Download network - Victoria College

Switching Basics and Intermediate
Routing CCNA 3
Chapter 1
www.ciscopress.com
VLSM
• Variable-length subnet masks were developed to
allow multiple levels of subnetted IP addresses
within a single network
• The routing protocol you use must support
VLSM
– Open Shortest Path First (OSPF)
– Enhanced Interior Gateway Routing Protocol (EIGRP)
– Routing Information Protocol version 2 (RIPv2)
• VLSM is crucial for an effective IP addressing
plan
www.ciscopress.com
VLSM
Prefix Length
• Prefix length is a shorthand way for
expressing the subnet mask for a
particular network
– Number of 1s in the binary representation of
the subnet mask
• When bits are taken from the host part of an
address and added to the network part, the
number of the bits in the host part decreases
– You create additional subnets at the expense of the
number of host devices on each network segment
www.ciscopress.com
VLSM
Prefix Length
• Number of subnets can be calculated using the
2s formula, where s is the number of bits by
which the default mask is extended
• In IOS releases prior to 12.0, you must explicitly
allow subnet 0
• In IOS releases 12.0 and later, subnet 0 is
enabled by default
• The all-1s subnet has always been allowed
www.ciscopress.com
VLSM
Prefix Length
• Bits that are not part of the network or
subnetwork portions of the address are the
range of host address
• Use the 2h – 2 formula (where h is the number of
host bits) to calculate available host addresses;
all 0s in host portion is the subnet identifier
address, all 1s in host portion is the subnet
broadcast address
www.ciscopress.com
VLSM
Prefix Length
Network Mask and IP Address for the Range 192.168.1.64
Through 192.168.1.79, with Host Bits Shaded
• In the IP network number that accompanies the network
mask, the following are true:
– When the host bits are all binary 0s, that address is the
beginning of the address range
– When the host bits are all binary 1s, that address is at the end of
the address range
www.ciscopress.com
VLSM
Prefix Length
Fourth Octet
for the
Range
192.168.1.64
Through
192.168.1.79
(continued
on next slide)
www.ciscopress.com
VLSM
Prefix Length (continued)
Fourth Octet
for the
Range
192.168.1.64
Through
192.168.1.79
(continued)
www.ciscopress.com
VLSM
Prefix Length
• In this example, PCs use the prefix length of 28 (the
subnet mask 255.255.255.240) to determine which other
devices on their local network have their first 28 bits in
common
– A 28-bit prefix length permits 14 hosts per subnet
• The PC uses ARP to find the corresponding destination
MAC address if communication with any of these devices
is necessary
• If the destination IP address is not in the range for the
subnet, the packet is forwarded to the default gateway
www.ciscopress.com
VLSM
Prefix Length
• A router works in a similar manner when it makes
a routing decision
– It compares the destination IP address of the packet to
network entries in the routing table
– The network entries have a prefix length associated
with them
– The router uses the prefix length to determine how
many destination bits must match to send the packet
out the corresponding outbound interface that is
associated with the network number in the routing table
www.ciscopress.com
VLSM
Prefix Length
• The router determines from the table where to send the
packet destined for 192.168.1.67
– In this table, there are four entries for network 192.168.1.0
– The third entry is for the 192.168.1.64 subnet, which is the subnet
to which 192.168.1.67 belongs
– Note that the next subnet, 192.168.1.80, begins with a number
larger than 192.168.1.67
www.ciscopress.com
VLSM
Benefits of VLSM
• More efficient use of IP addresses
– Without use of VLSM, a single subnet mask must be
implemented with an entire Class A, B, or C network
• Greater capacity to use router summarization
(discussed later in this chapter)
– Allows more hierarchical levels within an addressing
plan
• Isolation of topology changes from other routers
www.ciscopress.com
VLSM
Benefits of VLSM
VLSM Permits Flexible, Efficient Subnet Address Allocation
www.ciscopress.com
VLSM
VLSM Calculations
• VLSM is used to maximize number of possible IP
addresses available for a network
– Point-to-point serial links require only two host
addresses, so a /30 subnet does not waste scarce
subnet addresses
• With VLSM, you can subnet a subnet!
• Next slide will show how the subnet
172.16.32.0/20 is further subnetted with a /26
prefix
www.ciscopress.com
VLSM
VLSM Calculations
Further Subnetting 172.16.32.0/20 to /26 Prefixes
www.ciscopress.com
VLSM
VLSM Example
VLSM Used to Define Subnets of 172.16.32.0 Across the
Boundary Between Octets Three and Four
www.ciscopress.com
VLSM
CIDR and Route Summarization
• The definition of classless inter-domain routing (CIDR):
– Allocation of one or more blocks of Class C network numbers to
each network service provider
– Organizations using the network service provider for Internet
connectivity are allocated bitmask-oriented subsets of the
provider’s address space as required
• CIDR (“cider”) was developed to address the problem of IP
address space running out and core Internet routers
running out of capacity
• Route summarization is the representation by a single
network of a group of contiguous networks
www.ciscopress.com
VLSM
CIDR and Route Summarization
Route
Summarization
of Contiguous
Subnets of a
Class B Network
www.ciscopress.com
VLSM
CIDR and Route Summarization
Route Summarization of Contiguous Subnets of a Class B
Network (continued)
• Router D in previous slide has these networks in its
routing table
–
–
–
–
172.16.12.0/24
172.16.13.0/24
172.16.14.0/24
172.16.15.0/24
• To calculate the summary route:
– Find the number of highest-order bits that match in all addresses
– Locate where the common pattern of digits ends
– Count the number of common bits; this is the length of the
summary route
www.ciscopress.com
VLSM
CIDR and Route Summarization
Route Summarization of Contiguous Subnets of a Class B
Network (continued)
• Follow these guidelines when calculating summary routes:
– Addresses that do not share the same number of bits as the prefix
length of the summary route are not included in the summarization
block
– The IP addressing plan is hierarchical in nature to allow router to
aggregate the largest number of IP addresses into a single
summary route
– IP networks can only be summarized in 2n networks (for some n),
where the last octet of the first network in the sequence is divisible
by 2n
www.ciscopress.com
VLSM
Route Aggregation
• By using a prefix length instead of an address class to
determine the network portion of the address, CIDR allows
routers to aggregate routing information
– Shrinks routing table
– One address and mask combination can represent the routes to
multiple networks
• Route aggregation is used more loosely than CIDR;
describes the summarization of classful networks
• Without CIDR, routers must maintain tables for individual
networks
www.ciscopress.com
VLSM
Route Aggregation
CIDR Permits the Aggregation of Contiguous Class B
Networks
www.ciscopress.com
VLSM
Route Aggregation
Summarization Employs the Furthest-to-the-Right Principle
www.ciscopress.com
VLSM
Route Aggregation
• In previous slide, the router can summarize routes
to these networks using a 13-bit prefix which these
8 networks share
– 10101100 00011000 00000000 00000000 = 172.24.0.0
– 11111111 11111000 00000000 00000000 = 255.248.0.0
• A single address and mask define a classless prefix
that summarizes routes to the eight networks:
172.24.0.0/13
www.ciscopress.com
VLSM
Route Aggregation
• Using a prefix to summarize routes results in
the following:
– More efficient routing
– A reduced number of CPU cycles when calculating
a routing table or sorting through routing table
entries to find a match
– Reduced router memory requirements
www.ciscopress.com
VLSM
Supernetting
• The practice of using a summary network to group
multiple classful networks into a single address is
called supernetting
– Subnetting breaks down a classful network
– Supernetting pastes together classful networks
• With Class A and B address space almost exhausted,
large organizations requested multiple Class C
network addresses from their service providers
• A block of contiguous Class C addresses can appear
as a single large network, or supernet
www.ciscopress.com
VLSM
Supernetting
• Supernetting and route aggregation are similar
– Route aggregation is used in the context of
summarizing routes with BGP
– Supernetting is a term used when the summarized
networks are under common administrative control
• Many networking professionals use the terms
“route summarization” and “route aggregation”
interchangeably
www.ciscopress.com
VLSM
CIDR Example
CIDR Permits the Aggregation of Several Classful
Networks into a Single Route Advertisement
www.ciscopress.com
Classful and Classless Routing
• Behavior of classful routing is limited
compared to classless routing
– Classful routing protocols(RIPv1, IGRP) cannot do
VLSM
• Make routing decisions and send routing updates
according to Class A, B, and C constructs
– Classless routing protocols work independently of
Class A, B, and C addresses
• In the “real world,” classful routing protocols
are close to becoming irrelevant
www.ciscopress.com
Classful and Classless Routing
Classful Routing
• RIPv1 and IGRP are the two classful routing protocols
– Rare to see either of these employed on a router today
– Classful routing protocols do not include subnet mask
information in their updates
• The router applies two options when receiving a
routing update packet
– If the routing update information contains the same major
network number as configured on the receiving interface, the
router applies the subnet mask that is configured on that
interface
– If the routing update information contains a different major
network than the one configured on the the receiving
interface, the router applies the default subnet mask
www.ciscopress.com
Classful and Classless Routing
Classful Routing
• The router applies two options when receiving
a routing update packet (continued)
– The default classful masks are:
• Class A: 255.0.0.0
• Class B: 255.255.0.0
• Class C: 255.255.255.0
• All subnets of the same major network
(Classes A, B, and C) must use the same
mask when using a classful routing protocol
www.ciscopress.com
Classful and Classless Routing
Classful Routing
• Routers running a classful routing protocol perform
automatic route summarization across network
boundaries
– They make assumptions about networks based on their IP
address class
– These assumptions lead to automatic summarization of routes
when routers send routing updates across major classful
network boundaries
• Routers send update packets to other connected
routers
– Routers sends entire subnet address (without mask); assume
the network and the interface use the same subnet mask
www.ciscopress.com
Classful and Classless Routing
Classful Routing
• Router receiving the update makes the same
assumption
– If different masks are used, router would have wrong
information in routing table
– Important to use the same subnet mask on all interfaces that
belong to the same classful network
• When a router using a classful protocol sends an
update regarding information of a subnet of a classful
network across an interface belonging to a different
classful network, the router assumes the remote router
will use the default subnet mask for that IP address
class
www.ciscopress.com
Classful and Classless Routing
Classful Routing
Automatic Summarization Occurs at Classful
Boundaries with RIPv1 and IGRP
www.ciscopress.com
Classful and Classless Routing
Classful Routing
• The process in the previous slide is automatic
summarization across the network boundary
– Router sends a summary of all the subnets by
sending only major network information
– Classful routing protocols automatically create a
classful summary route at major network
boundaries
– Classful routing protocols do not allow
summarization at other points within the major
network space
www.ciscopress.com
Classful and Classless Routing
Classful Routing
• The router that receives the updates behaves
in a similar fashion
– When a routing update contains information about
a different classful network than the one that is in
use on its interface, the router applies the default
classful mask to that update
• When using classful routing protocols,
assigning the same subnet mask to all subnets
is called fixed-length subnet masking (FLSM)
– sometimes called static-length subnet
masking
www.ciscopress.com
Classful and Classless Routing
Discontiguous Subnets
• A classical problem with classful routing
protocols:
– Discontiguous subnets occur when a major network
separates subnets of a major network
– This can cause erroneous entries in routing tables
– Traffic will not always reach its destination
• Do not permit the use of discontiguous
networks when using a classful routing
protocol
www.ciscopress.com
Classful and Classless Routing
Discontiguous Subnets
Discontiguous Subnets Present a Problem with
Classful Routing
www.ciscopress.com
Classful and Classless Routing
Default Routes
• Routers learn paths to destinations in three
ways:
– The system administrator defines static routes via
an attached interface or the next hop to a
destination
– The network engineer manually defines default
routes as the path to take when no known route
exists to the destination; default routes minimize
the size of the routing table
– Dynamic routing occurs when the router learns of
paths to destinations by receiving routing updates
from other routers via a routing protocol
www.ciscopress.com
Classful and Classless Routing
Default Routes
• You can define a static route with the ip route
command:
• You can define a default route with the
ip default-network command:
www.ciscopress.com
Classful and Classless Routing
Default Routes
A Default Network is Configured Pointing Toward the
Internet
www.ciscopress.com
Classful and Classless Routing
Default Routes
• You can define a default route to work with either static
or dynamic routing:
• The 0s represent any destination with any mask
• Default routes are often referred to as quad-zero
routes
www.ciscopress.com
Classful and Classless Routing
Classful Routing Table
• What does a router running a classful routing
protocol do with packets that lie in subnets that
have no entry in the routing table?
– The router discards the packets!
• This can be overcome by using the ip classless
command
– Causes the router using a classful routing protocol to
evaluate all packets using the longest-match criterion
– As a last resort, the router uses a configured default
route
www.ciscopress.com
Classful and Classless Routing
Classless Routing
• All routing protocols except RIPv1 and IGRP are
classless routing protocols
• RIPv2, OSPF, IS-IS, EIGRP, and BGPv4 are
classless routing protocols that support VLSM
and CIDR
• With classless routing protocols, different
subnets in the same major network can have
different subnet masks
– Maximizes use of addresses
www.ciscopress.com
Classful and Classless Routing
Classless Routing
• Classful routing protocols automatically
summarize to the classful network boundary;
classless routing protocols allow you to control
the route summarization process manually
(might be needed to limit size of routing tables)
• Classless routing protocols do not automatically
advertise every subnet
• By default, classless routing protocols perform
automatic network summarization at classful
boundaries, just like classful protocols
www.ciscopress.com
Classful and Classless Routing
Classless Routing
• Difference between classless routing protocols
and their predecessors is that you can manually
turn off automatic summarization
– Use the no auto-summary command
– Not needed with OSPF or IS-IS
• Automatic summarization can cause problems in
networks with discontiguous subnets
– This can be fixed by turning off automatic
summarization
www.ciscopress.com
Classful and Classless Routing
Classless Routing
Discontiguous Subnets Presenting a Problem with
Classless Routing
www.ciscopress.com
Classful and Classless Routing
Effect of Auto-Summary and No Auto-Summary
• Beginning with IOS Release 12.2(8)T, EIGRP and BGP
had auto-summary enabled by default
• RIPv2 has always had auto-summary enabled by default
Default Behavior of RIPv2 is to Automatically Summarize at
the Network Boundary
www.ciscopress.com
Classful and Classless Routing
Effect of Auto-Summary and No Auto-Summary
RIPv2 Supports VLSM with Automatic
Summarization Disabled
www.ciscopress.com
Classful and Classless Routing
Effect of Auto-Summary and No Auto-Summary
• To disable auto-summary in RIPv2, use the
no auto-summary command as seen below
www.ciscopress.com
RIP Version 2
• RIP Version 1 characteristics
– Uses hop count as the metric for path selection
– Maximum allowable hop count is 15, so infinite
distance equals 16 hops
– Uses hold-down timers to prevent routing loops with a
default of 180 seconds
– Employs split horizon to prevent routing loops
– Failure to receive routing updates in a timely manner
results in removal of routes previously learned from a
neighbor
www.ciscopress.com
RIP Version 2
• RIP Version 1 characteristics (continued)
– The administrative distance is 120
– Routing updates are broadcast every 30
seconds by default
– Is capable of load-balancing over as many as
six equal-cost paths; four is the default
– Does not support authentication
– Does not support VLSM because it is a
classful routing protocol
www.ciscopress.com
RIP Version 2
• RIP Version 2 characteristics
– Uses hop count as the metric for path selection
– Maximum allowable hop count is 15, so infinite
distance equals 16 hops
– Uses hold-down timers to prevent routing loops with a
default of 180 seconds
– Employs split horizon to prevent routing loops
– Failure to receive routing updates in a timely manner
results in removal of routes previously learned from a
neighbor
www.ciscopress.com
RIP Version 2
• RIP Version 2 characteristics (continued)
– The administrative distance is 120
– Routing updates are multicast every 30 seconds by
default
– Is capable of load-balancing over as many as six
equal-cost paths; four is the default
– Supports clear text and Message Digest 5 (MD5)
authentication
– Supports VLSM because it is a classless routing
protocol
– Supports manual route summarization
www.ciscopress.com
RIP Version 2
• Major improvements with RIPv2:
– Support of authentication
• Clear text is the default
• MD5 used to encrypt enable secret passwords
– VLSM use
– Sending subnet masks in updates
– Multicasting routing updates
• Uses 224.0.0.9 as destination
• Keeps PCs and servers from having to process the
broadcast
www.ciscopress.com
RIP Version 2
– Multicasting routing updates (continued)
• Keeps PCs and servers from having to process the
broadcast (continued)
– IP sends the packet to the User Datagram Protocol (UDP)
and UDP checks whether RIP port 520 is available; most
PCs and servers do not have a process running on this
port and discard the packet
– Sometimes it is running as a gateway discovery
technique in TCP/IP services, such as UNIX or Windows
www.ciscopress.com
RIP Version 2
• Broadcast disadvantages of RIPv1
– RIPv1 can fit up to 25 networks/subnets in
each update; updates are sent every 30
seconds
• If the routing table has 1000 subnets, 40 packets
will be sent every 30 seconds
• Each of these broadcasts will have to be looked at
by all devices on the network
www.ciscopress.com
RIP Version 2
• Multicast advantages of RIPv2
– The IP multicast address for RIPv2 has its own MAC
address: 0x0100.5e00.0009
– Devices such as PCs and servers read this MAC
address and determine it is not for them; they discard
the frame
– If a device can’t distinguish this MAC address, the
packet will be discarded at the IP layer (OSI network
layer) as the multicast IP address is not the IP address
of the device
www.ciscopress.com
RIPv2 Configuration
• The router rip command starts a RIP
routing process; the network command
causes the implementation of these three
functions:
– Routing updates are multicast out an interface
– Routing updates are processed if they enter
that same interface
– The subnet that is directly connected to that
interface is advertised
www.ciscopress.com
RIPv2 Configuration
Sample Network and Configuration of RIPv2
www.ciscopress.com
RIPv2 Configuration
• In the previous slide, these commands
were used to configure Router A:
– Enable RIP as the routing protocol: router RIP
– Identify Version 2 as the RIP being used:
version 2
– Specifying a directly connected network:
network 172.16.0.0
– Specifying a directly connected network:
network 10.0.0.0
www.ciscopress.com
Verifying RIP Configuration
Sample Network for Verifying RIP
Configuration
www.ciscopress.com
Verifying RIP Configuration
• Most common commands for verifying RIP
Configuration:
– Display parameters for routing protocols: show ip
protocols
– Summary of IP information and status of all interfaces:
show ip interface brief
– Ensure that appropriate commands are configured for
the RIP network: show running-config
– Display contents of routing table: show ip route
www.ciscopress.com
Verifying RIP Configuration
www.ciscopress.com
Verifying RIP Configuration
www.ciscopress.com
Verifying RIP Configuration
Fields in the Routing Table Defined
www.ciscopress.com
Troubleshooting RIP Configuration
Sample Network for Troubleshooting RIP
Configuration
The debug ip rip command displays real-time RIP routing updates
as they are sent and received
To turn off debugging, use the no debug ip rip or the undebug all
(u all) commands
www.ciscopress.com
Troubleshooting RIP Configuration
The debug ip rip command
www.ciscopress.com
Troubleshooting RIP Configuration
Sample debug ip rip output
www.ciscopress.com
Summary
• Classless IP addressing is implemented with:
– VLSM: the ability to subnet a subnet and use different
subnet masks in the same classful network
– CIDR: the allocation of blocks of contiguous address
space to customers by ISPs
– Route summarization: a generic term that describes
the use of a single network to represent a sequence of
logically contiguous networks
– Route aggregation: a generalized form of supernetting
– Supernetting: pasting together classful networks into
supernets
www.ciscopress.com
Summary
• Classful routing protocols:
– RIPv1
– IGRP
• Classless routing protocols:
–
–
–
–
–
RIPv2
EIGRP
OSPF
IS-IS
BGPv4
www.ciscopress.com
Summary
• RIPv2, EIGRP, and BGPv4 can turn automatic route
summarization on and off
• RIPv2 is an improvement to RIPv1
– Adds authentication, VLSM support, passing of subnet
masks in routing updates, and multicasting of routing
updates
• Configuring RIPv2 requires adding the version 2
command; adding no auto-summary is recommended
• All connected networks participating in RIP are defined
with the network command in the form of classful
networks
www.ciscopress.com
Summary
• RIP configuration can be verified with several
commands: show ip protocols, show ip
interface brief, show running-config, and
show ip route
• You can troubleshoot RIP with the debug ip rip
command
www.ciscopress.com