TCP/IP Layer 4 Protocols

TCP and UDP TCP provides error recovery, but to do so, it consumes more bandwidth and uses more processing cycles. UDP does not perform error recovery, but it takes less bandwidth and uses fewer processing cycles

TCP TCP is defined in RFC 793 TCP Header Fields

TCP Acknowledgment Without Errors

TCP Acknowledgment with Errors

TCPFlow Control Using Windowing TCP implements flow control by taking advantage of the Sequence and Acknowledgment fields in the TCP header, The size of the window changes over time, so it is sometimes called a dynamic window. Additionally, because the actual sequence and acknowledgment numbers grow over time, the window is sometimes called a sliding window

TCP Windowing

Connection Establishment TCP connection establishment occurs before any of the other TCP features can begin their work. This three-way connection establishment flow must end before data transfer can begin.

UDP

Network Security

Threats Virus Spam Spyware Phishing URLs

Anti-x Anti-virus Anti-spyware Anti-spam Anti-phishing URL filtering filtering

LAN Switching

Bridge Creates Two Collision Domains and Two Shared Ethernets

Switch Creates Four Collision Domains and Four Ethernet Segments

Switching Logic Unicast addresses: MAC addresses that identify a single LAN interface card. Broadcast addresses: A frame sent with a destination address of the broadcast address (FFFF.FFFF.FFFF) implies that all devices on the LAN should receive and process the frame. Multicast addresses: Multicast MAC addresses are used to allow a dynamic subset of devices on a LAN to communicate.

LAN switch The primary job of a LAN switch is to receive Ethernet frames and then make a decision, either forward the frame out some other port(s), or ignore the frame. To accomplish this primary mission, transparent bridges perform three actions:

LAN switch Deciding when to forward a frame or when to filter (not forward) a frame, based on the destination MAC address. Learning MAC addresses by examining the source MAC address of each frame received by the bridge. Creating a (Layer 2) loop-free environment with other bridges by using Spanning TreeProtocol (STP).

The Forward Versus Filter Decision To decide whether to forward a frame, a switch uses a dynamically built table that lists MAC addresses and outgoing interfaces Switches compare the frame’s destination MAC address to this table to decide whether the switch should forward a frame or simply ignore it.

Sample Switch Forwarding and Filtering Decision

Sample Switch Filtering Decision

Switch Learning

Avoiding Loops Using Spanning Tree Protocol The third primary feature of LAN switches is loop prevention, as implemented by SpanningTree Protocol (STP). Without STP, frames would loop for an indefinite period of time in Ethernet networks with physically redundant links To prevent looping frames, STP blocks some ports from forwarding frames

STP To avoid Layer 2 loops, all switches need to use STP. STP causes each interface on a switch to settle into either a blocking state or a forwarding state Forwarding means that the interface can send and receive data frames Blocking means that the interface cannot forward or receive data frames

Network with Redundant Links But Without STP Larry sends a single unicast frame to Bob’s MAC address, but Bob is powered off, so none of the switches has learned Bob’s MAC address yet. Bob’s MAC address would be an unknown unicast address at this point in time. Therefore, frames destined for Bob’s MAC address are forwarded by each switch out every port

Internal Processing on Cisco Switches Cut-through processing, the switch starts sending the frame out the output port as soon as possible. Although this might reduce latency, it also propagates errors. Fragment-free processing works similarly to cut-through, but it tries to reduce the number of errored frames that it forwards. it waits to receive the first 64 bytes before forwarding a frame

Internal Processing on Cisco Switches Store-andforward: With many links to the desktop running at 100 Mbps, uplinks at 1 Gbps, and faster, today’s switches typically use store-andforward processing, because the improved latency of the other two switching methods is negligible at these speeds.

Switch Internal Processing

Virtual LANs (VLAN)

Without VLANs, a switch considers all interfaces on the switch to be in the same broadcast domain. In other words, all connected devices are in the same LAN. So, instead of all ports on a switch forming a single broadcast domain, the switch separates them into many, based on configuration. It’s really that simple.

Sample Network with Two Broadcast Domains and No VLANs

Sample Network with Two VLANs Using One Switch

Virtual LANs (VLAN)

Campus LAN with Design Terminology Listed

Roles of campus switches Access: Provides a connection point (access) for end-user devices. Does not forward frames between two other access switches under normal circumstances. Distribution: Provides an aggregation point for access switches, forwarding frames between switches, but not connecting directly to end-user devices. Core: Aggregates distribution switches in very large campus LANs, providing very high forwarding rates.

Operating Cisco LAN Switches

Cisco 2960 Catalyst Switch Series Cisco refers to a switch’s physical connectors as either interfaces or ports.

Cisco 2960 On a 2960, the number before the / is always 0. The first 10/100 interface on a 2960 is numbered starting at 0/1, the second is 0/2, and so on. The interfaces also have names; for example, “interface FastEthernet 0/1” is the first of the 10/100 interfaces. Any Gigabit- capable interfaces would be called “GigabitEthernet” interfaces. For example, the first 10/100/1000 interface on a 2960 would be “interface gigabitethernet 0/1.”

Switch Status from LEDs

Accessing the Cisco IOS CLI

CLI Configuration Mode Versus Exec Modes

Navigating Between Different Configuration Modes ■ Using a hostname Fred global configuration command to configure the switch’s name ■ Movement from global configuration mode to console line configuration mode (using the line console 0 command) ■ Setting the console’s simple password to hope (using the password hope line subcommand) ■ Movement from console configuration mode to interface configuration mode (using the interface command) ■ Setting the speed to 100 Mbps for interface Fa0/1 (using the speed 100 interface subcommand) ■ Movement from interface configuration mode back to global configuration mode (using the exit command)

Storing Switch Configuration Files RAM: Sometimes called DRAM for Dynamic Random- Access Memory, RAM is used by the switch just as it is used by any other computer: for working storage. The running (active) configuration file is stored here. ROM: Read-Only Memory (ROM) stores a bootstrap (or boothelper) program that is loaded when the switch first powers on. This bootstrap program then finds the full Cisco IOS image and manages the process of loading Cisco IOS into RAM, at which point Cisco IOS takes over operation of the switch.

Storing Switch Configuration Files Flash memory: Either a chip inside the switch or a removable memory card, Flash memory stores fully functional Cisco IOS images and is the default location where the switch gets its Cisco IOS at boot time. Flash memory also can be used to store any other files, including backup copies of configuration files. NVRAM: Nonvolatile RAM (NVRAM) stores the initial or startup configuration file that is used when the switch is first powered on and when the switch is reloaded.

Cisco Switch Memory Types

Names and Purposes of the Two Main Cisco IOS Configuration Files