Title: Switching%20Basics%20and%20Intermediate%20Routing%20CCNA%203%20Chapter%201
1Switching Basics and Intermediate Routing CCNA
3Chapter 1
2VLSM
- 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
3VLSMPrefix 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
4VLSMPrefix 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
5VLSMPrefix 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
6VLSMPrefix 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
7VLSMPrefix Length
- Fourth Octet for the Range 192.168.1.64
Through 192.168.1.79 (continued on next slide)
8VLSMPrefix Length (continued)
- Fourth Octet for the Range 192.168.1.64
Through 192.168.1.79 - (continued)
9VLSMPrefix 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
10VLSMPrefix 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
11VLSMPrefix 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
12VLSMBenefits 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
13VLSMBenefits of VLSM
- VLSM Permits Flexible, Efficient Subnet Address
Allocation
14VLSMVLSM 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
15VLSMVLSM Calculations
- Further Subnetting 172.16.32.0/20 to /26 Prefixes
16VLSMVLSM Example
- VLSM Used to Define Subnets of 172.16.32.0 Across
the Boundary Between Octets Three and Four
17VLSMCIDR 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 providers
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
18VLSMCIDR and Route Summarization
- Route Summarization of Contiguous Subnets of a
Class B Network
19VLSMCIDR 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
20VLSMCIDR 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
21VLSMRoute 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
22VLSMRoute Aggregation
- CIDR Permits the Aggregation of Contiguous Class
B Networks
23VLSMRoute Aggregation
- Summarization Employs the Furthest-to-the-Right
Principle
24VLSMRoute 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
25VLSMRoute 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
26VLSMSupernetting
- 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
27VLSMSupernetting
- 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
28VLSMCIDR Example
- CIDR Permits the Aggregation of Several Classful
Networks into a Single Route Advertisement
29Classful 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
30Classful and Classless RoutingClassful 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
31Classful and Classless RoutingClassful 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
32Classful and Classless RoutingClassful 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
33Classful and Classless RoutingClassful 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
34Classful and Classless RoutingClassful Routing
- Automatic Summarization Occurs at Classful
Boundaries with RIPv1 and IGRP
35Classful and Classless RoutingClassful 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
36Classful and Classless RoutingClassful 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
37Classful and Classless RoutingDiscontiguous
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
38Classful and Classless RoutingDiscontiguous
Subnets
- Discontiguous Subnets Present a Problem with
Classful Routing
39Classful and Classless RoutingDefault 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
40Classful and Classless RoutingDefault Routes
- You can define a static route with the ip route
command
- You can define a default route with the
- ip default-network command
41Classful and Classless RoutingDefault Routes
- A Default Network is Configured Pointing Toward
the Internet
42Classful and Classless RoutingDefault 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
43Classful and Classless RoutingClassful 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
44Classful and Classless RoutingClassless 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
45Classful and Classless RoutingClassless 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
46Classful and Classless RoutingClassless 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
47Classful and Classless RoutingClassless Routing
- Discontiguous Subnets Presenting a Problem with
Classless Routing
48Classful and Classless RoutingEffect 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
49Classful and Classless RoutingEffect of
Auto-Summary and No Auto-Summary
- RIPv2 Supports VLSM with Automatic Summarization
Disabled
50Classful and Classless RoutingEffect of
Auto-Summary and No Auto-Summary
- To disable auto-summary in RIPv2, use the
- no auto-summary command as seen below
51RIP 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
52RIP 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
53RIP 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
54RIP 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
55RIP 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
56RIP 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
57RIP 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
58RIP 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 cant 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
59RIPv2 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
60RIPv2 Configuration
- Sample Network and Configuration of RIPv2
61RIPv2 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
62Verifying RIP Configuration
- Sample Network for Verifying RIP Configuration
63Verifying 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
64Verifying RIP Configuration
65Verifying RIP Configuration
66Verifying RIP Configuration
- Fields in the Routing Table Defined
67Troubleshooting 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
68Troubleshooting RIP Configuration
69Troubleshooting RIP Configuration
- Sample debug ip rip output
70Summary
- 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
71Summary
- Classful routing protocols
- RIPv1
- IGRP
- Classless routing protocols
- RIPv2
- EIGRP
- OSPF
- IS-IS
- BGPv4
72Summary
- 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
73Summary
- 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