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© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
ITE PC v4.0
Chapter 1 1
VLSM and CIDR

ITE PC v4.0
Chapter 1 2© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public
Objectives
 Compare and contrast classful and classless IP addressing.
 Review VLSM and explain the benefits of classless IP
addressing.
 Describe the role of the Classless Inter-Domain Routing
(CIDR) standard in making efficient use of scarce IPv4
addresses

ITE PC v4.0
Introduction
 Prior to 1981, IP addresses used only the first 8 bits to specify
the network portion of the address
 In 1981, RFC 791 modified the IPv4 32-bit address to allow
for three different classes
 IP address space was depleting rapidly
the Internet Engineering Task Force (IETF) introduced
Classless Inter-Domain Routing (CIDR)
–CIDR uses Variable Length Subnet Masking (VLSM)
to help conserve address space.
-VLSM is simply subnetting a subnet

ITE PC v4.0
Classful and Classless IP Addressing
 Classful IP addressing
 As of January 2007, there are over 433 million hosts on
internet
 Initiatives to conserve IPv4 address space include:
-VLSM & CIDR notation (1993, RFC 1519)
-Network Address Translation (1994, RFC 1631)
-Private Addressing (1996, RFC 1918)

ITE PC v4.0
 The High Order Bits
These are the leftmost bits in a 32 bit address

ITE PC v4.0
 Classes of IP addresses are identified by the decimal number of
the 1st octet
Class A address begin with a 0 bit
Range of class A addresses = 0.0.0.0 to 127.255.255.255
Class B address begin with a 1 bit and a 0 bit
Range of class B addresses = 128.0.0.0 to 191.255.255.255
Class C addresses begin with two 1 bits & a 0 bit
Range of class C addresses = 192.0.0.0 to 223.255.255.255.

ITE PC v4.0
 The IPv4 Classful Addressing Structure (RFC 790)
An IP address has 2 parts:
-The network portion
Found on the left side of an IP address
-The host portion
Found on the right side of an IP address

ITE PC v4.0

ITE PC v4.0
 Purpose of a subnet mask
It is used to determine the network portion of an IP
address

ITE PC v4.0
 Classful Routing Updates
-Recall that classful routing protocols (i.e. RIPv1)
do not send subnet masks in their routing updates
The reason is that the Subnet mask is
directly related to the network address

ITE PC v4.0
 Classless Inter-domain Routing (CIDR – RFC 1517)
Advantage of CIDR :
-More efficient use of IPv4 address
space
-Route summarization
Requires subnet mask to be included in routing update
because address class is meaningless
Recall purpose of a subnet mask:
-To determine the network and host portion
of an IP address

ITE PC v4.0
 Classless Routing Protocol
 Characteristics of classless routing protocols:
-Routing updates include the subnet mask
-Supports VLSM
Supports Route Summarization

ITE PC v4.0
Routing
Protocol
Routing
updates
Include
subnet
Mask
Supports
VLSM
Ability to send
Supernet routes
Classful No No No
Classless Yes Yes Yes
 Classless Routing Protocol

ITE PC v4.0
 Route summarization done by CIDR
-Routes are summarized with masks that are less
than that of the default classful mask
-Example:
172.16.0.0 / 13 is the summarized
route for the 172.16.0.0 / 16 to
172.23.0.0 / 16 classful networks

ITE PC v4.0
 Steps to calculate a route
summary
-List networks in binary
format
-Count number of left
most matching bits to
determine summary route’s
mask
-Copy the matching
bits and add zero bits to
determine the summarized
network address

ITE PC v4.0
Chapter 1 16
Distance Vector Routing
Protocols

ITE PC v4.0
Objectives
 Identify the characteristics of distance vector routing
protocols.
 Describe the network discovery process of distance
vector routing protocols using Routing Information
Protocol (RIP).
 Describe the processes to maintain accurate routing
tables used by distance vector routing protocols.
 Identify the conditions leading to a routing loop and
explain the implications for router performance.

ITE PC v4.0
Why dynamic routing protocol

ITE PC v4.0
Distance Vector Routing Protocols
 Examples of Distance Vector routing protocols:
Routing Information Protocol (RIP) v1,2
Interior Gateway Routing Protocol (IGRP)
Enhanced Interior Gateway Routing Protocol
(EIGRP)

ITE PC v4.0
 Distance Vector Technology
–The Meaning of Distance Vector:
•A router using distance vector routing protocols
knows 2 things:
Distance to final destination
Vector, or direction, traffic should be
directed

ITE PC v4.0
Characteristics of Distance Vector routing protocols:
 Periodic updates
 Broadcast updates
 Entire routing table is included into routing update

ITE PC v4.0
 Routing Protocol Algorithm:
-Defined as a procedure for accomplishing a certain task

ITE PC v4.0
Routing Protocol Characteristics
–Criteria used to compare routing protocols includes
Time to convergence
Scalability
Resource usage
Implementation & maintenance

ITE PC v4.0

ITE PC v4.0
Network Discovery
 Router initial start up
-Initial network discovery
Directly connected networks are initially placed in
routing table

ITE PC v4.0
Network Discovery
 Initial Exchange of Routing Information
–If a routing protocol is configured then
-Routers will exchange routing information
 Routing updates received from other routers
-Router checks update for new information
If there is new information:
-Metric is updated
-New information is
stored in routing table

ITE PC v4.0
Network Discovery
 Exchange of Routing Information
–Router convergence is reached when
-All routing tables in the network contain the same
network information
–Routers continue to exchange routing information
-If no new information is found then Convergence is
reached

ITE PC v4.0
Network Discovery
 Convergence must be reached before a network is
considered completely operable
 Speed of achieving convergence consists of 2 independent
categories
-Speed of broadcasting routing information
-Speed of calculating routes

ITE PC v4.0
Routing Table Maintenance
 Periodic Updates: RIPv1 & RIPv2
These are time intervals in which a router sends
out its entire routing table.

ITE PC v4.0
 RIP uses 4 timers
-Update timer
-Invalid timer
-Hold-down timer
-Flush timer

ITE PC v4.0
 Invalid Timer:- If an update has not been received to refresh
an existing route after 180 seconds (the default), the route is
marked as invalid by setting the metric to 16. The route is
retained in the routing table until the flush timer expires.
 Flush Timer:- By default, the flush timer is set for 240 seconds,
which is 60 seconds longer than the invalid timer. When the
flush timer expires, the route is removed from the routing table.

ITE PC v4.0
 EIGRP routing updates are
-Partial updates (The Update Contain only the changed
Routes)
-Triggered by topology changes (Once a change happen,
Update is sent)
-Bounded (Update sent only to Routers who need it )
-Non periodic

ITE PC v4.0
 Triggered Updates
–Conditions in which triggered updates are sent
-Interface changes state
-Route becomes unreachable
-Route is placed in routing table

ITE PC v4.0
Routing Loops
 Routing loops are
A condition in
which a packet is
continuously
transmitted within
a series of routers
without ever
reaching its
destination.

ITE PC v4.0
Routing Loops
 Routing loops may be caused by:
-Incorrectly configured static routes
-Incorrectly configured route redistribution
-Slow convergence
-Incorrectly configured discard routes
 Routing loops can create the following issues
-Excess use of bandwidth
-CPU resources may be strained
-Network convergence is degraded
-Routing updates may be lost or not processed in a timely
manner

ITE PC v4.0
Routing Loops problems
 Count to Infinity
This is a routing loop whereby packets bounce
infinitely around a network.

ITE PC v4.0
Routing Loops problems
 Distance Vector routing protocols set a specified
metric value to indicate infinity
Once a router “counts to infinity” it marks the route
as unreachable

ITE PC v4.0
Routing Loops
 Preventing loops with holddown timers
-Holddown timers allow a router to not accept any changes to a
route for a specified period of time.
-Point of using holddown timers
Allows routing updates to propagate through network with
the most current information. Holddown timers are used to
prevent regular update messages from inappropriately
reinstaling a route that may have gone bad.

ITE PC v4.0
Routing Loops
 Holddown Timer. This timer stabilizes routing
information and helps prevent routing loops during
periods when the topology is converging on new
information. Once a route is marked as unreachable
(Invalid), it must stay in holddown long enough for all
routers in the topology to learn about the unreachable
network. By default, the holddown timer is set for 180
seconds.

ITE PC v4.0
Update, holddown & flush timers
Last update
about certain
network
Invalid timer Flush timerHolddown timer
180 sec 180 sec 240 sec

ITE PC v4.0
Update, holddown & flush timers
Last update
about certain
network
Update about the
same network with
metric=16
Invalid timer Flush timerHolddown timer
180 sec 180 sec 240 sec

ITE PC v4.0
Routing Loops
 The Split Horizon Rule is used to prevent routing
loops
 Split Horizon rule:
A router should not advertise a network through the
interface from which the update came.

ITE PC v4.0
Routing Loops
 Split horizon with poison
reverse
The rule states that
once a router learns of
an unreachable route
through an interface,
advertise it as
unreachable back
through the same
interface

ITE PC v4.0
Route poisoning & Poison reverse
 Route poisoning: sending update about the
unreachable network with setting the metric (hop count)
to 16
 Poison reverse: after receiving the poisoned update the
router will suspend the split horizon rule and send the
same poisoned update again across the same interface
then resume split horizon rule again

ITE PC v4.0
Routing Loops
 IP & TTL
–Purpose of the TTL field
The TTL field is found in an IP header and is
used to prevent packets from endlessly
traveling on a network
 How the TTL field works
-TTL field contains a numeric value
The numeric value is decreased by one by
every router on the route to the destination.
If numeric value reaches 0 then Packet
is discarded.

ITE PC v4.0
Routing Protocols Today
 Factors used to determine whether to use RIP or EIGRP
include
-Network size
-Compatibility between models of routers
-Administrative knowledge

ITE PC v4.0
Routing Protocols Today
 RIP
Features of RIP:
-Supports split horizon & split horizon with
poison reverse
-Capable of load balancing
-Easy to configure
-Works in a multi vendor router environment

ITE PC v4.0
Chapter 1 48
RIP version 1

ITE PC v4.0
Objectives
 Describe the functions, characteristics, and operation
of the RIPv1 protocol.
 Configure a device for using RIPv1.
 Verify proper RIPv1 operation.
 Describe how RIPv1 performs automatic
summarization.
 Configure, verify, and troubleshoot default routes
propagated in a routed network implementing RIPv1.
 Use recommended techniques to solve problems
related to RIPv1

ITE PC v4.0
RIPv1
 RIP Characteristics
-A classful, Distance Vector (DV) routing protocol
-Metric = hop count
-Routes with a hop count > 15 are unreachable
-Updates are broadcast every 30 seconds

ITE PC v4.0
RIP v1 message

ITE PC v4.0
RIPv1
 RIP Operation
–RIP uses 2 message types:
Request message
-This is sent out on startup by each RIP
enabled interface
-Requests all RIP enabled neighbors to send
routing table
Response message
-Message sent to requesting router
containing routing table

ITE PC v4.0
Basic RIPv1 Configuration
 Router RIP Command
–To enable RIP enter:
-Router rip at the global configuration prompt
-Prompt will look like R1(config-router)#

ITE PC v4.0
Basic RIPv1 Configuration
 Specifying Networks
–Use the network command
to:
-Enable RIP on all
interfaces that
belong to this
network
-Advertise this
network in RIP
updates
sent to other
routers
every 30 seconds

ITE PC v4.0
Verification and Troubleshooting
 To verify and troubleshoot routing
-Use the following
commands:
-show ip route
-show ip protocols
-debug ip rip

ITE PC v4.0
Interpreting RIP route

ITE PC v4.0
 show ip
protocols
command
-Displays
routing protocol
configured
on router

ITE PC v4.0
 Debug ip rip command
-Used to display RIP routing updates as they are
happening

ITE PC v4.0
 Passive interface command
-Used to prevent a router from sending updates through
an interface
-Example:
Router(config-router)#passive-interface interface-type interface-number

ITE PC v4.0
 Passive interfaces

ITE PC v4.0
Automatic Summarization
 Boundary Routers
–RIP automatically summarizes classful networks
–Boundary routers summarize RIP subnets from one
major network to another.

ITE PC v4.0
Processing RIP Updates
 2 rules govern RIPv1 updates:
-If a routing update and the interface it’s
received on belong to the same
network then
The subnet mask of the
interface is applied to the
network in the routing update
-If a routing update and the interface it’s
received on belong to a different
network then
The classful subnet mask of the
network is applied to the
network in the routing update.

ITE PC v4.0
 Sending RIP Updates
–RIP uses automatic summarization to reduce the
size of a routing table.

ITE PC v4.0
 Advantages of automatic
summarization:
–The size of routing updates is
reduced
–Single routes are used to
represent multiple routes which
results in faster lookup in the
routing table.
–Stability in routing table

ITE PC v4.0
 Disadvantage of Automatic Summarization:
-Does not support discontiguous networks

ITE PC v4.0
 Discontiguous
Topologies do not
converge with RIPv1
 A router will only
advertise major
network addresses
out interfaces that do
not belong to the
advertised route.

ITE PC v4.0
Default Route and RIPv1
 Default routes
Packets that are not defined specifically in a routing
table will go to the specified interface for the default
route
Example: Customer routers use default routes to
connect to an ISP router.
Command used to configure a default route is
ip route 0.0.0.0 0.0.0.0 s0/0/1

ITE PC v4.0
Default Route and RIPv1
 Propagating the Default Route in RIPv1
 Default-information originate command
-This command is used to specify that the router is to originate
default information, by propagating the static default route in
RIP update.

ITE PC v4.0
Summary: Commands used by RIP
Command Command’s purpose
Rtr(config)#router rip Enables RIP routing process
Rtr(config-router)#network Associates a network with a RIP routing process
Rtr#debug ip rip used to view real time RIP routing updates
Rtr(config-router)#passive-interface fa0/0 Prevent RIP updates from going out an interface
Rtr(config-router)#default-information originate Used by RIP to propagate default routes
Rtr#show ip protocols Used to display timers used by RIP

ITE PC v4.0
Chapter 1 70
RIPv2

ITE PC v4.0
Introduction
 Chapter focus
-Difference between RIPv1 & RIPv2
RIPv1
-A classful distance vector routing protocol
-Does not support discontiguous subnets
-Does not support VLSM
-Does not send subnet mask in routing update
-Routing updates are broadcast
RIPv2
-A classless distance vector routing protocol that is an
enhancement of RIPv1’s features.
-Next hop address is included in updates
-Routing updates are multicast
-The use of authentication is an option

ITE PC v4.0
Introduction
 Similarities between RIPv1 & RIPv2
-Use of timers to prevent routing loops
-Use of split horizon or split horizon with poison
reverse
-Use of triggered updates
-Maximum hop count of 15

ITE PC v4.0
RIPv1 Limitations
Examining the routing tables
-To examine the contents of
routing updates use the
debug ip rip command
-If RIPv1 is
configured then
Subnet masks will not be
included with the
network address

ITE PC v4.0
Configuring RIPv2
 Enabling and Verifying RIPv2
 Configuring RIP on a Cisco router
By default it is running RIPv1

ITE PC v4.0
Configuring RIPv2
 Configuring RIPv2 on a
Cisco router
-Requires using the
version 2 command
-RIPv2 ignores RIPv1
updates
 To verify RIPv2 is
configured use the
show ip protocols
command

ITE PC v4.0
Configuring RIPv2
 Auto-Summary & RIPv2
 RIPv2 will automatically
summarize routes at
major network
boundaries and can
also summarize routes
with a subnet mask that
is smaller than the
classful subnet mask

ITE PC v4.0
 Disabling Auto-
Summary in RIPv2
 To disable automatic
summarization issue
the no auto-summary
command
Configuring RIPv2

ITE PC v4.0
VLSM & CIDR
 Networks using a VLSM IP
addressing scheme
Use classless
routing protocols (i.e.
RIPv2) to disseminate
network addresses
and their subnet
masks

ITE PC v4.0
Verifying & Troubleshooting RIPv2
 Basic Troubleshooting steps
-Check the status of all links
-Check cabling
-Check IP address & subnet mask configuration
-Remove any unneeded configuration commands
 Commands used to verify proper operation of RIPv2
–Show ip interfaces brief
–Show ip protocols
–Debug ip rip
–Show ip route

ITE PC v4.0
 Common RIPv2 Issues
 When trouble shooting RIPv2 examine the following issues:
Version
Check to make sure you are using version 2
Network statements
Network statements may be incorrectly typed
or missing
Automatic summarization
If summarized routes are not needed then disable
automatic summarization

ITE PC v4.0
 Reasons why it’s good to authenticate routing information
-Prevent the possibility of accepting invalid routing
updates
-Contents of routing updates are encrypted
 Types of routing protocols that can use authentication
-RIPv2
-EIGRP
-OSPF
-IS-IS
-BGP

ITE PC v4.0
Summary
Routing
Protocol
Distance
Vector
Classless
Routing
Protocol
Uses
Hold-
Down
Timers
Use of
Split
Horizon
or
Split
Horizon
w/
Poison
Reverse
Max
Hop
count
= 15
Auto
Summary
Support
CIDR
Supports
VLSM
Uses
Authen-
tication
RIPv1 Yes No Yes Yes Yes Yes No No No
RIPv2 Yes Yes Yes Yes Yes Yes Yes Yes Yes

ITE PC v4.0

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