Routing Protocols and Concepts: Chapter 8 doc

Topics ® The Routing Table Structure Routing Behavior * Topology * Classful and Classless Routing Behavior * Routing Table Entries * Classful Routing Behavior: no * Level 1 Routes I

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Routing Protocols and Concepts

CCNA Exploration Companion Guide

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For further information

This presentation is an

overview of what Is

covered in the

curriculum/book

For further explanation

and details, please read

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Topics

® The Routing Table Structure Routing Behavior

* Topology * Classful and Classless

Routing Behavior

* Routing Table Entries * Classful Routing Behavior: no

* Level 1 Routes Ip Classless a

* Parent and Child Routes: * Classless Routing Behavior: ip

classless

Classful Networks

*« Parent and Child Routes:

Classless Networks

* Steps in the Route Table

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The Houting Table Structure

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The Routing Table Structure

C 172.16.2.0 is directly connected, Serial0/0/0

C 172.16.3.0 is directly connected, FastEthernet0/0

10.0.0.0/16 is subnetted, 1 subnets

S 10.1.0.0 is directly connected, Serial0/0/1

C 192.168.1.0/24 1s directly connected, Serial0/0/1

5 192.168.100.0/24 ¡is directly connected, Serial0/0/1

® The structure of the routing table might seem obvious

@ Help you verify and troubleshoot routing issues because you will

understand the routing table lookup process

@® You will Know exactly what the Cisco IOS software does when it

searches for a route

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“= —f Fa0/0 = DCE ( Aron @==

@ Notice that R3 also has a 172.16.4.0/24 subnet that is disconnected, or

discontiguous, from the 172.16.0.0 network that R1 and R2 share

@ The effects of this discontiguous subnet are examined later in this chapter

when you look at the route lookup process

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Topology: Interface Configurations for R1 and R3

) # ) #

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Sample Routing Table Entries

@ Route entries from the following sources:

Directly connected networks Static routes

Dynamic routing protocols

® The source of the route does not affect the structure of the

routing table

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Sample Routing Table Entries

@ The routing table hierarchy in Cisco IOS software was originally

implemented with the classful routing scheme

@ Although the routing table incorporates both classful and

classless addressing, the overall structure is still built around

this classful scheme

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RT: add 192.168.1.0/24 via 0.0.0.0, connected metric [0/0]

RT: interface Serial0/0/1 added to routing table

R2(config-if)# end

R2# undebug all

@® Serial 0/0/1 interface for R2 is configured with the 192.168.1.1/24 address

® Assoonasno shutdown is entered, the output from debug ip routing

shows that this route has been added to the routing table

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Level 1 Houtes

RT: add 192.168.1.0/24 via 0.0.0.0, connected metric [0/0]

RT: interface Serial0/0/1 added to routing table

® The routing table is actually a hierarchical structure that is used to

speed up the lookup process when locating routes and forwarding

packets

@ Within this structure, the hierarchy includes several levels

@ For simplicity, we discuss all routes as one of two levels: level 1

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@ A level 7 route is a route with a subnet mask equal to or less than the

classful mask of the network address

192.168.1.0/24 is a level 1 network route because the subnet mask is

equal to the network’s classful mask

/24 is the classful mask for Class C networks, such as the 192.168.1.0

network

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Level 1 Ro ute S We will be using this chart throughout this chapter

C 192.168.1.0/24 is directly connected, Serial0/0/1

Next-hop IP address

Level 1 Routes Level 2 Routes and/or Exit Interface

Level 1 Route Examples

@ A level 1 route can function as any of the following:

e Default route: A default route is a static route with the address 0.0.0.0/0

e Supernet route: A supernet route is a network address with a mask less than

the classful mask

e Network route: A network route is a route that has a subnet mask equal to that

of the classful mask

e A network route can also be a parent route (next) 13

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Level 1 Houtes

eG 192.168.1.0/24 is directly connected, Serial0/0O/1

Next-hop IP address

aa Ultimate Route (eae

@ An ultimate route is a route that includes one or both of the following:

° A next-hop IP address (another path)

e An exit interface

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Level 1 Houtes

eG 192.168.1.0/24 is directly connected, Serial0/0O/1

Next-hop IP address

‘Default Ultimate Route “Path! Interface

“Network Ultimate Route -Path/ Interface

@® Directly connected network 192.168.1.0/24 is a

e level 1 network route - subnet mask that is the same as its

classful mask

e ultimate route - contains the exit interface Serial 0/0/1

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Parent and Child Routes: Classful Networks

172.16.0.0/24 is subnetted, 1 subnets < Level 1 Farent Roue

C 172.16.3.0 is directly connected, FastEthernet0/0 — Child Route

® Another type oí level † network route, a parent roulte

® VWVhen the 172.16.3.0 subnet was added to the routing table,

Another route, 172.16.0.0 also added

@® First entry: no next-hop IP address or exit interface information

This route is known as a level 7 parent route 16

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172.16.3.0 is directly connected, FastEthernet0/0 *— Child Rout

192.168.1.0/24 is directly connected, Serial0/O0/1

Level 1 Parent Rout

Level 2

e

@ A parent route is a heading:

Indicates the presence of level 2 routes, also known as child routes

to the routing table

classful mask is entered into the routing table

A level 1 parent route is automatically created any time a subnet is added

A parent route is created whenever a route with a mask greater than the

The subnet 172.16.3.0 is the level 2 child route of the parent route 172.16.0.0

1 7

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172.16.0.0/24 is subnetted, 1 subnets 4 Level 1 Parent Route

C 172.16.3.0 is đirectly connected, FastEthernet0/0 4—— Level 2

@ A level 2 route \s a route that is a subnet of a classful network address

® Like a level 1 route, the source of a level 2 route can be a:

° directly connected network

e static route

° dynamic routing protocol

@ Note: [his is the case even if a classless routing protocol is the source

of the subnet route

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Subnet mask for child routes

Specific child route Exit interface

¬¬———> SOtuirce is connected route

® Parenf route:

172.16.0.0: The classful network address for our subnet

/24: The subnet mask for all the child routes If the child routes have

variable-length subnet masks (VLSM), the subnet mask will be excluded

from the parent route and included with the individual child routes

(later)

is subnetted, 1 subnets: This part of the route specifies that this is a

parent route and in this case has one child route (that is, one subnet)

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————>> SOtUuirce is connected route

® Child route:

C: The route code for a directly connected network

172.16.3.0: The specific route entry

is directly connected: Along with the route code of C, this specifies that

this is a directly connected network with an administrative distance of 0

FastEthernet0/0: The exit interface for forwarding packets that match this specific route entry

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¬¬———> SOtuirce is connected route

A level 2 child route contains the route source and the network address of

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Adding another child route

172.16.2.0 is directly connected, Serial0/0/0

172.16.3.0 is directly connected, FastEthernet0/0

192.168.1.0/24 is directly connected, Serial0/O0/1

Because both child routes have the same subnet mask, the parent

route still maintains the /24 mask but now shows two subnets

Later we will see the role of the parent route

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Adding another child route

lf child routes are = 172.16.2.0 is directly connected, Serial0/0/0

deleted there Is c 172.16.3.0 is directly connected, FastEthernet0/0

no parent route

Next-hop IP address

@ If there is only a single level 2 child route and that route is removed, the

level 1 parent route is automatically deleted

@ A level 1 parent route exists only when there is at least one level 2 child route <9

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Parent and Child Routes: Classless Networks

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variably subnetted, 3 subnets, 2 masks

is directly connected, Serial0/0/0

is directly connected, Serial0/0/1

is directly connected, FastEthernet0/0

@ All three subnets belong to the classful network 172.16.0.0/16 and are

therefore level 2 child routes

@ Notice that the child routes do not share the same subnet mask, as was the

case in the classful example

Implementing a network addressing scheme with VLSM

@ Whenever there are two or more child routes with different subnet masks

belonging to the same classful network, the routing table presents a

slightly different view, which states that this parent network is variably

subnetted

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Classful Number of subnets and parent route masks for this parent route

Child routes have

Classful different masks

4

172.16.0.0/16 is variably subnetted, 3 subnets, 2 masks

C Ha 96t 1s directly connected, EastEthernet0/0

Masks for the child routes

The parent route of 172.16.0.0 now contains the classful mask /16

In the classful example shown earlier the classful mask was not displayed Parent route states that the child routes are variably subnetted

Includes the number of different masks of the child routes (2 masks)

Each child route now contains the subnet mask for that specific route

In the non-VLSM example both child routes shared the same subnet mask

and the parent displayed their common subnet mask

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Classful Number of subnets and parent route masks for this parent route

Child routes have

mask † Parent route

® Parent route:

172.16.0.0: The parent route, the classful network address associated

with all child routes

/16: The classful subnet mask of the parent route

variably subnetted: States that the child routes are variably subnetted

and that there are multiple masks for this classful network

3 subnets, 2 masks: Indicates the number of subnets and the number of

different subnet masks for the child routes under this parent route

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Classful Number of subnets and

parent route masks for this parent route

Child routes have

child routes Exit interfaces

@ Child route:

C: The route code for a directly connected network

172.16.1.4: The specific route entry

/30: The subnet mask for this specific route

is directly connected: Along with the route code of C, specifies that this

is a directly connected network with an administrative distance of 0

Serial0/0/0: The exit interface for forwarding packets that match this

specific route entry

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Houting Table Lookup Process

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Houting Table Lookup Process

VERS LEN Type of Service Total Length

Identification Flags Fragment Offset Ro ut In g Tab le

TTL Protocol Header Checksum Router# show jp route

s IP Add 172.16.0.0/24 1s gubnsttsd, 4 subnets woe rose s 172.16.4.0 is directly connected, Serial0/0/1

R 172.16.1.0 [120/1] via 172.16.2.1, 00:00:08, Serial0/0/0 Destination IP Address Cc 172.16.2.0 is directly connected, Serial0/0/0

T 7 T C 172.16.3.0 is directly connected, FastEthernet0/0

Options Padding 10.0.0.0/16 is subnetted, 1 subnets

| rea aR oh Se a rn BI ca i es tae om fal ns ce os a VI CỤ, 8 10.1.0.0 is directly connected, Serial0/0/1

Cc 192.168.1.0/24 is directly connected, Serial0/0/1

_— s 192.168.100.0/24 is directly connected, Serial0/0/1

@ When a router receives a packet on one of its interfaces

@ The routing table lookup process compares the destination IP

address of the incoming packet with the entries in the routing table

® The best match between the packet's destination IP address and

the route in the routing table is used to determine to which interface

to forward the packet

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172 16.3.0/24

steps in the Route Table

Lookup Process

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steps in the Route Table Lookup Process

R3# show ip route

@ Neither R1 nor R2 has a route to 172.16.4.0

@ Also, R3 does not have routes to subnets 172.16.1.0/24, 172.16.2.0/24, or

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® The router examines level 1 routes, including network routes and

Supernet routes, for the best match with the destination address of

the IP packet

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® lí the best match Is a level 1 ultimate route—a classful network,

Supernet, or default route—this route is used to forward the packet

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Step 2

® The router examines child routes (the subnet routes) of the parent

route for a best match

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The Route Lookup Process

Next-hop IP address

=

‘Supernct — — Ultimate Route _Pathflnterface —`

® lf there is a match with a level 2 child route, that subnet is used to

forward the packet

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Step 3

@ Is the router implementing classful or classless routing behavior

(later)

° Router (config)# ip classless

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a Tưng

® lí classful routing behavior is in effect, terminate the lookup

process and drop the packet

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Tiêu đề	The Routing Table: A Closer Look
Tác giả	Rick Graziani, Allan Johnson
Trường học	Unknown
Thể loại	Lecture notes

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Số trang	87
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