The document provides an overview of the OSI model and TCP/IP protocols. It describes the seven layers of the OSI model from the physical layer to the application layer and their functions. It also explains the four layers of the TCP/IP model and some of the common protocols used in each layer such as IP, TCP, UDP, HTTP, FTP etc. Additionally, it summarizes the Address Resolution Protocol (ARP), which maps IP addresses to MAC addresses when a host needs to deliver a packet on a local network.

1. OSI MODEL & TCP/IP

2. INTRODUCTION OSI
• The Open System Interconnection Reference
Model (OSI Reference Model or OSI Model) is an
abstract description for layered communications
and computer network protocol design.
• It divides network architecture into seven layers
which, from top to bottom, are the Application,
Presentation, Session, Transport, Network, Data
Link, and Physical Layers. It is therefore often
referred to as the OSI Seven Layer Model.
2

3. OSI LAYERS OSI Model
Data unit Layer Function
Host
layers
Data
7. Application Network process to application
6. Presentation
Data representation,
encryption and decryption
5. Session Interhost communication
Segment
s
4. Transport
End-to-end connections and
reliability, Flow control
Media
layers
Packet 3. Network
Path determination and logical
addressing
Frame 2. Data Link Physical addressing
Bit 1. Physical
Media, signal and binary
transmission
3
Going from layer 1 to 7: Please Do Not Throw Sausage Pizza
Away
Going from layer 7 to 1: All People Seem To Need Data
Processing

4. LAYER1: PHYSICAL LAYER
• The Physical Layer defines the electrical and
physical specifications for devices. In particular,
it defines the relationship between a device
and a physical medium.
• This includes the layout of pin, voltages, cable
specification, hubs, repeaters, network
adapters, host bus adapters, and more.
4

5. LAYER1: PHYSICAL LAYER
• The major functions and services
performed by the Physical Layer are:
• Establishment and termination of a
connection to a communication medium.
• Participation in the process whereby the
communication resources are effectively
shared among multiple users. For
example, flow control.
• Modulation, or conversion between the
representation of digital data in user
equipment and the corresponding signals
transmitted over a communications channel.
These are signals operating over the
physical cabling (such as copper and optical
fiber) or over a radio link.
5

6. LAYER 2: DATA LINK LAYER
• The Data Link Layer provides the
functional and procedural means to
transfer data between network entities
and to detect and possibly correct
errors that may occur in the Physical
Layer.
• Originally, this layer was intended for
point-to-point and point-to-multipoint
media, characteristic of wide area media
in the telephone system.
• The data link layer is divided into two
sub-layers by IEEE. 6

7. LAYER 2: DATA LINK LAYER
• One is Media Access Control (MAC) and another is
Logical Link Control (LLC).
• Mac is lower sub-layer, and it defines the way about
the media access transfer, such as
CSMA/CD/CA(Carrier Sense Multiple
Access/Collision Detection/Collision Avoidance)
• LLC provides data transmission method in different
network. It will re-package date and add a new
header.
7

8. LAYER 3: NETWORK LAYER
• The Network Layer provides the functional and
procedural means of transferring variable
length data sequences from a source to a
destination via one or more networks, while
maintaining the quality of service requested by
the Transport Layer.
8

9. LAYER 3: NETWORK LAYER
• The Network Layer performs
• network routing functions,
• perform fragmentation and reassembly,
• report delivery errors.
• Routers operate at this layer—sending data
throughout the extended network and making
the Internet possible.
9

10. LAYER 4: TRANSPORT LAYER
• The Transport Layer provides transparent
transfer of data between end users, providing
reliable data transfer services to the upper
layers.
• The Transport Layer controls the reliability of a
given link through flow control,
segmentation/desegmentation, and error
control.
10

11. LAYER 4: TRANSPORT LAYER
Feature Name TP0 TP1 TP2 TP3 TP4
Connection oriented network Yes Yes Yes Yes Yes
Connectionless network No No No No Yes
Concatenation and separation No Yes Yes Yes Yes
Segmentation and reassembly Yes Yes Yes Yes Yes
Error Recovery No Yes No Yes Yes
Reinitiate connection (if an
excessive number of PDUs are
unacknowledged)
No Yes No Yes No
multiplexing and demultiplexing
over a single virtual circuit
No No Yes Yes Yes
Explicit flow control No No Yes Yes Yes
Retransmission on timeout No No No No Yes
Reliable Transport Service No Yes No Yes Yes
11

12. LAYER 5: SESSION LAYER
• The Session Layer controls the dialogues
(connections) between computers.
• It establishes, manages and terminates the
connections between the local and remote
application.
• It provides for full-duplex, half-duplex,
or simplex operation, and establishes
checkpointing, adjournment, termination, and
restart procedures.
12

13. LAYER 5: SESSION LAYER
• The OSI model made this layer responsible for
graceful close of sessions, which is a property
of the Transmission Control Protocol, and also
for session check pointing and recovery, which
is not usually used in the Internet Protocol
Suite. The Session Layer is commonly
implemented explicitly in application
environments that use remote procedure calls.
13

14. LAYER 6: PRESENTATION LAYER
• The Presentation Layer establishes a context
between Application Layer entities, in which
the higher-layer entities can use different
syntax and semantics, as long as the
presentation service understands both and
the mapping between them.
• This layer provides independence from
differences in data representation (e.g.,
encryption) by translating from application to
network format, and vice versa.
• This layer formats and encrypts data to be
sent across a network, providing freedom
from compatibility problems.
• It is sometimes called the syntax layer.
14

15. LAYER 7: APPLICATION LAYER
• The application layer is the OSI layer closest to
the end user, which means that both the OSI
application layer and the user interact directly
with the software application.
• Application layer functions typically include:
• identifying communication partners,
• determining resource availability,
• synchronizing communication.
15

16. LAYER 7: APPLICATION LAYER
• Identifying communication partners
• Determines the identity and availability of
communication partners for an application
with data to transmit.
• Determining resource availability
• Decide whether sufficient network or the
requested communication exist.
• Synchronizing communication
• All communication between applications
requires cooperation that is managed by
the application layer.
16

17. LAYER 7: APPLICATION LAYER
• Some examples of application layer
implementations include
• Hypertext Transfer Protocol (HTTP)
• File Transfer Protocol (FTP)
• Simple Mail Transfer Protocol (SMTP)
17

18. OSI FEATURE
• Open system standards over the world
• Rigorously defined structured, hierarchical
network model
• Complete description of the function
• Provide standard test procedures
18

19. INTRODUCTION TCP/IP
• The Internet Protocol Suite (commonly
known as TCP/IP) is the set
of communications protocols used for
the Internet and other similar networks.
• It is named from two of the most important
protocols in it:
• the Transmission Control Protocol (TCP) and
• the Internet Protocol (IP), which were the first
two networking protocols defined in this
standard. 19

20. TCP/IP ENCAPSULATION
20

21. TCP/IP SOME PROTOCOL
Layer Protocol
Application
DNS, TFTP, TLS/SSL, FTP, Gopher, HTTP, IMAP, IRC, NNTP,
POP3, SIP, SMTP, SMPP, SNMP, SSH, Telnet, Echo, RTP, PN
RP, rlogin, ENRP
Routing protocols like BGP and RIP which run over TCP/UDP,
may also be considered part of the Internet Layer.
Transport TCP, UDP, DCCP, SCTP, IL, RUDP, RSVP
Internet
IP (IPv4, IPv6), ICMP, IGMP, and ICMPv6
OSPF for IPv4 was initially considered IP layer protocol since
it runs per IP-subnet, but has been placed on the Link
since RFC 2740.
Link ARP, RARP, OSPF (IPv4/IPv6), IS-IS, NDP 21

22. ADDRESS RESOLUTION
PROTOCOL (ARP)

23. WHAT ADDRESS ?------ IP ADDRESSE
▶ IP Addressing occurs at layer 2 (data link ) & layer 3 (network )of open system
interconnection (OSI) reference model .
layers (seven) specific particular network function such as addressing , for
control ,error control ,encapsulation and reliable message .
▶ layer 2 addresses used for local /directly connected device.

24. WHAT ADDRESS ?------ IP ADDRESSE
▶ Layer 3 addresses used for inter network /indirectly connected device.
Why How 48 Bit MAC Addresses
Media access protocol
To identify & group devices so that datagram transmition can be send and receive

25. ADDRESS RESOLUTION
PROTOCOL (ARP)
Definition : Address Resolution Protocol (ARP) is
a protocol for mapping an Internet Protocol
address (IP address) to a physical machine
address(MAC address) that is recognized in the
local network. For example, in IP Version 4, the
most common level of IP in use today, an
address is 32 bits long.

26. HOW ARP
WORKS
When an incoming packet destined for a host machine
on a particular local area network arrives at a gateway,
the gateway asks the ARP program to find a physical
host or MAC address that matches the IP address. The
ARP program looks in the ARP cache and, if it finds the
address
Server then creates data link Header & Trailer that
encapsulates packets & proceeds to transfer data

27. . If no entry is found for the IP address, ARP broadcasts a
request packet in a special format to all the machines on
the LAN to see if one machine knows that it has that IP
address associated with it. A machine that recognizes
the IP address as its own returns a reply so indicating.
ARP updates the ARP cache for future reference and
then sends the packet to the MAC address that replied.
HOW ARP WORKS

29. In an Ethernet local area network, however, addresses for
attached devices are 48 bits long. (The physical machine
address is also known as a Media Access Control or MAC
address.) A table, usually called the ARP cache, is used to
maintain a correlation between each MAC address and its
corresponding IP address. ARP provides the protocol rules for
making this correlation and providing address conversion in
both directions.
ADDRESS RESOLUTION
PROTOCOL (ARP)

30. Since protocol details differ for each type
of local area network, there are separate
ARP Requests for Comments (RFC) for
Ethernet, ATM, Fiber Distributed-Data
Interface, HIPPI, and other protocols.
HOW ARP
WORKS

31. ADDRESS
RESOLUTION
Subnet
subnet address?
Problem
Router knows that destination host is on its subnet based on the IP
address of an arriving packet
Does not know the destination host’s subnet address, so cannot
deliver the packet across the subnet
Destination Host
128.171.17.13

32. ARP IS LEVEL 3 PROTOCOL TO PERFORM……
MAPPING

33. ADDRESS MAPPING
• The delivery of a packet to a host
or a router requires two levels of
addressing: logical and physical. We
need to be able to map a logical
address to its corresponding
physical address and vice versa.
These can be done using either
static or dynamic mapping.

34. ARP IS LEVEL 3 PROTOCOL TO PERFORM……
MAPPING
▶ Map Addresses must be mapped to an IP addresses.
▶ Some other layer 3 protocols (I)ARP (ii)Reverse ARP
(iii)Serial live ARP (iv)Inverse ARP.
▶ Level 3 devices need ARP to map IP network address
to MAC hardware addresses so that IP packets come
be sent access network.

35. ARP IS LEVEL 3 PROTOCOL TO PERFORM……
MAPPING
▶ Before datagram is send from 1 devices to 2nd. It
1ooks in its ARP cache to see if use is mac add and
corresponding IP addresses for destination device.
▶ Only device with matching IP address replier to
sending device with packet containing mac address
for device.

36. ARP IS LEVEL 3 PROTOCOL TO PERFORM……
MAPPING
▶ MAC address hardware addresses of host.
▶ ARP maintains cache (table) in which MAC add are mapped
to IP address.

37. FRAGMENTATION
• Fragmentation is basically a process in which free
memory space is broken into little pieces.
• In this, memory blocks cannot be allocated to
processes due to their small size and such blocks
remain unused.
• It usually occurs in dynamic memory allocation
system when many of free blocks are too small to
satisfy any request. 37

38. SEGMENTATION
• Segmentation is basically a memory management
technique that supports user’s view of memory and
is also known as non-contiguous memory allocation
technique.
• In this, each process is divided into number of
segments and detail about each segment can be
stored in table that is known as segment table.
38

39. • It is basically a process that creates variable-sized
address spaces in computer storage for related
data.
• Segments are not fixed in size.
39

40. FRAGMENTATION
• In this, storage space is used inefficiently
that in turn reduce capacity and
performance.
• Types of fragmentation includes internal
and external fragmentation.
• Its main purpose is to help operating
system use the available space on
storage device.
• It is generally associated with IP.
• It reduces efficiency in memory
management.
• It is an unwanted problem that causes
wastage of memory and inflexibility.
SEGMENTATION
• In this, memory is divided into variable
size parts usually known as segments.
• Types of segmentation includes virtual
memory and simple segmentation.
• Its main purpose is to give user’s view of
process.
• It is generally associated with TCP.
• It simply allows for better efficiency in
memory management.
• Its advantages include less overhead,
larger segment size than actual page
size, no internal fragmentation, etc.
40

41. IPADDRESSING AND
SUBNETTING
41

42. OBJECTIVES
(CONTINUED)
42
• Discuss advanced routing concepts such as
CIDR(Classless Inter-Domain Routing),
summarization, and VLSM(Variable Length Subnet
Masking)
Convert between decimal, binary, and hexadecimal
numbering systems
Explain the differences between IPv4 and IPv6
•
•

43. IP
ADDRESSING
43
•
•
An IP address has 32 bits divided into four octets
To make the address easier to read, people use
decimal numbers to represent the binary digits
– Example: 192.168.1.1
Dotted decimal notation
– When binary IP addresses are written in decimal
format
•

44. IP ADDRESSING
(CONTINUED)
44

45. MAC TO IP ADDRESS
COMPARISON
45
• MAC address
–Identifies a specific NIC in a computer on a network
–Each MAC address is unique
–TCP/IP networks can use MAC addresses in
communication
Network devices cannot efficiently route traffic using
MAC addresses because they:
–Are not grouped logically
–Cannot be modified
–Do not give information about physical or logical
network configuration
•

46. MAC TO IP ADDRESS
COMPARISON
46
•
IP addressing
–Devised for use on large networks
IP addresses have a hierarchical structure and do
provide logical groupings
–IP address identifies both a network and a host
•

47. IP CLASSES
47
• Class A
–Reserved for governments and large corporations
throughout the world
–Each Class A address supports 16,777,214 hosts
Class B
–Addresses are assigned to large- and medium-sized
companies
–Each Class B address supports 65,534 hosts
•

48. IP CLASSES
(CONTINUED)
48

49. IP CLASSES
(CONTINUED)
49
• Class C
–Addresses are assigned to groups that do not meet
the qualifications to obtain Class A or B addresses
–Each Class C address supports 254 hosts
Class D
–Addresses (also known as multicast addresses) are
reserved for multicasting
–Multicasting is the sending of a stream of data
(usually audio and video) to multiple computers
simultaneously
•

50. IP CLASSES
(CONTINUED)
50

51. IP CLASSES
(CONTINUED)
51
• Class E
–Addresses are reserved for research, testing, and
experimentation
–The Class E range starts where Class D leaves off
Private IP ranges
–Many companies use private IP addresses for their
internal networks
• Will not be routable on the Internet
–Gateway devices have network interface connections
to the internal network and the Internet
• Route packets between them
•

52. IP CLASSES
(CONTINUED)
52

53. NETWORK
ADDRESSING
53
• IP addresses identify both the network and the host
–The division between the two is not specific to a
certain number of octets
Subnet mask
–Indicates how much of the IP address represents the
network or subnet
Standard (default) subnet masks:
–Class A subnet mask is 255.0.0.0
–Class B subnet mask is 255.255.0.0
–Class C subnet mask is 255.255.255.0
•
•

54. NETWORK ADDRESSING
(CONTINUED)
54
• TCP/IP hosts use the combination of the IP address
and the subnet mask
–To determine if other addresses are local or remote
–The binary AND operation is used to perform the
calculation
Subnetting
–Manipulation of the subnet mask to get more network
numbers
•

55. NETWORK ADDRESSING
(CONTINUED)
55
• Subnet address
–Network is identified by the first, or first few, octets
–A TCP/IP host must have a nonzero host identifier
Broadcast address
–When the entire host portion of an IP address is all
binary ones
– Examples: 190.55.255.255 and 199.192.65.63
•

56. NETWORK ADDRESSING
(CONTINUED)
56

57. BROADCAST
TYPES
57
• Flooded broadcasts
–Broadcasts for any subnet
–Use use the IP address 255.255.255.255
–A router does not propagate flooded broadcasts
because they are considered local
Directed broadcasts are for a specific subnet
–Routers can forward directed broadcasts
–For example, a packet sent to the Class B address
129.30.255.255 would be a broadcast for network
129.30.0.0
•

58. SUBDIVIDING IP
CLASSES
58
• Reasons for subnetting
–To match the physical layout of the organization
–To match the administrative structure of the
organization
–To plan for future growth
–To reduce network traffic

59. SUBDIVIDING IP CLASSES
(CONTINUED)
59

60. SUBNET
MASKING
60
• When network administrators create subnets
–They borrow bits from the original host field to make a
set of subnetworks
–The number of borrowed bits determines how many
subnetworks and hosts will be available
Class C addresses also can be subdivided
–Not as many options or available masks exist because
only the last octet can be manipulated with this class
•

61. 61

62. SUBNET MASKING
(CONTINUED)
62

63. SUBNET MASKING
(CONTINUED)
63

64. LEARNING TO
SUBNET
• Suppose you had a network with:
–Five different segments
–Somewhere between 15 and 20 TCP/IP hosts on
each network segment
• You just received your Class C address from
ARIN (199.1.10.0)
• Only one subnet mask can handle your network
configuration: 255.255.255.224
–This subnet mask will allow you to create eight
subnetworks and to place up to 30 hosts per
network
64
•
•
•

65. LEARNING TO SUBNET
(CONTINUED)
65
• Determine the subnet identifiers (IP addresses)
–Write the last masking octet as a binary number
–Determine the binary place of the last masking digit
Calculate the subnets
–Begin with the major network number (subnet zero)
and increment by 32
–Stop counting when you reach the value of the mask
Determine the valid ranges for your hosts on each
subnet
–Take the ranges between each subnet identifier
–Remove the broadcast address for each subnet
•
•

66. LEARNING TO SUBNET
(CONTINUED)
66

67. LEARNING TO SUBNET
(CONTINUED)
67

68. LEARNING TO SUBNET
(CONTINUED)
68

69. SUBNETTING
FORMULAS
69
• Consider memorizing the following two formulas:
2y = # of usable subnets (where y is the number of bits
borrowed)
2x – 2 = # of usable hosts per subnet (where x is the
number of bits remaining in the host field after
borrowing)

70. SUBNETTING FORMULAS
(CONTINUED)
70

71. SUBNETTING FORMULAS
(CONTINUED)
71

72. CIDR
72
• Classless Inter-Domain Routing (CIDR)
–Developed to slow the exhaustion of IP addresses
–Based on assigning IP addresses on criteria other
than octet boundaries
CIDR addressing method allows the use of a prefix
to designate the number of network bits in the mask
–Example: 200.16.1.48 /25 (CIDR notation)
–The first 25 bits in the mask are network bits (1s)
The prefix can be longer than the default subnet
mask (subnetting) or it can be shorter than the
default mask (supernetting)
•
•

73. VARIABLE LENGTH SUBNET
MASKS(VLSM)
73
• Variable length subnet masking (VLSM)
–Allows different masks on the subnets
–Essentially done by subnetting the subnets
Basic routing protocols such as RIP version 1 and
IGRP
–Do not support VLSM because they do not carry
subnet mask information in their routing table updates
–Are classful routing protocols
RIP version 2, OSPF, or EIGRP are classless
protocols
•
•

74. 74

75. VARIABLE LENGTH SUBNET
MASKS (CONTINUED)
75

76. VARIABLE LENGTH SUBNET
MASKS (CONTINUED)
76

77. IPV4 VERSUS
IPV6
77
• IP version 4 (IPv4)
–The version of IP currently deployed on most systems
today
IP version 6 (IPv6)
–Originally designed to address the eventual depletion
of IPv4 addresses
CIDR has slowed the exhaustion of IPv4 address
space and made the move to IPv6 less urgent
–However, CIDR is destined to become obsolete
because it is based on IPv4
•
•

78. IPV4 VERSUS IPV6
(CONTINUED)
78
• Network address translation (NAT)
–Another technique developed in part to slow the
depletion of IPv4 addresses
–Allows a single IP address to provide connectivity for
many hosts
NAT is CPU intensive and expensive
–Some protocols do not work well with NAT, such as
the IP Security Protocol (IPSec)
IPv4 does not provide security in itself
–Has led to security issues with DNS and ARP
•
•

79. VIRTUAL LANS

80. VLAN INTRODUCTION
VLANs logically segment switched networks based
on the functions, project teams, or applications of
the organization regardless of the physical location
or connections to the network.
All workstations and servers used by a particular
workgroup share the same VLAN, regardless of the
physical connection or location.

81. VLAN INTRODUCTION
A workstation in a VLAN group is restricted to
communicating with file servers in the same VLAN group.
VLANs function by logically segmenting the network into
different broadcast domains so that packets are only
switched between ports that are designated for the same
VLAN.

82. VLAN INTRODUCTION
Routers in VLAN
topologies provide
broadcast filtering,
security, and traffic flow
management.

83. VLAN INTRODUCTION
VLANs address scalability, security, and network
management.
Switches may not bridge any traffic between
VLANs, as this would violate the integrity of the
VLAN broadcast domain.
Traffic should only be routed between VLANs.

84. BROADCAST DOMAINS WITH VLANS AND
ROUTERS
A VLAN is a broadcast domain created by one or
more switches.
Layer 3 routing allows the router to send packets to
the three different broadcast domains.

85. BROADCAST DOMAINS WITH VLANS AND
ROUTERS

86. BROADCAST DOMAINS WITH VLANS AND
ROUTERS
Implementing VLANs on a switch causes the following
to occur:
The switch maintains a separate bridging table for each
VLAN.
If the frame comes in on a port in VLAN 1, the switch
searches the bridging table for VLAN 1.
When the frame is received, the switch adds the source
address to the bridging table if it is currently unknown.
The destination is checked so a forwarding decision can be
made.
For learning and forwarding the search is made against the
address table for that VLAN only.

87. VLAN OPERATION
Each switch port could be assigned to a different VLAN.
Ports assigned to the same VLAN share broadcasts.
Ports that do not belong to that VLAN do not share these
broadcasts.

88. VLAN OPERATION
Users attached to the same shared segment, share
the bandwidth of that segment.
Each additional user attached to the shared medium
means less bandwidth and deterioration of network
performance.
VLANs offer more bandwidth to users than a shared
network.
The default VLAN for every port in the switch is the
management VLAN.
The management VLAN is always VLAN 1 and may
not be deleted. All other ports on the switch may be

89. VLAN OPERATION
Dynamic VLANs allow for membership based on the
MAC address of the device connected to the switch port.
As a device enters the network, it queries a database
within the switch for a VLAN membership.

90. VLAN OPERATION
In port-based or port-centric VLAN membership, the port
is assigned to a specific VLAN membership independent
of the user or system attached to the port.
All users of the same
port must be in the same
VLAN.

91. VLAN OPERATION
Network administrators are responsible for
configuring VLANs both manually and statically.

92. BENEFITS OF VLANS
The key benefit of VLANs is that they permit the network
administrator to organize the LAN logically instead of
physically.

93. WHAT IS A SPANNING TREE
• “Given a connected,
undirected graph, a
spanning tree of that
graph is a subgraph
which is a tree and
connects all the
vertices together”.
• A single graph can
have many different
spanning trees.

94. SPANNING TREE PROTOCOL
• The purpose of the protocol is to have bridges
dynamically discover a subset of the topology
that is loop-free (a tree) and yet has just
enough connectivity so that where physically
possible, there is a path between every switch

95. SPANNING TREE PROTOCOL
• Several flavors:
• Traditional Spanning Tree (802.1d)
• Rapid Spanning Tree or RSTP (802.1w)
• Multiple Spanning Tree or MSTP (802.1s)

96. TRADITIONAL SPANNING
TREE (802.1D)
• Switches exchange messages that allow them
to compute the Spanning Tree
• These messages are called BPDUs (Bridge Protocol
Data Units)
• Two types of BPDUs:
• Configuration
• Topology Change Notification (TCN)

97. TRADITIONAL SPANNING
TREE (802.1D)
• First Step:
• Decide on a point of reference: the Root Bridge
• The election process is based on the Bridge ID,
which is composed of:
• The Bridge Priority: A two-byte value that is
configurable
• The MAC address: A unique, hardcoded address that
cannot be changed.

98. ROOT BRIDGE SELECTION
(802.1D)
• Each switch starts by sending out BPDUs
with a Root Bridge ID equal to its own
Bridge ID
• I am the root!
• Received BPDUs are analyzed to see if a
lower Root Bridge ID is being
announced
• If so, each switch replaces the value of the
advertised Root Bridge ID with this new

99. ROOT BRIDGE SELECTION
(802.1D)
Switch B Switch C
Swtich A
32678.0000000000AA
32678.0000000000BB 32678.0000000000CC
• All switches have the same priority.
• Who is the elected root bridge?

100. ROOT PORT SELECTION
(802.1D)
• Now each switch needs to figure out where it
is in relation to the Root Bridge
• Each switch needs to determine its Root Port
• The key is to find the port with the lowest Root Path
Cost
• The cumulative cost of all the links leading to the Root
Bridge

101. ROOT PORT SELECTION
(802.1D)
• Each link on a switch has a Path Cost
• Inversely proportional to the link speed
• e.g. The faster the link, the lower the cost
Link Speed STP Cost
10 Mbps 100
100 Mbps 19
1 Gbps 4
10 Gbps 2

102. ROOT PORT SELECTION
(802.1D)
• Root Path Cost is the accumulation of a link’s
Path Cost and the Path Costs learned from
neighboring Switches.
• It answers the question: How much does it cost to
reach the Root Bridge through this port?

103. ROOT PORT SELECTION
(802.1D)
1. Root Bridge sends out BPDUs with a Root
Path Cost value of 0
2. Neighbor receives BPDU and adds port’s Path
Cost to Root Path Cost received
3. Neighbor sends out BPDUs with new
cumulative value as Root Path Cost
4. Other neighbor’s down the line keep adding
in the same fashion

104. ROOT PORT SELECTION
(802.1D)
• On each switch, the port where the lowest Root
Path Cost was received becomes the Root Port
• This is the port with the best path to the Root Bridge

105. ROOT PORT SELECTION
(802.1D)
Switch B Switch C
Swtich A
1 2
1 1
2 2
Cost=19 Cost=19
Cost=19
32678.0000000000AA
32678.0000000000BB 32678.0000000000CC
• What is the Path Cost on each Port?
• What is the Root Port on each switch?

106. ROOT PORT SELECTION
(802.1D)
Switch B Switch C
Swtich A
1 2
1 1
2 2
Cost=19 Cost=19
Cost=19
32678.0000000000AA
32678.0000000000BB 32678.0000000000CC
Root Port
Root Port

107. ELECTING DESIGNATED
PORTS (802.1D)
• OK, we now have selected root ports but
we haven’t solved the loop problem yet,
have we
• The links are still active!
• Each network segment needs to have only one
switch forwarding traffic to and from that
segment
• Switches then need to identify one
Designated Port per link

108. ELECTING DESIGNATED
PORTS(802.1D)
• Which port should be the Designated Port on each
segment?
Switch B Switch C
Swtich A
1 2
1 1
2 2
Cost=19 Cost=19
Cost=19
32678.0000000000AA
32678.0000000000BB 32678.0000000000CC

109. ELECTING DESIGNATED
PORTS (802.1D)
• Two or more ports in a segment having
identical Root Path Costs is possible,
which results in a tie condition
• All STP decisions are based on the
following sequence of conditions:
• Lowest Root Bridge ID
• Lowest Root Path Cost to Root Bridge
• Lowest Sender Bridge ID
• Lowest Sender Port ID

110. ELECTING DESIGNATED
PORTS(802.1D)
Switch B Switch C
Swtich A
1 2
1 1
2 2
Cost=19 Cost=19
Cost=19
32678.0000000000AA
32678.0000000000BB 32678.0000000000CC
Designated
Port
Designated
Port
Designated
Port
In the B-C link, Switch B has the lowest
Bridge ID, so port 2 in Switch B is the
Designated Port

111. BLOCKING A PORT
• Any port that is not elected as either a Root
Port, nor a Designated Port is put into the
Blocking State.
• This step effectively breaks the loop and
completes the Spanning Tree.

112. DESIGNATED PORTS ON EACH
SEGMENT (802.1D)
Switch B Switch C
Swtich A
1 2
1 1
2 2
Cost=19 Cost=19
Cost=19
32678.0000000000AA
32678.0000000000BB 32678.0000000000CC
• Port 2 in Switch C is then put into the Blocking State because it
is neither a Root Port nor a Designated Port
✕

113. SPANNING TREE PROTOCOL
STATES
• Disabled
• Port is shut down
• Blocking
• Not forwarding frames
• Receiving BPDUs
• Listening
• Not forwarding frames
• Sending and receiving BPDUs

114. SPANNING TREE PROTOCOL
STATES
• Learning
• Not forwarding frames
• Sending and receiving BPDUs
• Learning new MAC addresses
• Forwarding
• Forwarding frames
• Sending and receiving BPDUs
• Learning new MAC addresses

115. STP TOPOLOGY CHANGES
• Switches will recalculate if:
• A new switch is introduced
• It could be the new Root Bridge!
• A switch fails
• A link fails

116. ROOT BRIDGE PLACEMENT
• Using default STP parameters might result in
an undesired situation
• Traffic will flow in non-optimal ways
• An unstable or slow switch might become the root
• You need to plan your assignment of bridge
priorities carefully

117. BAD ROOT BRIDGE PLACEMENT
Switch D
Switch C
Swtich B
32678.0000000000BB 32678.0000000000DD
32678.0000000000CC Switch A 32678.0000000000AA
Root
Bridge
Out to router

118. GOOD ROOT BRIDGE
PLACEMENT
Switch D
Switch C
Swtich B
1.0000000000BB 0.0000000000DD
32678.0000000000CC Switch A 32678.0000000000AA
Alernative
Root Bridge
Out to active
router
Root Bridge
Out to standby
router

119. PROTECTING THE STP
TOPOLOGY
• Some vendors have included features that
protect the STP topology:
• Root Guard
• BPDU Guard
• Loop Guard
• UDLD
• Etc.

120. STP DESIGN GUIDELINES
• Enable spanning tree even if you don’t have
redundant paths
• Always plan and set bridge priorities
• Make the root choice deterministic
• Include an alternative root bridge
• If possible, do not accept BPDUs on end user
ports
• Apply BPDU Guard or similar where available