Chapter 5 – Sections & Objectives

• 5.1 Ethernet Protocol
  • Explain how the Ethernet sublayers are related to the frame fields.
  • Describe the Ethernet MAC address.
• 5.2 LAN Switches
  • Explain how a switch operates.
  • Explain how a switch builds its MAC address table and forwards frames.
  • Describe switch forwarding methods.
  • Describe the types of port settings available for Layer 2 switches.
• 5.3 Address Resolution Protocol
  • Compare the roles of the MAC address and the IP address.
  • Describe the purpose of ARP.
  • Explain how ARP requests impact network and host performance.

5.1 Ethernet Protocol

Ethernet Frame

• Ethernet Encapsulation
  • Ethernet operates in the data link layer and the physical layer.
  • Ethernet supports data bandwidths from 10Mbps through 100Gbps.
  • Ethernet standards define both the Layer 2 protocols and the Layer 1 technologies.
• MAC Sublayer
  • MAC constitutes the lower sublayer of the data link layer.
  • Responsible for Data encapsulation and Media access control.
• Ethernet Evolution
  • Ethernet has been evolving since its creation in 1973.
  • The Ethernet frame structure adds headers and trailers around the Layer 3 PDU to encapsulate the message being sent.
• Ethernet Frame Fields
  • The minimum Ethernet frame size is 64 bytes and the maximum is 1518 bytes.
  • Frame smaller than the minimum or greater than the maximum are dropped.
  • Dropped frames are likely to be the result of collisions or other unwanted signals and are therefore considered invalid.

Ethernet MAC Addresses

• • MAC Addresses and Hexadecimal

– MAC address is 48-bit long and expressed as 12 hexadecimal digits.

• MAC Addresses: Ethernet Identity
  – IEEE requires a vendor to follow two simple rules:
  1. Must use that vendor’s assigned OUI as the first three bytes.
  2. All MAC addresses with the same OUI must be assigned a unique value in the last three bytes.
• Frame Processing
  • The NIC compares the destination MAC address in the frame with the device’s physical MAC address stored in RAM.
  • If there is a match, the framed is passed up the OSI layers.
  • If there is no match, the device discards the frame.
• MAC Address Representations
  • MAC addresses can be represented with colons, dashes or dots and are case-insensitive.
  • 00-60-2F-3A-07-BC, 00:60:2F:3A:07:BC, 0060.2F3A.07BC and 00-60-2f-3a-07-bc are all valid representations of the same MAC address.
• Unicast MAC Address
  • Unique address used when a frame is sent from a single transmitting device to a single destination device.
  • The source MAC address must always be a unicast.
• Broadcast MAC Address
  • Used to address all nodes in the segment.
  • The destination MAC address is the address of FF-FF-FF-FF-FF-FF in hexadecimal (48 ones in binary).
• Multicast MAC Address
  • Used to address a group of nodes in the segment.
  • The multicast MAC address is a special value that begins with 01-00-5E in hexadecimal.
  • The remaining portion of the multicast MAC address is created by converting the lower 23 bits of the IP multicast group address into 6 hexadecimal characters.

5.2 LAN Switches

The MAC Address Table

• Switch Fundamentals
  • An Ethernet Switch is a Layer 2 device.
  • It uses MAC addresses to make forwarding decisions.
  • The MAC address table is sometimes referred to as a content addressable memory (CAM) table.
• Learning MAC Addresses
  • Switches dynamically build the CAM by monitoring source MACs.
  • Every frame that enters a switch is checked for new addresses.
  • The frame is forwarded based on the CAM.
• Filtering Frames
  • Since the switch knows where to find a specific MAC address, it can filter the frames to that port only.
  • Filtering is not done is the destination MAC is not present in the CAM.

Switch Forwarding Methods

• Frame Forwarding Methods on Cisco Switches
  • Store-And-Forward
  • Cut-Through
• Cut-Through Switching
  • Fast-forward switching
  • Lowest level of latency immediately forwards a packet after reading the destination address.
  • Typical cut-through method of switching.
  • Fragment-free switching
  • Switch stores the first 64 bytes of the frame before forwarding.
  • Most network errors and collisions occur during the first 64 bytes.
• Memory Buffering on Switches
  • Port-based memory
  • Share memory

Switch Port Settings

• Duplex and Speed Settings
  • Full-duplex – Both ends of the connection can send and receive simultaneously.
  • Half-duplex – Only one end of the connection can send at a time.
  • A common cause of performance issues on Ethernet links is when one port on the link operates at half-duplex and the other on full-duplex.
• Auto-MDX
  • Detects the type of connection required and configures the interface accordingly.
  • Helps reducing configuration errors.

5.3 Address Resolution Protocol

MAC and IP

• The combination of MAC and IP facilitate the End-to-End communication.
• Layer 2 addresses are used to move the frame within the local network
• Layer 3 addresses are used to move the packets through remote networks.
• Destination on Same Network
  – Physical address (MAC address) is used for Ethernet NIC to Ethernet NIC communications on the same network.
• Destination on Remote Network
  – Logical address (IP address) is used to send the packet from the original source to the final destination.

ARP

• Introduction to ARP
  • ARP allows the source to request the MAC address of the destination.
  • The request is based upon the layer 3 address of the destination (known by the source).
• ARP Functions
  • Resolving IPv4 addresses to MAC addresses
  • Maintaining a table of mappings
  • ARP uses ARP Request and ARP Reply to perform its functions.
• Removing Entries from an ARP Table
  • Entries are removed from the device’s ARP table when its cache timer expires.
  • Cache timers are OS dependent.
  • ARP entries can be manually removed via commands.
• ARP Tables
  • On IOS: show ip arp
  • On Windows PCs: arp -a

ARP Issues

• ARP Broadcasts
  – ARP requests can flood the local segment.
• ARP Spoofing
  – Attackers can respond to requests and pretend to be providers of services. Example: default gateway

5.4 Chapter Summary

Chapter Summary Summary

• Explain the operation of Ethernet.
• Explain how a switch operates.
• Explain how the address resolution protocol enables communication on a network.

Section 5.1 New Terms and Commands

Section 5.2 New Terms and Commands

Section 5.3 New Terms and Commands

• ARP Table
• ARP Cache
• ARP Request
• ARP Reply
• show ip arp, arp –a
• ARP spoofing

Download Slide PowerPoint (pptx):