 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.
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.
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.