3.1 layer2 switching

8/7/2019 3.1 Layer2 Switching

http://slidepdf.com/reader/full/31-layer2-switching 1/25

Switching Concepts



Switching Concepts

Segmentation allows network congestion to

be significantly reduced within each

segment.



Switching Concepts



Switch Forwarding Methodsy Switches used one of the following forwarding methods for

switching data between network ports: store-and-forward orcut-through switching.yHowever, store-and-forward is the sole forwardingmethod used on current models of Cisco Catalystswitches.

y Store-and-Forward SwitchingyIn store-and-forward switching, when the switch receivesthe frame, it stores the data in buffers until the completeframe has been received.yStore-and-forward switching is required for QoS analysison converged networks where frame classification for trafficprioritization is necessary.

y Cut-through SwitchingyIn cut-through switching, the switch acts upon the data assoon as it read the destination MAC address so that it candetermine which port to forward the data.y The destination MAC address is located in the first 6

bytes of the frame.

yCut-through switching is faster than store-and-forwardswitching.y However, because the switch does not perform any error

checking, it forwards corrupt frames throughout thenetwork.

y The corrupt frames consume bandwidth while they are

being forwarded.



Switch Forwarding Methodsy There are two variants of cut-through switching:y

Fast-forward switching: Fast-forward switching offers thelowest level of latency.yFast-forward switching is the typical cut-through method of switchingyFast-forward switching immediately forwards a packet afterreading the destination address.yBecause fast-forward switching starts forwarding before theentire packet has been received, there may be times when packetsare relayed with errors.

y

Fragment-free switching: In fragment-free switching, theswitch stores the first 64 bytes of the frame before forwarding.yFragment-free switching is the compromise between store-and-forward switching and cut-through switching.yThe reason fragment-free switching stores only the first 64 bytesof the frame is that most network errors and collisions occurduring the first 64 bytes.yFragment-free switching tries to enhance cut-through switchingby performing a small error check on the first 64 bytes of theframe to ensure that a collision has not occurred before

forwarding the frame.y Some switches are configured to perform cut-through

switching on a per-port basis until a user-defined errorthreshold is reached and then they automatically change tostore-and-forward.

yWhen the error rate falls below the threshold, the portautomatically changes back to cut-through switching.



Switching Concepts



Prepare to Configure the Switch

y The initial startup of a Catalyst switch requires thecompletion of the following steps:

y Step 1. Before starting the switch, verify thefollowing:

yAll network cable connections are secure.

yYour PC is connected to the console port.

y

Your terminal emulator application, such asHyperTerminal, is running and configured correctly.

y Step 2. Attach the power cable plug to the switchpower supply socket.

ySome Catalyst switches, including the Cisco Catalyst2960 series, do not have power buttons.

y

Step 3. Observe the boot sequence as follows:yWhen the switch is on, the POST begins.

yDuring POST, the LEDs blink while a series of testsdetermine that the switch is functioning properly.

yIf the switch fails POST, the SYST LED turns amber.



Configure Management Interfacey To manage a switch remotely using TCP/IP, you need to assign

the switch an IP address.yAn access layer switch is much like a PC in that you need toconfigure an IP address, a subnet mask, and a default gateway.

yThis IP address is assigned to a virtual interface called a VLAN

yIt is necessary to ensure the VLAN is assigned to a specific port orports on the switch.

yThe default configuration on the switch is to have the

management of the switch controlled through VLAN 1. However, abest practice for basic switch configuration is to change themanagement VLAN to a VLAN other than VLAN 1.

y Note that a Layer 2 switch, such as the Cisco Catalyst 2960,only permits a single VLAN interface to be active at a time.

yThis means that the Layer 3 interface, interface VLAN 99, isactive, but the Layer 3 interface, interface VLAN 1, is not active.



Configure Duplex and Speedy

You can use the duplexinterface configurationcommand to specify the duplexmode of operation for switchports.

yYou can manually set theduplex mode and speed of switch ports to avoid inter-vendor issues with

autonegotiation.yAlthough there can be issueswhen you configure switchport duplex settings to auto.



Tabley Switches use MAC address tables to determine how to

forward traffic between ports.yMAC tables include dynamic and static addresses.yshow mac-address-table commandyThe MAC address table was also referred to as contentaddressable memory (CAM) table.

y Dynamic MAC addresses: The source MAC addressesthe switch learns and ages when they are not in use.

yYou can change the aging time setting for MAC addresses.The default time is 300 seconds.ySetting too short an aging time can cause addresses to beprematurely removed from the table.y Then, when the switch receives a packet for an unknown

destination, it f loods the packet to all ports.y The unnecessary flooding can impact performance.

y static MAC addresses: A network administrator canspecifically assign static MAC addresses to certain ports.

Static addresses are not aged out.yOnly those devices that are known to the networkadministrator can connect to the network.yTo create a static MAC address table, use the mac-address-table static <MAC address> vlan {1-4096, ALL}interfaceinterface-idcommand.

The maximum size of the

MAC address table varieswith different switches.

±For example, theCatalyst 2960 seriesswitch can store up to8,192 MAC addresses.



Using the Show Commandsy When you need to verify the configuration of your

Cisco switch, the show command is very useful.y One of the more valuable show commands is the

show running-config command.

yThis command displays the configuration currently running on the switch. The output of the S1 switch:

y Fast Ethernet 0/18 interface configured with the management

VLAN 99y VLAN 99 configured with an IP address of 172.17.99.11

255.255.0.0

y Default gateway set to 172.17.50.1

y HTTP server configured

y Another commonly used command is the showinterfaces command.

yThe show interfaces command is used frequently whileconfiguring and monitoring network devices.

yThe first highlighted line in the figure indicates that theFast Ethernet 0/1 interface is up and running.

yThe next highlighted line shows that the duplex is auto-duplex and the speed is auto-speed.



Configure Encrypted Passwordsy When configuring passwords in Cisco IOS CLI, by

default all passwords, except for the enable secretpassword, are stored in clear text format within thestartup-config and running-config.

yIt is universally accepted that passwords should beencrypted and not stored in clear text format.yThe Cisco IOS command service password-encryption enables service password encryption.

y When the service password-encryptioncommandis entered from global configuration, all systempasswords are stored in an encrypted form.

yAs soon as the command is entered, all currently passwords are converted to encrypted passwords.

y If you want to remove the requirement to store allsystem passwords in an encrypted format, enter

the no service password-encryption command.R emoving password encryption does not convertencrypted passwords back into readable text.However, all newly set passwords are stored in cleartext format.



---- End of presentationEnd of presentation ----

3.1 layer2 switching

Documents