day4 ch3 switching

8/10/2019 Day4 Ch3 Switching

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What is switching ?

In a telecommunications network, a switching

is a technique that channels incoming data

from any of multiple input ports to the specific

output port that will take the data toward its

intended destination.


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Type switching

Two major packet switching modes exist;

(1) connectionlesspacket switching, also

known as datagramswitching

(2) connection-orientedpacket switching,

also known as virtual circuitswitching.
http://en.wikipedia.org/wiki/Connectionless_communicationhttp://en.wikipedia.org/wiki/Datagramhttp://en.wikipedia.org/wiki/Connection-oriented_communicationhttp://en.wikipedia.org/wiki/Virtual_circuithttp://en.wikipedia.org/wiki/Virtual_circuithttp://en.wikipedia.org/wiki/Connection-oriented_communicationhttp://en.wikipedia.org/wiki/Connection-oriented_communicationhttp://en.wikipedia.org/wiki/Connection-oriented_communicationhttp://en.wikipedia.org/wiki/Datagramhttp://en.wikipedia.org/wiki/Connectionless_communication


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Switching Services

bridges use software to create and manage a filtertable.

switches use application specific integrated circuits

(ASICs) to build and maintain their filter tables. A layer 2 switch is similar to a multi port bridge

Bridges are self managed while switches are

manageable. Layer 2 switches and bridges are faster than routers

because Layer 2 switches do not look at thenetwork layer header and hence faster.


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Limitations of Layer 2 Switching

Bridged networks break up collision domains,

but the network is one large broadcast

domain.

Broadcasts and multicasts, along with the slow

convergence time of spanning trees, can

considerably slow down the network


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Bridging vs. LAN Switching

Bridges are software based, while switches are hardware based because

they use ASIC chips to help make filtering decisions.

Bridges can have only one spanning-tree instance per bridge, while

switches can have many.

Switches have a higher number of ports than most bridges. Both bridges and switches forward layer 2 broadcasts.

Bridges and switches learn MAC addresses by examining the source

address of each frame received.

Both bridges and switches make forwarding decisions based on layer 2

addresses.


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Three Switch Functions at Layer 2

Address learning,

forward/filter decisions, and

loop avoidance.


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Switch Operation

When a device transmits and an interfacereceives a frame, the switch places the framessource address in the MAC forward/filter table

The switch floods the network with this frameout of every port except the source port

If a device answers this flooded frame and

sends a frame back, then the switch will takethe source address from that frame and placethat MAC address in its database


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How switches learn hosts locations


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Forward/Filter Decisions

When a frame arrives at a switch interface, thedestination hardware address is compared to theforward/filter MAC database.

If the destination hardware address is known andlisted in the database, the frame is sent out onlythe correct exit interface.

The switch doesnt transmit the frame out any

interface except for the destination interface. This preserves bandwidth on the other network

segments and is calledframe filtering.


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Loop Avoidance

Redundant links between switches help prevent

complete network failures in the event one link

stops working.

redundant links can also cause problems

Frames can be flooded down all redundant links

simultaneously, creating network loops

If no loop avoidance schemes are put in place,the switches will flood broadcasts endlessly

throughout the inter-network. [broadcast storm]


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Loop Avoidance

A device can receive multiple copies of thesame frame, since that frame can arrive fromdifferent segments at the same time.

The MAC address filter table will be totallyconfused about the devices location

switch could get so caught up in constantly

updating the MAC filter table with sourcehardware address locations that it may fail toforward a frame [MAC table thrashing].


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Broadcast storm


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Multiple frame copies


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Spanning Tree Protocol (STP)

Digital Equipment Corporation (DEC) createdthe original version of Spanning Tree Protocol(STP)

The IEEE later created its own version of STPcalled 802.1D.

All Cisco switches run the IEEE 802.1D version

of STP STPs main task is to stop network loops from

occurring on your layer 2 network


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Spanning Tree Protocol (STP)

STP monitors all network links & ensures that

no loops occur, by shutting down any

redundant links.

STP uses the spanning-tree algorithm (STA) to

first create a topology database, then search

out and destroy redundant links.

With STP running, frames will be forwarded

only on the premium, STP-picked links.


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A switched network with switching loops


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Spanning Tree Terms

STP is a bridge protocol that uses the STA to findredundant links dynamically and create aspanning-tree topology database.

Bridges periodically exchange BPDU messageswith other bridges to detect loops

Root bridge The root bridge is the bridge with thelowest(best) bridge ID.

All the switches in the network elect a rootbridge that becomes the focal point in thenetwork.


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Spanning Tree Terms

BPDU All the switches exchange information

to use in the selection of the root switch, as

well as in subsequent configuration of the

network.

Each switch compares the parameters in the

Bridge Protocol Data Unit (BPDU) that they

send / receive from neighbors


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Spanning Tree Terms

Bridge ID STP keeps track of all the switches inthe network using bridge IDs .

It is determined by a combination of the

bridge priority (32,768 by default on all Ciscoswitches) and the base MAC address.

The bridge with the lowest bridge ID becomes

the root bridge in the network. Non-root bridge All bridges except the root

bridge are Non-root bridges.


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Spanning Tree Terms

Root port The root port is always the link directly connected

to the root bridge, or the shortest path to theroot bridge.

If more than one link connects to the rootbridge, then a port cost is determined bychecking the bandwidth of each link.

The lowest-cost port becomes the root port.

If multiple links have the same cost, the bridgewith the lower advertising bridge ID is used. If multiple links are from the same device, the

lowest port number will be used.


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Spanning Tree Terms

Designated port A designated port is one that

has been determined as having the best

(lowest) cost.

A designated port will be marked as a

forwarding port.

Port cost The cost of a link is determined by

the bandwidth of a link.


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Spanning Tree Terms

Non-designated port A non-designated port is onewith a higher cost than the designated port.

Nondesignated ports are put in blocking mode

Forwarding port A forwarding port forwardsframes.

Blocked port A blocked port is the port thatwill not forward frames, in order to preventloops.

a blocked port will always listen to frames.


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Spanning Tree Operations

STP elects a root bridge that will forward throughall ports and act as a point of reference for allother devices in the STP domain.

Every bridge finds its one, and only one, root port

Each and every link between two switches musthave one, and only one, designated port.

Every port on the root switch is a designatedport,

Any port that is not either a root port or adesignated port is placed in the blocking state,thus breaking the switching loop.


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Selecting the Root Bridge

The bridge ID is used to elect the root bridge in the STPdomain as well as to determine the root port for eachof the remaining devices in the STP domain.

Bridge ID is 8 bytes long, and includes both the priority

[2 bytes] and the MAC address [6 bytes] of the device.

The default priority on all devices running the IEEE STPversion is 32,768.

If two switches or bridges happen to have the samepriority value, then the MAC address becomes thetiebreaker


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Selecting the Root Bridge

BPDUs are sent every two seconds, by default

The bridges ID can be changed by lowering its

priority so that it will become a root bridge

automatically.

Changing the default priority is the best way

to choose a root bridge.

core switch in the network should be root

bridge for STP to converge quickly.


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Spanning tree commands

Switch#sh spanning-tree

VLAN0001

Spanning tree enabled protocol ieee

Root ID Priority 32768

Address 0009.7ccf.a880

Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

command to change a bridge priority on a Catalyst 2950 or 3550 switch:

Switch(config)#spanning-tree vlan 1 priority ?

bridge priority in increments of 4096

Switch(config)#spanning-tree vlan 1 priority 4096

Switch(config)#spanning-tree vlan 1 root ?

primary Configure this switch as primary root for this spanning tree(pri=28673)

secondary Configure switch as secondary root (priority =24577)


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Selecting the Root Port

If more than one link leads to the root bridge,then cumulative outbound port costs alongthe path to the root bridge is considered

The STP cost is an accumulated total path costbased on the rated bandwidth of each of thelinks

The IEEE 802.1D specification assigns a defaultport cost value to each port based onbandwidth.


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Typical Costs of Different Ethernet Networks


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Spanning-Tree Port States

The ports on a bridge or switch running STP cango through five different states:

Blocking

Listening Learning

Forwarding

Disabled Blocked ports can still receive BPDUsthey just

dont send out any frames.


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Convergence

Convergence occurs when all ports on bridges

and switches have transitioned to either the

forwarding or blocking modes.

No data is forwarded until convergence is

complete.

It usually takes 50 seconds to go from blocking

to forwarding mode,


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LAN Switch Types

LAN switch types decide how a frame is handledwhen its received on a switch port.

Latencythe time it takes for a frame to be sent

out an exit port once the switch receives theframedepends on the chosen switching mode.

There are three switching modes:

Cut-through (Fast-Forward)

Fragment-Free (modified cut-through)

Store-and-forward


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Different switching modes within a frame


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Basic Tasks in Cisco Managed switch Configuration

Setting the passwords

Setting the hostname

Configuring the IP address and subnet mask

Setting a description on the interfaces

Setting port security

Erasing the switch configurations


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To set the user mode passwords for the 2950 switch,

Switch>enable Switch#config t Switch(config)#line vty 0 15 Switch(config-line)#login

Switch(config-line)#password telnet Switch(config-line)#line con 0 Switch(config-line)#login Switch(config-line)#password todd Switch(config-line)#exit Switch(config)#exit Switch#


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Setting the Enable Secret Password

The enable secret password is a more secure

password and it supersedes the enable password

(config)#enable secret todd2

on the 2950, the enable password and enable

secret must be different

Setting the Hostname

Switch(config)#hostname Todd2950

Todd2950(config)#


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Setting IP Information

Reasons to set the IP address information on

the switch:

To manage the switch via Telnet or other

management software

To configure the switch with different VLANs

and other network functions

By default, no IP address or default-gateway

information is set.


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Setting IP Information

every port on every switch is a member of VLAN1 bydefault.

Todd2950#config t

Todd2950(config)#int vlan1

Todd2950(config-if)#ip address 172.16.10.17255.255.255.0

Todd2950(config-if)#no shut

Todd2950(config-if)#exit Todd2950(config)#ip default-gateway 172.16.10.1

Todd2950(config)#


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Configuring Interface Descriptions

Todd2950(config)#int fastEthernet 0/1

Todd2950(config-if)#description Sales Printer

Todd2950(config-if)#int f0/12

Todd2950(config-if)#description Connection

to backbone

Todd2950(config-if)#^Z Todd2950#


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Setting Port Security on a Catalyst Switch

Ensures that only a certain device is plugged

into a particular switch port,

Switch(config-if)#switchport port-security

mac-address mac-address


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Erasing the Switch Configuration

Todd2950#erase startup-config

Erasing the nvram filesystem will remove all

files! Continue? [confirm] [Enter]

[OK]

Erase of nvram: complete

Todd2950#

day4 ch3 switching

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