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CCNA 3 v3.1 Module 1

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CCNA 3 Module 1

Introduction to Classless Routing

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Classfull Routing

• IP V4 – class A, B,C

• Limited number of unique network addresses

• No subnet information sent in routing

• A single network must use the same subnet mask

Network 192.168.187.0 - 255.255.255.0

• Classful routing protocols

Cannot support Variable Length Subnet Mask (VLSM)

RIP V1, IGRP, EGP, BGP3

• WAN and LAN links have the same

Number of bits in subnet mask

Number of possible hosts

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IPv4 Address Classes

• No medium size host networks

• In the early days of the Internet, IP addresses were allocated to organizations based on request rather than actual need.

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IPv4 Address Classes

Class D Addresses

• A Class D address begins with binary 1110 in the first octet

• First octet range 224 to 239

• Class D address can be used to represent a group of hosts called a host group, or multicast group

Class E Addresses

• First octet of an IP address begins with 1111

• Class E addresses are reserved for experimental purposes and should not be used for addressing hosts or multicast groups

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IP Addressing CRISIS

• Address Depletion – shortage of ip addresses

• Internet Routing Table Explosion – increase in the size of internet routing tables

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A Waste of Space

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Class C Address

• LAN link has 28-2 = 254 possible hosts

• WAN link has 28-2 = 254 possible hosts

WAN link only needs 2 hosts

252 wasted host addressesIPv6• 128 bit address space

• Possible addresses

2128-2

340,283,366,920,938,463,374,670,431,768,211,456

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Variable Length Subnet Mask?

•Short term extensions to IPv4

•Subnetting 1985

•Variable length subnetting 1987

•Classless Interdomain Routing 1993•Allows for route aggregation and suppernetting

•Private IP addresses

•Network Address Translation (NAT)

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Routing protocols that all Classless routing

• Can have a variable length subnet mask

• Protocols

OSPF

EIGRP

IS-IS

RIP V2

Static Routing

BGP4

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Route Aggregation with VLSMSummarisation

• Represents a collection of IP Addresses within a Single IP Address

• Place networks close to one another to save routing table space

keeping networks like 172.16.14.0 and 172.16.15.0 near one another so that the routers need only carry a route for 172.16.14.0/23

Without this the internet backbone would collapse

• Route summarization (or supernetting)

only possible if the routers run a classless routing protocol

E.g.s OSPF and IS-IS

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Aggregation Rules

• A router must know in detail the subnet numbers attached to it

• A router does not need to tell other routers about each individual subnet if the router can send one aggregate route for a set of routers

• A router using aggregate routes would have fewer entries in its routing table

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• VLSM allows for

the summarization of routes and increases flexibly

• Bases summarization entirely on the higher-order bits shared on the left

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variable-sized networks / subnetworks is summarized at various pointsusing a prefix addressuntil the entire network is advertised as a single aggregate route

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What is a VLSM?

• Variable Length Subnet Mask

Use address space more efficiently

Use a long mask on networks with few hosts

Use a short mask on subnets with many hosts

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• VLSM allows a single autonomous system to have networks with different subnet masks

a 30-bit subnet mask on network connections

255.255.255.252 for networks with 2 nodes

Used for point-to-point connections

a 24-bit mask for user networks

255.255.255.0 for networks up to 252 users

Wasteful for point-to-point networks

even a 22-bit mask for user networks

255.255.252.0 for networks with up to 1000 users

• Do a class example

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When to use VLSM• When designing an addressing scheme

should allows for growth

not involve wasting addresses

• VLSM helps to manage IP Addresses

Set subnet mask to suit the link or the segment requirements

• To prevent waste of addresses use VLSM

Large subnets

Created for addressing LANs

Very small subnets

A 30-bit mask for subnets with only 2 valid host addresses

For a point-to-point connection

For WAN links

• Example on board

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First and Last Subnet Mask

• Previously it was recommended not to use

First subnet – zero subnet – network address

Last subnet - all-ones subnet – broadcast address

• VLSM allows

first and last subnets to be used

If management decide not to use subnet 0 – 7 useable subnetsno ip subnet-zero

If management decides to use subnet zero, it has 8 useable subnets ip subnet-zero

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RIP V1• Distance vector protocol

• Broadcasts entire routing table to each neighbor router

• Broadcasts at intervals of 30 seconds

• Metric is hop count

• Classful routing – Class A, B, C

• Prevents routing loops using

Maximum hop count – maximum 15 hops – after that packet is dropped

Split horizon – don’t teach the teacher

Holddown timers – ignore poorer metric information for 180 seconds

• Popularity is based on

Simplicity and Load balancing over 6 equal-cost paths (4 paths default)

• Limitations:

It does not send subnet mask information in its updates

It sends updates as broadcasts on 255.255.255.255.

It does not support authentication

It is not able to support VLSM or classless interdomain routing (CIDR)

• Configuration

Router(config)#router rip

Router(config-router)#network <network number>

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RIP V2

• Distance vector protocol

• Metric is hop count

• Prevents routing loops using

Uses a hop count metric- max 15 - 16 hops for infinite distance

Uses holddown timers to prevent routing loops – default 180sec

It uses split horizon to prevent routing loops

• Provides prefix routing

Send out subnet mask information with route update

• Supports classless routing (VLSM)

Different subnetworks can use different subnet masks

• Provides for authentication in its updates

Clear text authentication key is the default

Message-Digest 5 (MD5) encryption

Used to authenticate the source of a routing update

• It multicasts routing updates

Send routing updates to 224.0.0.9 instead of 255.255.255.255

• Uses External routing tags to separate RIP routes from externally learned routes

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Configuring RIP Version 2

• Router(config)#router rip

• Router(config-router)#version 2

• Router(config-router)#network <network no>

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Verify RIP V2 with show ip protocols

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Verifying Rip V2 using show ip route

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Other verifications for RIP V2

• Show ip interface brief

• Show running-config

• Show ip protocols

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Troubleshooting RIP V2

• debug ip rip

Displays rip routing updates as they are sent/recieved

• No debug all

Turns off debugging

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Default Routes

• By default, routers learn paths to destinations three different ways:

• Static routes – System administrator manually defines the static routes as the next hop to a destination - useful for security and traffic reduction, as no other route is known.

ip route <destination network> <subnet mask> <next hop>

• Default routes – System administrator manually defines default routes as the path to take when there is no known route to the destination - keep routing tables shorter

ip route 0.0.0.0 0.0.0.0 <next hop>

• Dynamic routes – Router learns of paths to destinations by receiving periodic updates from other routers

ip default-network <default network number>

All the packets that are not defined in the routing table will go to the nominated interface of the default router