CIT 384: Network Administration Slide #1

CIT 384: Network Administration

Switches

CIT 384: Network Administration Slide #2

Topics

1. TCP/IP Architecture

2. OSI Reference Model

3. Cisco certification overview

CIT 384: Network Administration Slide #3

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 LinkPhysical

CIT 384: Network Administration Slide #4

HubsAdvantages 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.

CIT 384: Network Administration Slide #5

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.

CIT 384: Network Administration Slide #6

Bridges

Bridges reduce collisions + add bandwidth.

CIT 384: Network Administration Slide #7

SwitchesA 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.

CIT 384: Network Administration Slide #8

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

CIT 384: Network Administration Slide #9

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.)

CIT 384: Network Administration Slide #10

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.

CIT 384: Network Administration Slide #11

CIT 384: Network Administration Slide #12

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.

CIT 384: Network Administration Slide #13

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.

CIT 384: Network Administration Slide #14

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.

CIT 384: Network Administration Slide #15

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.

CIT 384: Network Administration Slide #16

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.

CIT 384: Network Administration Slide #17

Collision Domains

• All devices share the available bandwidth.

• Bandwidth may be inefficiently used due to effects of collision, especially under high use.

CIT 384: Network Administration Slide #18

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.

CIT 384: Network Administration Slide #19

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.

CIT 384: Network Administration Slide #20

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

CIT 384: Network Administration Slide #21

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.

CIT 384: Network Administration Slide #22

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.

CIT 384: Network Administration Slide #23

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.

CIT 384: Network Administration Slide #24

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

CIT 384: Network Administration Slide #25

Distribution Switches

• Provide path through which access switches can forward traffic to each other.

• Models– Catalyst 6500– Catalyst 4500

CIT 384: Network Administration Slide #26

Core Switches

• Aggregate distribution switches. Not needed on smaller networks.

• Models– Catalyst 6500– Nexus 7000

CIT 384: Network Administration Slide #27

CIT 384: Network Administration Slide #28

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)

CIT 384: Network Administration Slide #29

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

CIT 384: Network Administration Slide #30

Accessing the IOS CLI

CIT 384: Network Administration Slide #31

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.

CIT 384: Network Administration Slide #32

CLI configuration

Access Type Configuration

Console line console 0

login

password spam

telnet line vty 0 15

login

password eggs

CIT 384: Network Administration Slide #33

User and Enable Mode

User Exec mode can read most data.

Enable mode is needed to reconfigure switch.

CIT 384: Network Administration Slide #34

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

CIT 384: Network Administration Slide #35

CLI Help FeaturesWhat 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 parm<tab> Command completion.

command parm1 ? Help describing all of the second options for command where parm1 is the first parameter.

CIT 384: Network Administration Slide #36

CLI EditingKeystroke 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.

CIT 384: Network Administration Slide #37

Configuration Mode

Commands in Configuration Mode update the active configuration file.

CIT 384: Network Administration Slide #38

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

CIT 384: Network Administration Slide #39

Example CLI SessionSwitch> enableSwitch# configure terminalSwitch(config)# enable secret letmeinSwitch(config)# interface FastEthernet 0/1Switch(config-if)# speed 100Switch(config-if)# ip address 10.1.1.1 255.0.0.0Switch(config-if)# no shutdownSwitch(config-if)# exit Switch(config)# line console 0Switch(config-line)# password spamSwitch(config-line)# exitSwitch(config)# copy running-config startup-configSwitch(config)# disableSwitch>

CIT 384: Network Administration Slide #40

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.

CIT 384: Network Administration Slide #41

Switch Configurations

ConfigurationsRunning: currently used config in RAM.

Startup: will be used at next reboot.

Viewingshow running-config

show startup-config

Saving running-config (replaces old startup)copy running-config startup-config

CIT 384: Network Administration Slide #42

References1. James Boney, Cisco IOS in a Nutshell, 2nd edition, O’Reilly, 2005. 2. Cisco, Catalyst 2960 series switches,

http://www.cisco.com/en/US/products/ps6406/index.html, 2008.3. Cisco, Cisco Connection Documentation,

http://www.cisco.com/univercd/home/home.htm4. Cisco, Internetworking Basics,

http://www.cisco.com/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.