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© 2007 Cisco Systems, Inc. All rights reserved. Cisco PublicITE PC v4.0Chapter 1 1

VLSM and CIDR

ITE PC v4.0Chapter 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


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


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)


Classful and Classless IP Addressing

The High Order Bits

These are the leftmost bits in a 32 bit address


Classful and Classless IP Addressing 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.



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





Purpose of a subnet mask

It is used to determine the network portion of an IP address


Classful and Classless IP Addressing 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


Classful and Classless IP Addressing Classless Inter-domain Routing (CIDR – RFC 1517)

Advantage of CIDR :

-More efficient use of IPv4 address space

-Route summarizationRequires 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


Classful and Classless IP Addressing Classless Routing Protocol

Characteristics of classless routing protocols:

-Routing updates include the subnet mask

-Supports VLSM

Supports Route Summarization



RoutingProtocol

Routing updatesInclude subnet Mask

Supports VLSM

Ability to sendSupernet routes

Classful No No No

Classless Yes Yes Yes

Classless Routing Protocol


Classless Inter-Domain Routing (CIDR) 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


Classless Inter-Domain Routing (CIDR) 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


Distance Vector Routing Protocols


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.


Why dynamic routing protocol


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)


Distance Vector Routing Protocols 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


Distance Vector Routing ProtocolsCharacteristics of Distance Vector routing protocols:

Periodic updates

Broadcast updates

Entire routing table is included into routing update


Distance Vector Routing Protocols Routing Protocol Algorithm:

-Defined as a procedure for accomplishing a certain task


Distance Vector Routing ProtocolsRouting Protocol Characteristics

–Criteria used to compare routing protocols includes

Time to convergence

Scalability

Resource usage Implementation & maintenance


Distance Vector Routing Protocols


Network Discovery Router initial start up

-Initial network discovery

Directly connected networks are initially placed in routing table


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


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


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


Routing Table Maintenance Periodic Updates: RIPv1 & RIPv2

These are time intervals in which a router sends out its entire routing table.


Routing Table Maintenance RIP uses 4 timers

-Update timer

-Invalid timer

-Hold-down timer

-Flush timer


Routing Table Maintenance 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.


Routing Table Maintenance 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


Routing Table Maintenance Triggered Updates

–Conditions in which triggered updates are sent

-Interface changes state

-Route becomes unreachable

-Route is placed in routing table


Routing Loops

Routing loops are

A condition in which a packet is continuously transmitted within a series of routers without ever reaching its destination.


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


Routing Loops problems Count to Infinity

This is a routing loop whereby packets bounce infinitely around a network.


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


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 timersAllows 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.


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.


Update, holddown & flush timers

Last update about certain

network

Invalid timer Flush timerHolddown timer

180 sec 180 sec 240 sec


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


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.


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


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


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.


Routing Protocols Today Factors used to determine whether to use RIP or EIGRP

include

-Network size

-Compatibility between models of routers

-Administrative knowledge


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


RIP version 1


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


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


RIP v1 message


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


Basic RIPv1 Configuration

Router RIP Command

–To enable RIP enter:

-Router rip at the global configuration prompt

-Prompt will look like R1(config-router)#


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


Verification and Troubleshooting

To verify and troubleshoot routing

-Use the following

commands:

-show ip route

-show ip protocols

-debug ip rip


Interpreting RIP route



show ip protocols command

-Displays routing protocol configured on router


Verification and Troubleshooting Debug ip rip command

-Used to display RIP routing updates as they are happening



Passive interface command

-Used to prevent a router from sending updates through an interface

-Example:

Router(config-router)#passive-interface interface-type interface-number


Verification and Troubleshooting Passive interfaces


Automatic Summarization Boundary Routers

–RIP automatically summarizes classful networks

–Boundary routers summarize RIP subnets from one major network to another.


Automatic Summarization

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.


Automatic Summarization Sending RIP Updates

–RIP uses automatic summarization to reduce the size of a routing table.



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


Automatic Summarization Disadvantage of Automatic Summarization:

-Does not support discontiguous networks



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.


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


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.


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


RIPv2


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


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


RIPv1 LimitationsExamining 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


Configuring RIPv2

Enabling and Verifying RIPv2

Configuring RIP on a Cisco router

By default it is running RIPv1


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


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


Disabling Auto-Summary in RIPv2

To disable automatic summarization issue the no auto-summary command

Configuring RIPv2


VLSM & CIDR

Networks using a VLSM IP addressing scheme

Use classless routing protocols (i.e. RIPv2) to disseminate

network addresses and their subnet

masks


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


Verifying & Troubleshooting RIPv2 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


Verifying & Troubleshooting RIPv2 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


Summary

RoutingProtocol

DistanceVector

ClasslessRoutingProtocol

UsesHold-DownTimers

Use ofSplit

Horizon or

Split Horizon

w/Poison

Reverse

MaxHop

count= 15

AutoSummary

SupportCIDR

SupportsVLSM

Uses Authen-tication

RIPv1 Yes No Yes Yes Yes Yes No No No

RIPv2 Yes Yes Yes Yes Yes Yes Yes Yes Yes
