Switches

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CIT 384 Network Administration

• Switches

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Topics

1. TCP/IP Architecture
2. OSI Reference Model
3. Cisco certification overview

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Switching at Data Link Layer

• Forwards Ethernet frames between two devices on
  same LAN.
• Logical link control
• Flow control
• Error correction
• Media access control
• Who can transmit when.
• Breaks data into frames.

Application
Presentation
Session
Transport
Network
Data Link
Physical
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Hubs

• Advantages over 10BASE2 bus
• Problem on 1 cable didnt impact entire network.
• Easier to run cables for star topology.
• Disadvantages of hubs
• Collisions
• Shared bandwidth, only one device can send at a
  time.
• Broadcasts seen by all hosts.

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Bridges

• A bridge segments a network into two parts.
• Each part is a collision domain.
• Frames in domain A can collide with each other.
• But they cannot collide with frames in domain B.
• Bridges buffer packets.
• If domain A is busy, then frame is buffered until
  the segment is no longer busy.

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Bridges

• Bridges reduce collisions add bandwidth.

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Switches

• A switch works like a collection of bridges.
• Each port is its own collision domain.
• If only one device connected to a port, the
  switch can communicate at full duplex.

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Ethernet Address Types

• Unicast addresses represent a single device.
• Multicast addresses represent a subset of devices
  on the LAN. They begin with 0100.5E
• The broadcast address represents all devices on
  the LAN. It is
• FFFF.FFFF.FFFF

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Switch Functions

1. Decide whether to forward a frame or not, based
   on its destination MAC address.
2. Learn MAC addresses by examining source MAC
   addresses received.
3. Create a layer 2 loop-free environment using
   Spanning Tree Protocol (STP.)

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Forwarding Frames

1. Switch receives frame on a port.
2. Extracts destination MAC address from Ethernet
   header.
3. Looks up destination MAC address in internal
   address table.
4. Forwards frame to port specified in table unless
   that port is the same port on which the frame was
   received.
5. If no entry exists in table for MAC address,
   forward the frame out all interfaces except the
   port it originated from.

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Learning MAC Addresses

1. Switch receives frame on a post.
2. Retrieves source MAC address from Ethernet header
   of frame.
3. Searches address table for MAC address.
4. If MAC address not found, add the (MAC address,
   port) pair to the table.

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MAC Address Table

• Switches have a finite amount of memory.
• Can only store a fixed of addresses.
• Maintain an inactivity timer for each address.
• Timer reset to 0 each time address is seen.
• If switch runs out of space, oldest entries are
  removed from table.

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Loops

• Bob is off, so no switch has his address.
• Therefore switches flood frames to Bob.
• Switches receive flood from other switches, which
  they flood again, looping indefinitely.

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Avoiding Loops

• Dont have physically redundant switches.
• Redundancy is needed for reliability.
• Its easy for someone to add a switch and create
  a loop in a complex campus network.
• Create a logical network without loops STP.
• Dynamically disable switch ports that could allow
  loops.
• Re-enable disabled ports when a link fails so
  that network retains benefit of redundancy.

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Collision Domains

• A collision domain is the set of devices whose
  frames can collide with each other.
• Bridges, switches, and routers separate the
  network into collision domains.

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Collision Domains

• All devices share the available bandwidth.
• Bandwidth may be inefficiently used due to
  effects of collision, especially under high use.

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Broadcast Domains

• A broadcast domain is the set of devices for
  which, when one device sends a broadcast, all
  devices receive the broadcast.
• Routers separate the network into broadcast
  domains.

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Broadcast Domains

• Bandwidth is not shared.
• Some bandwidth and processing may be wasted by
  broadcasts in large domains.
• A LAN consists of all devices in the same
  broadcast domain.

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Segmenting Device Features
Feature Hub Switch Router
Greater cabling distances Yes Yes Yes
Multiple collision domains No Yes Yes
Increases bandwidth No Yes Yes
Multiple broadcast domains No No Yes
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Virtual LANs (VLANs)

• Without VLANs, switches consider all devices on
  same broadcast domain (LAN.)
• With VLANs
• Switch can support multiple broadcast domains.
• Assign ports to specific VLANs.

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VLAN Purposes

• Create more flexible network designs, without
  having more switches.
• Segment devices into smaller broadcast domains to
  increase performance.
• Reduce STP workload by limiting VLAN to a single
  access switch.
• To enforce security by aggregating sensitive
  hsots on specific VLANs.
• To separate VOIP traffic from PC traffic.

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Switch Design Hierarchy

• Three layers
• Access
• Distribution
• Core
• Differences
• Whether switch connects to end-user devices.
• Whether switch should forward frames between
  other switches by connecting to multiple
  different switches.

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Switch Layers

• Access switches
• Connect directly to end-user devices.
• Send traffic to connected end-user devices.
• Send other traffic to distribution layer.
• Models
• Catalyst 3750
• Catalyst 3560
• Catalyst 2960

Catalyst 3750 switches
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Distribution Switches

• Provide path through which access switches can
  forward traffic to each other.
• Models
• Catalyst 6500
• Catalyst 4500

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Core Switches

• Aggregate distribution switches. Not needed on
  smaller networks.
• Models
• Catalyst 6500
• Nexus 7000

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Catalyst 2960

• Ethernet ports
• 29600-24TT-L 24 10/100 2 10/100/1000 ports
• Series ranges from 81 to 48 gigabit ports.
• Operating System Cisco IOS
• Other features
• Some models support Power over Ethernet
• QoS support for IP phones
• Security features (MAC binding, NAC)

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Catalyst 2960 LEDs
Name Description
1 SYST greenup amberPOST failed offpowered off
2 RPS Status of redundant power supply
3 STAT If on, each port onup, no traffic, flash greenup, traffic, flashing amberport is disabled
4 DUPLX If on, each port on is full duplex, off is half duplex
5 SPEED If on, each port off10, solid100, flashing1000
7 Port Individual port LED see above for meanings
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Accessing the IOS CLI
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CLI Access Methods

• Console
• 9600 bps 8N1 serial connection
• Available before switch has been configured.
• Available during problems down network, OS.
• Only one connection at a time.
• ssh and telnet
• Switch must be configured with an IP address.
• Provides remote access from anywhere on net.
• Up to 16 simultaneous connections.
• All data (including passwords) revealed by
  telnet, so ssh is preferred.

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CLI configuration
Access Type Configuration
Console line console 0 login password spam
telnet line vty 0 15 login password eggs
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User and Enable Mode

• User Exec mode can read most data.
• Enable mode is needed to reconfigure switch.

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Cisco IOS

• Internetwork Operating System
• Multitasking, no memory protection
• CLI copied by many network vendors.
• IOS is available with different feature sets
• IP data
• Voice data
• Security VPN
• Older switches run CatOS

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CLI Help Features
What you Type Help provided
? Help for all commands in this mode.
help Message describing how to get help.
command ? Help describing all of the first options for command.
command parm? Help describing all of the first options for command that begin with parm.
command parmlttabgt Command completion.
command parm1 ? Help describing all of the second options for command where parm1 is the first parameter.
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CLI Editing
Keystroke Effect
up arrow Command history (backward)
down arrow Command history (forward)
left arrow Move backward one character in current command line.
right arrow Move forward one character in current command line.
backspace Move backward and delete one character.
ctrl-a Move cursor to first character of command line.
ctrl-e Most cursor to end of the command line.
ctrl-r Redisplays command line with all characters. Useful when console messages clutter the screen.
ctrl-d Deletes a single character
esc-b Move cursor backward one word.
esc-f Move cursor forward one word.
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Configuration Mode

• Commands in Configuration Mode update the active
  configuration file.

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Configuration Submodes
Prompt Mode Name Command to reach
host(config) Global configure terminal
host(config-line) Line line console 0 line vty 0 15
host(config-if) Interface interface type
host(config-router) Router router ospf router igrp
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Example CLI Session

• Switchgt enable
• Switch configure terminal
• Switch(config) enable secret letmein
• Switch(config) interface FastEthernet 0/1
• Switch(config-if) speed 100
• Switch(config-if) ip address 10.1.1.1 255.0.0.0
• Switch(config-if) no shutdown
• Switch(config-if) exit
• Switch(config) line console 0
• Switch(config-line) password spam
• Switch(config-line) exit
• Switch(config) copy running-config
  startup-config
• Switch(config) disable
• Switchgt

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Switch Memory Types

• RAM Working storage, includes the active
  configuration file.
• ROM Stores boot program that finds Cisco IOS
  image and loads into RAM.
• Flash Stores IOS image and other files.
• NVRAM Stores startup configuration file that is
  used when switch is booted.

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Switch Configurations

• Configurations
• Running currently used config in RAM.
• Startup will be used at next reboot.
• Viewing
• show running-config
• show startup-config
• Saving running-config (replaces old startup)
• copy running-config startup-config

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References

1. James Boney, Cisco IOS in a Nutshell, 2nd
   edition, OReilly, 2005.
2. Cisco, Catalyst 2960 series switches,
   http//www.cisco.com/en/US/products/ps6406/index.h
   tml, 2008.
3. Cisco, Cisco Connection Documentation,
   http//www.cisco.com/univercd/home/home.htm
4. Cisco, Internetworking Basics, http//www.cisco.co
   m/univercd/cc/td/doc/cisintwk/ito_doc/introint.htm
   
5. Wendell Odom, CCNA Official Exam Certification
   Library, 3rd edition, Cisco Press, 2007.
6. Priscilla Oppenheimer and Joseph Bardwell,
   Troubleshooting Campus Networks, Addison-Wesley,
   2002.
7. W. Richard Stevens, TCP/IP Illustrated,
   Addison-Wesley, 1994.