CIT 384: Network Administration
Switches
CIT 384: Network Administration
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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
Application
Presentation
Session
• Forwards Ethernet
frames between two
devices on same LAN.
• Logical link control
Transport
– Flow control
– Error correction
Network
• Media access control
Data Link
Physical
CIT 384: Network Administration
– Who can transmit when.
– Breaks data into frames.
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Hubs
Advantages over 10BASE2 bus
– Problem on 1 cable didn’t 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
Don’t have physically redundant switches.
– Redundancy is needed for reliability.
– It’s 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 enduser devices.
– Send traffic to connected
end-user devices.
– Send other traffic to
distribution layer.
Models
– Catalyst 3750
– Catalyst 3560
– Catalyst 2960
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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 8+1 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
green=up; amber=POST failed; off=powered off
2 RPS
Status of redundant power supply
3 STAT
If on, each port: on=up, no traffic, flash green=up,
traffic, flashing amber=port is disabled
4 DUPLX
If on, each port: on is full duplex, off is half duplex
5 SPEED
If on, each port: off=10, solid=100, flashing=1000
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
command ?
Message describing how to get help.
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 parm<tab> 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
host(config)#
Mode Name
Global
Command to reach
configure terminal
host(config-line)#
Line
host(config-if)#
Interface
line console 0
line vty 0 15
interface type #
host(config-router)# Router
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router ospf
router igrp
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Example CLI Session
Switch> 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
Switch>
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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.
2.
3.
4.
5.
6.
7.
James Boney, Cisco IOS in a Nutshell, 2nd edition, O’Reilly, 2005.
Cisco, Catalyst 2960 series switches,
http://www.cisco.com/en/US/products/ps6406/index.html, 2008.
Cisco, Cisco Connection Documentation,
http://www.cisco.com/univercd/home/home.htm
Cisco, Internetworking Basics,
http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/introint.ht
m
Wendell Odom, CCNA Official Exam Certification Library, 3rd
edition, Cisco Press, 2007.
Priscilla Oppenheimer and Joseph Bardwell, Troubleshooting Campus
Networks, Addison-Wesley, 2002.
W. Richard Stevens, TCP/IP Illustrated, Addison-Wesley, 1994.
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