Transcript Module 1

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
IPv6 252 wasted host addresses
• 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 VLSM
Summarisation
• 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
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A router must know in detail the subnet
numbers attached to it
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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
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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 points
using a prefix address
until 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
designing
an addressing scheme
When
to use
VLSM
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
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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
decides to use
subnet zero, it
has 8 useable
subnets
ip subnet-zero
If management decide not to use subnet 0 – 7 useable subnets
no ip subnet-zero
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•
Distance vector protocol
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Broadcasts entire routing table to each neighbor router
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Broadcasts at intervals of 30 seconds
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Metric is hop count
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Classful routing – Class A, B, C
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Prevents routing loops using
RIP V1
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
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Popularity is based on
Simplicity and Load balancing over 6 equal-cost paths (4 paths default)
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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)
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Configuration
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• Distance vector protocol
• Metric is hop count
RIP V2
• 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
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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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erify 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
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the nominated interface of the default router