NETWORK DESIGNchapter5 part1.pptx

Designing IP Addressing and selecting
Routing Protocols
compiled by Tizita obssa
1
chapter5

Planning the IP Addressing
Hierarchy
2
 The IP addressing hierarchy influences network routing.
 This section describes IP addressing hierarchy and how it
reduces routing overhead.
 This section discusses the issues that influence the IP
addressing plan and the routing protocol choice, including
summarization, fixed-length subnet masking, variable-
length subnet masking,and class ful and classless routing
protocols.

Hierarchical Addressing
3
 The telephone numbering system is a hierarchical
system.
For example, the North American Numbering Plan
includes the country code, the area code, and the line
number.
 The telephone architecture has handled prefix routing,
or routing based only on the prefix part of the address,
for many years.
 For example, a telephone switch in Detroit, Michigan
does not have to know how to reach a specific line in
Portland, Oregon

Cont…
4
 It must simply recognize that the call is not local. A long-
distance carrier must recognize that area code 503 is
for Oregon, but it does not have to know the details of
how to reach the specific line in Oregon.
 The IP addressing scheme is also hierarchical, and
prefix routing is not new in the IP environment either.
 As in the telephone example, IP routers make
hierarchical decisions.

Cont…
5
 Recall that an IP address comprises a
prefix(network) part and a host part.
 A router has to know only how to reach the next hop;
it does not have to know the details of how to reach
an end node that is not local.
 Routers use the prefix to determine the path for a
destination address that is not local.
 The host part is used to reach local hosts.

Route Summarization
6
 With route summarization, also referred to as route
aggregation or super netting, one route in the routing
table represents many other routes.
 Summarizing routes reduces the routing update
traffic and reduces the number of routes in the routing
table and overall router overhead in the router receiving
the routes.
 In a hierarchical network design, effective use of route
summarization can limit the impact of topology changes
to the routers in one section of the network.

Cont…
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 If the Internet had not adapted route
summarization by standardizing on classless inter
domain routing (CIDR), it would not have survived.
 CIDR is a mechanism developed to help alleviate
the problem of IP address exhaustion and growth of
routing tables.
 The idea behind CIDR is that blocks of multiple
addresses (for example, blocks of Class C address)
can be combined, or aggregated, to create a larger
(that is, more hosts allowed), classless set of IP
addresses.

Cont…
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 These multiple Class C addresses can then be
summarized in routing tables, resulting in fewer route
advertisements.
 (Note that the CIDR mechanism can be applied to
blocks of Class A, B, and C addresses; it is not restricted
to Class C.)
 Classless Inter-Domain Routing (CIDR): An Address
Assignment and Aggregation Strategy.

Cont…
9
For summarization to work correctly, the following
requirements must be met:
 Multiple IP addresses must share the same leftmost bits.
 Routers must base their routing decisions on a 32-bit IP
address.
 Routing protocols must carry the prefix length with the 32-
bit IP address.
For example, assume that a router has the following
networks behind it:

Cont…
10
 For example, assume that a router has the following
networks behind it:
192.168.168.0/24 192.168.169.0/24
192.168.170.0/24 192.168.171.0/24
192.168.172.0/24 192.168.173.0/24
192.168.174.0/24 192.168.175.0/24
 Each of these networks could be advertised
separately; however, this would mean advertising eight
routes(which is not recommended)
 Instead, this router can summarize the eight routes
into one route and advertise 192.168.168.0/21.

Cont…
11
 By advertising this one route, the router is saying, "Route
packets to me if the destination has the first 21 bits the
same as the first 21 bits of 192.168.168.0.“
Figure 6-5 illustrates how this summary route is
determined.
 The addresses all have the first 21 bits in common and
include all the combinations of the other 3 bits in the
network portion of the
address; therefore, only the first 21 bits are needed to
determine whether the router can route to one of these
specific addresses.
Figure 6-5 Find the Common Bits to Summarize Routes
 192.168 192.168 192.168 192.168 192.168 192.168

Cont…
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168.0 = 169.0 = 170.0 = 171.0 = 172.0 = 173.0 =
174.0 = 175.0 =
11000000 10101000 10101
000 00000000
11000000 10101000 10101 001 00000000
11000000 10101000 10101 010 00000000
11000000 10101000 10101 011 00000000
11000000 10101000 10101 100 00000000
11000000 10101000 10101 101 00000000
11000000 10101000 10101 110 00000000
11000000 10101000 10101 111 00000000

Cont…
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 Number of Common Bits = 21
 Number of Non-Common Network Bits = 3
 Number of Host Bits = 8

IP Addressing Hierarchy Criteria
14
IP addressing hierarchy has an important impact on the
routing protocol choice, and vice versa.
The decision about how to implement the IP addressing
hierarchy is usually based on the following questions:
■Is hierarchy needed within the IP addressing plan?
■What are the criteria for dividing the network into route
summarization groups?

Cont…
15
■How is route summarization performed, and what is
the correlation with routing?
■Is a hierarchy of route summarization groups
required?
■ How many end systems does each route
summarization group or subgroup contain?

Benefits of Hierarchical
Addressing
16
 A network designer decides how to implement the IP
addressing hierarchy based on the network's size,
geography, and topology.
 In large networks, hierarchy within the IP addressing
plan is mandatory for a stable network (including stable
routing tables).
 For the following reasons, a planned, hierarchical IP
addressing structure, with room for growth, is
recommended for networks of all sizes:

Cont…
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 An IP addressing plan influences the network's overall
routing.
 Before allocating blocks of IP addresses to various
parts of the network and assigning IP addresses to
devices, consider the criteria for an appropriate and
effective IP addressing scheme.
 Routing stability, service availability, network scalability,
and modularity are some crucial and preferred network
characteristics and are directly affected by IP address
allocation and deployment.

Cont…
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 Modular design and scalable solutions: Whether
building a new network or adding a new service on top
of an existing infrastructure, a modular design helps to
deliver a long-term, scalable solution.
 IP addressing modularity allows the aggregation of
routing information on a hierarchical basis.

What is Route aggregation?
19
 Route aggregation is used to reduce routing
overhead and improve routing stability and scalability.
 However, to implement route aggregation, a
designer must be able to divide a network into
contiguous IP address areas and must have a solid
understanding of IP address assignment, route
aggregation, and hierarchical routing.

Summarization Groups
20
 To reduce the routing overhead in a large network, a
multilevel hierarchy might be required.
 The depth of hierarchy depends on the network size
and the size of the highest-level summarization group.
Figure 6-6 shows an example of a network hierarchy.
 Figure 6-6 IP Addressing Hierarchy

Cont… 192.168.26.0/24
21

Cont…
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A typical organization has up to three levels of hierarchy:
■ First level: Network locations typically represent the first level of
hierarchy in enterprise networks.
Each location typically represents a group of summarized subnets, known
as a summarization group.
■ Second level: A second level of hierarchy can be done within first-level
summarization groups.
For example, a large location can be divided into smaller summarization
groups that represent the buildings or cities within that location.
Not all first-level summarization groups require a second level of hierarchy.
■ Third level: To further minimize the potential routing overhead and
instability, a third level of hierarchy can exist within the second-level
summarization group. For example, sections or floors within individual
buildings can represent the third-level summarization group.

Impact of Poorly Designed IP
Addressing
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 A poorly designed IP addressing scheme usually results in IP
addresses that are randomly assigned on an as-needed basis.
 In this case, the IP addresses are most likely dispersed through
the network with no thought as to whether they can be grouped
or summarized.
 A poor design provides no opportunity for dividing the network
into contiguous address areas, and therefore no means of
implementing route summarization.

CONT…
24
 Figure 6-7 is a sample network with poorly designed IP
addressing; it uses a dynamic routing protocol.
Suppose that a link in the network is flapping (changing its state
from UP to DOWN, and vice versa) ten times per minute.
Because dynamic routing is used, the routers that detect the
change send routing updates to their neighbours, those
neighbours send it to their neighbours, and so on.
Because aggregation is not possible, the routing update is
propagated throughout the entire network, even if there is no
need for a distant router to have detailed knowledge of that link.
Figure 6-7 A Poorly Designed IP Addressing Scheme Results in
Excess Routing Traffic

CONT…
25

CONT…
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Impacts of poorly designed IP addressing include the
following:
 Excess routing traffic consumes bandwidth: When any route
changes, routers send routing updates. Without
summarization, more updates are sent, and the routing
traffic consumes more bandwidth.
 Increased routing table recalculation: Routing updates
require routing table recalculation, which affects the router's
performance and ability to forward traffic.
 Possibility of routing loops: When too many routing
changes prevent routers from converging with their
neighbours, routing loops might occur, which might have
global consequences for an organization.

Benefits of Route Aggregation
27
 Implementing route aggregation on border routers between
contiguously addressed areas controls routing table size.
 Figure 6-8 shows an example of implementing route
summarization (aggregation) on the area borders in a
sample network.
 If a link within an area fails, routing updates are not
propagated to the rest of the network, because only the
summarized route is sent to the rest of the network, and it
has not changed; the route information about the failed link
stays within the area.
 This reduces bandwidth consumption related to routing
overhead and relieves routers from unnecessary routing
table recalculation.

Addressing Plan Results in Reduced
Routing Traffic
28

CONT…
29
 Efficient aggregation of routing advertisements
narrows the scope of routing update propagation
 and significantly decreases the cumulative frequency
of routing updates

Fixed- and Variable-Length Subnet
Masks
30
 Another consideration when designing the IP addressing
hierarchy is the subnet mask to use— either the same mask for
the entire major network or different masks for different parts of
the major network.
 A major network is a Class A, B, or C network.
 Fixed-Length Subnet Masking (FLSM) is when all subnet masks
in a major network must be the same.
 Variable-Length Subnet Masking (VLSM) is when subnet masks
within a major network can be different.
 In modern networks, VLSM should be used to conserve the IP
addresses.

CONT…
31
 Some routing protocols require FLSM; others allow VLSM.
 FLSM requires that all subnets of a major network have the
same subnet mask, which therefore results in less efficient
address space allocation.
 For example, in the top network shown in Figure 6-9,
network 172.16.0.0/16 is sub netted using FLSM. Each
subnet is given a /16 mask.
 Because FLSM is used, all subnets have the same subnet
mask. This is inefficient, because even though only two
addresses are needed on the point-to-point links, a /24
subnet mask with 254 available host addresses is used.
 Figure 6-9 Fixed-Length Versus Variable-Length Subnet
Mask
 FLSM 172.16.0.0/24
 VLSM 172.16.0.0/24

CONT…
32
 VLSM makes it possible to subnet with different subnet masks and
therefore results in more efficient address space allocation. VLSM
also provides a greater capability to perform route summarization,
because it allows more hierarchical levels within an addressing
plan.
 VLSM requires prefix length information to be explicitly sent with
each address advertised in a routing update.
For example, in the lower network shown in Figure 6-9, network
172.16.0.0/16 is subnet ted using VLSM.
 The network is composed of multiple LANs that are connected by
point-to-point WAN links.
 The point-to-point links have a subnet mask of /30, providing only
two available host addresses, which is all that is needed on these
links.
 The LANs have a subnet mask of /24 because they have more
hosts that require addresses.

Routing Protocol Considerations
33
To use VLSM, the routing protocol in use must be
classless. Classful routing protocols permit only FLSM.
KEY POINT
 With class full routing, routing updates do not carry the
subnet mask. With classless routing, routing updates do
carry the subnet mask.
Class full Routing Protocols
 As illustrated at the top of Figure 6-10, the following rules
apply when class ful routing protocols are used:
 The routing updates do not include subnet masks.

Cont…
34
When a routing update is received and the routing
information is about one of the following:
 Routes within the same major network as configured on
the receiving interface, the subnet mask configured on the
receiving interface is assumed to apply to the received
routes also.
Therefore, the mask must be the same for all subnets of
a major network. In other words, submitting must be done
with FLSM.
 Routes in a different major network than configured on
the receiving interface, the default major network mask is
assumed to apply to the received routes. Therefore,
automatic route summarization is performed across major
network (Class A, B, or C) boundaries, and sub netted
networks must be contiguous.
 Figure 6-10 Classful Versus Classless Routing Protocols

Automatic Route Summarization on
Network Boundary
35

Classful
No Automatic Route Summarization on
Network Boundaries Necessary
36
Routing Table
Destination Next Hop
172.16.1.0/24 172.16.2.0/24 BB

CONT…
37
 Figure 6-11 illustrates a sample network with a
discontiguous 172.16.0.0 network that runs a classful
routing protocol.
 Routers A and C automatically summarize across the
major network boundary, so both send routing
information about 172.16.0.0 rather than the individual
subnets (172.16.1.0/24 and 172.16.2.0/24).

Cont…
38
Consequently, Router B receives two entries for the
major network 172.16.0.0, and it puts both entries into
its routing table. Router B therefore might make
incorrect routing decisions.
Figure 6-11 Class ful Routing Protocols Do Not Send
the Subnet Mask in the Routing Update

CONT…
39
Routing Table
172.16.0.0 172.16.0.0 A C

CONT…
40
 Because of these constraints, class full routing is not
often used in modern networks. Routing Information
Protocol (RIP) version 1 (RIP vl) is an example of a
class full routing protocol.

Cont…
41
Classless Routing Protocols
 As illustrated in the lower portion of Figure 6-10, the
following rules apply when classless routing protocols
are used:
 The routing updates include subnet masks.
 Automatic route summarization at the major network
boundary is not required, and route summarization can
be manually configured.

Cont…
42
 Sub netted networks can be discontinuous.
 Consequently, all modern networks should use
classless routing. Examples of classless routing
protocols include
 RIP version 2 (RIPv2),
 Enhanced Interior Gateway Routing Protocol (EIGRP),
 OSPF,
 IS-IS,
 and Border Gateway Protocol (BGP).

Cont…
43
 NOTE The classless routing protocols do not all
behave the same regarding summarization.
 For example, RIPv2 and EIGRP automatically
summarize at the network boundary by default, but they
can be configured not to, and they can be configured to
summarize at other address boundaries.
 Open Shortest Path First (OSPF) and Intermediate
System-to-Intermediate System (IS-IS) do not
summarize at the network boundary by default; they
can be configured to summarize at other address
boundaries.

CONT…
44
 Figure 6-12 illustrates how dis contiguous networks are
handled by a classless routing protocol. This figure shows
the same network as in Figure 6-11, but running a
classless routing protocol that does not automatically
summarize at the network boundary.
 In this example, Router B learns about both sub
networks 172.16.1.0/24 and 172.16.2.0/24, one from
each interface; routing is performed correctly.

Cont…
45
NOTE Although using discontiguous subnets with
classless routing protocols does not pose the
routing issues demonstrated in Figure 6-11,
contiguous blocks of IP networks should be used
whenever possible to promote more efficient
summarization.

Cont….
46
 Figure 6-12 Classless Routing Protocols Send
the Subnet Mask in the Routing Update
 172.16.1.0/24 A ^192.168.1.0/24 B ^
192.168.2.0/24 C ^ 172.16.2.0/24

Cont…
47
Routing Table
172.16.1.0/24 172.16.2.0/24 A C

CONT…
48
 Hierarchical IP Addressing and Summarization Plan
Example
 Recall that the number of available host addresses on
a subnet is calculated by the formula 2h ,
 where” h” is the number of host bits (the number of
bits set to 0 in the subnet mask).
 The first two columns in Table 6-3 show the location
and number of IP addresses required at each location
for the sample network shown in Figure 6-4.

Cont…
49
 The third column in this table is the next highest power
of 2 from the required number of addresses; this value
is used to calculate the required number of host bits,
as shown in the fourth column.
 Assuming that the Class B address 172.16.0.0/16 is
used to address this network, the fifth column
illustrates sample address blocks allocated to each
location.
 remark: total network bit=32=network bits+ host
bits

50
Table 6-3 Address Blocks by
Location for the Topology in
Figure 6-4
Location
Number of IP
Addresses Required
Rounded Power of
2
2h
h is host bits
Number of
Host Bits
Address Block Assigned
San Francisco 1290 2048 11 172.16.0.0-172.16.7.255/21
Denver Région 1024 1024 10 172.16.8.0-172.16.11.255/22
Denver Campus 441 512 9 172.16.9.0-172.16.10.255/23
Remote Office 1 28 64 6 172.16.11.0/26
Remote Office 2 35 64 6 172.16.11.64/26
Houston Region 1024 1024 10 172.16.12.0-172.16.15.255/22
Houston Campus 329 512 9 172.16.16.0-172.16.17.255/23
Remote Office 3 21 64 6 172.16.11.0/26

CONT…
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Note that:
because the largest remote office needs 35
addresses and there is plenty of address space, 64
addresses are allocated to each remote office.
For the main campus, 2048 addresses are allocated;
11 host bits are required. This subnet is further divided
into smaller subnets supporting floors or wiring closets.
For the Denver region, 1024 addresses are allocated;
10 host bits are required. This address block is further
divided into smaller subnets supporting buildings,
floors, or wiring closets.

Cont…
52
Similarly, for the Houston region, 1024 addresses are
also allocated and further subdivided, as shown in
Table 6-3.
Figure 6-13 illustrates one of the links in the Denver
region going down and how summarization is
performed to reduce routing update traffic.
Figure 6-13 Hierarchical IP Addressing Plan Example

CONT….
53

READING ASSIGNMENT
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 Discuss the impact of poorly designed IP addressing.
 Demonstrate the Flsm & Vlsm with appropriate
examples (should be different from handout).
 Read about the different types of routing protocols

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