CIT 384: Network Administration Switches

CIT 384: Network Administration Slide #1

Topics

1. TCP/IP Architecture 2. OSI Reference Model 3. Cisco certification overview CIT 384: Network Administration Slide #2

Switching at Data Link Layer

Application Presentation Session Transport Network

Data Link

Physical

• 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.

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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

0100.5E

The addresses represent a subset of devices on the LAN. They begin with

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.) CIT 384: Network Administration Slide #9

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

Greater cabling distances Multiple collision domains

Hub

Yes No Increases bandwidth No Multiple broadcast domains No

Switch

Yes Yes Yes No

Router

Yes Yes Yes Yes CIT 384: Network Administration Slide #20

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 CIT 384: Network Administration Catalyst 3750 switches Slide #24

Distribution Switches

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

• Models – Catalyst 6500 – Catalyst 4500 CIT 384: Network Administration Slide #25

Core Switches

• Aggregate distribution switches. Not needed on smaller networks.

• Models – Catalyst 6500 – Nexus 7000 CIT 384: Network Administration Slide #26

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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) CIT 384: Network Administration Slide #28

# Name

1 SYST 2 RPS 3 STAT 4 DUPLX 5 SPEED 7 Port

Catalyst 2960 LEDs

Description

green=up; amber=POST failed; off=powered off Status of redundant power supply If on, each port: on=up, no traffic, flash green=up, traffic, flashing amber=port is disabled If on, each port: on is full duplex, off is half duplex If on, each port: off=10, solid=100, flashing=1000 Individual port LED; see above for meanings CIT 384: Network Administration Slide #29

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 CIT 384: Network Administration Slide #32

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 CIT 384: Network Administration Slide #34

CLI Help Features

What you Type

?

help command ?

Help provided

Help for all commands in this mode.

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 Command completion.

command parm1 ?

Help describing all of the second options for

command

where

parm1

the first parameter.

is CIT 384: Network Administration Slide #35

Keystroke

up arrow down arrow left arrow right arrow backspace ctrl-a ctrl-e ctrl-r ctrl-d esc-b esc-f

CLI Editing

Effect

Command history (backward) Command history (forward) Move backward one character in current command line.

Move forward one character in current command line.

Move backward and delete one character.

Move cursor to first character of command line.

Most cursor to end of the command line.

Redisplays command line with all characters. Useful when console messages clutter the screen.

Deletes a single character Move cursor backward one word.

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)# host(config-line)# host(config-if)#

Mode Name Command to reach

Global configure terminal Line Interface host(config-router)# Router line console 0 line vty 0 15 interface type # router ospf router igrp CIT 384: Network Administration Slide #38

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> CIT 384: Network Administration Slide #39

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 CIT 384: Network Administration Slide #41

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References

James Boney,

Cisco IOS in a Nutshell, 2 nd 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, 3 rd 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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