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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_communication8/10/2019 Day4 Ch3 Switching
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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#
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