Ch. 2 – Introduction to
Classless Routing
CCNA 3 – Spring 2008
Overview
IP Addressing
• Legacy Classful IP Addressing
– Depletion of IPv4 Address Space
– Subnetting
• Evolution
– Classless Addressing
– IPv6
• IPv4 Enhancements
– Classless interdomain routing (CIDR)
– Route summarization
– Network Address Translation (NAT/PAT)
– Variable length subnet masking (VLSM)
Routing Protocols
• Classful Routing Protocol (RIPv1)
• Classless Routing Protocol (RIPv2)
Rick Graziani graziani@cabrillo.edu
2
IP Addressing
•IP Addressing
•Legacy Classful IP Addressing
–Depletion of IPv4 Address Space
–Subnetting
•Evolution
–Classless Addressing
–IPv6
•IPv4 Enhancements
–Classless interdomain routing (CIDR)
–Route summarization
–Network Address Translation (NAT/PAT)
–Variable length subnet masking (VLSM)
Legacy IPv4 Classful Addresses
Rick Graziani graziani@cabrillo.edu
4
Classful IP Addressing
• In the early days of the Internet, IP addresses were allocated to
•
•
•
•
•
•
organizations based on request rather than actual need.
When an organization received an IP network address, that address
was associated with a “Class”, A, B, or C.
This class also determined the default or Major Mask for the
network.
This is known as Classful IP Addressing
The first octet of the address determined what class the network
belonged to and which bits were the network bits and which bits
were the host bits.
Until 1985, there were no subnet masks.
Formalized in 1985 (RFC 950), the subnet masks were introduced to
break a single class A, B or C network in to smaller pieces.
Rick Graziani graziani@cabrillo.edu
5
Classful: The class determines the Major Network Mask
Classful IP Addressing
• The Class determined the Major or Base Network Mask, also known as
the Default Mask.
• Classful IP Addressing the class determines the Major Network Mask.
194.168.1.3
Class C
Default Mask: 255.255.255.0
Network: 194.168.1.0
1.12.100.31
Class A
Default Mask: 255.0.0.0
Network: 1.0.0.0
150.30.77.5
Class B
Default Mask: 255.255.0.0
Network: 150.30.0.0
Rick Graziani graziani@cabrillo.edu
6
IP addressing crisis and Classless
Addressing
•
•
•
•
•
IPv4 Addressing Crisis
– Address Depletion
– Internet Routing Table explosion
1985 (RFC 950), the subnet masks
– Allowed organization to create their own
separate networks without requesting new
ones
1992 when the IETF introduced CIDR (Classless
Interdomain Routing), making the address class
meaning less.
– This is known as Classless IP Addressing.
Classless IP Addressing the ISP provides both
the network address and the major network
mask to the customer.
Today’s networks are classless, except for some
things like the structure of Cisco’s IP routing table
and Classful routing protocols like RIPv1 and
IGRP.
Rick Graziani graziani@cabrillo.edu
7
Classless: The ISP determines the mask
Classless IP Addressing
• The value of the first octet is meaningless.
• Classless IP Addressing the ISP provides the Major or Base (Default)
network mask.
194.168.1.3
Class C
Major Network Mask: 255.0.0.0
Network: 194.0.0.0
1.12.100.31
Class A
Major Network Mask: 255.255.0.0
Network: 1.12.0.0
150.30.77.5
Class B
Major Network Mask: 255.255.255.0
Network: 172.30.77.0
Rick Graziani graziani@cabrillo.edu
8
Subnetting
Classful IP Addressing
194.168.1.3
Class C
Default Mask: 255.255.255.0
Network: 194.168.1.0
Classless IP Addressing
194.168.1.3
Class C
Major Network Mask: 255.0.0.0
Network: 194.0.0.0
– Classless: Both the IP Address and Network Mask are provided
– Classful: The IP Address is provided and the Network Mask is
derived from the value of the first octet of the IP Address
Rick Graziani graziani@cabrillo.edu
9
All Zeros and All Ones Subnets
•
Number of usable subnets
– In the past, because of legacy equipment and software,
some devices could not use the all 0’s (first) and/or all
1’s (last subnets)
– In today’s networks, the all 0’s (first) and/or all 1’s (last
subnets) are usable subnets!
– To properly determine the number of usable subnets (for
example on an exam), it should be stated for clarity if
any of the subnets are not usable.
– It should never be assumed that the all 0’s (first) and/or
all 1’s (last subnets) are not usable subnets.
Rick Graziani graziani@cabrillo.edu
10
All Zeros and All Ones Subnets
Using the All Ones and All Zeroes Subnet
•
There is no command to enable or disable the use of the all-ones subnet, it is
enabled by default.
Router(config)#ip subnet-zero
•
The use of the all-ones subnet has always been explicitly allowed and the use
of subnet zero is explicitly allowed since Cisco IOS version 12.0.
RFC 1878 states, "This practice (of excluding all-zeros and all-ones subnets) is
obsolete! Modern software will be able to utilize all definable networks." Today,
the use of subnet zero and the all-ones subnet is generally accepted and most
vendors support their use, though, on certain networks, particularly the ones
using legacy software, the use of subnet zero and the all-ones subnet can lead
to problems.”
CCO: Subnet Zero and the All-Ones Subnet
http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note091
86a0080093f18.shtml
Rick Graziani graziani@cabrillo.edu
11
Need a Subnet Review?
•
If you need a review of subnets, please review the
following links:
•
•
•
•
•
Classless Subnetting Explained (PDF)
Worksheet: Classless Subnetting Worksheet (Excel spreadsheet)
Nutshell: Classless Subnetting in a Nutshell (Excel spreadsheet)
Nutshell: Classful Subnetting in a Nutshell
Article: Regional Internet Registry - How IP Addresses are
Allocated
Rick Graziani graziani@cabrillo.edu
12
Long Term Solution: IPv6 (coming)
•
•
•
•
•
•
IPv6, or IPng (IP – the Next Generation) uses a 128-bit address
space, yielding
340,282,366,920,938,463,463,374,607,431,768,211,456
possible addresses.
IPv6 has been slow to arrive
IPv4 revitalized by new features, making IPv6 a luxury, and not
a desperately needed fix
IPv6 requires new software; IT staffs must be retrained
IPv6 will most likely coexist with IPv4 for years to come.
For more information on IPv6, see:
–IPv6 Overview
Rick Graziani graziani@cabrillo.edu
13
IPv4 Enhancements
• Private Addressing - RFC 1918
• NAT/PAT (Network Address Translation / Port Address
•
•
Translation) – RFC
CIDR (Classless Inter-Domain Routing) – RFCs 1517,
1518, 1519, 1520
VLSM (Variable Length Subnet Mask) – RFC 1009
Rick Graziani graziani@cabrillo.edu
14
Private IP addresses (RFC 1918)
• If addressing any of the following, these private addresses can be used
instead of globally unique addresses:
– A non-public intranet (internal network)
– A test lab
– A home network
• This allows network administrators to assign many more IP Addresses
than what they may have been allocated by their provider.
Rick Graziani graziani@cabrillo.edu
15
IPv4 Enhancements
• Private Addressing - RFC 1918
• NAT/PAT (Network Address Translation / Port Address
•
•
Translation) – RFC
CIDR (Classless Inter-Domain Routing) – RFCs 1517,
1518, 1519, 1520
VLSM (Variable Length Subnet Mask) – RFC 1009
Rick Graziani graziani@cabrillo.edu
16
NAT Example
• Network Address Translation (NAT) allows Private IP Addresses to
•
•
•
be converted to public IP Addresses (one-to-one translation).
Port Address Translation (PAT) allows multiple private IP addresses
to be translated by a single public address (many-to-one translation).
This solves the limitation of NAT which is one-to-one translation.
We will examine NAT and PAT in more detail later this semester.
Rick Graziani graziani@cabrillo.edu
17
IPv4 Enhancements
• Private Addressing - RFC 1918
• NAT/PAT (Network Address Translation / Port Address
•
•
Translation) – RFC
CIDR (Classless Inter-Domain Routing) – RFCs 1517,
1518, 1519, 1520
VLSM (Variable Length Subnet Mask) – RFC 1009
Rick Graziani graziani@cabrillo.edu
18
CIDR (Classless Inter-Domain Routing)
• By 1992, members of the IETF were having serious concerns about the
•
•
•
•
•
exponential growth of the Internet and the scalability of Internet routing
tables.
The IETF was also concerned with the eventual exhaustion of 32-bit
IPv4 address space.
Projections were that this problem would reach its critical state by 1994
or 1995.
IETF’s response was the concept of Supernetting or CIDR, “cider”.
To CIDR-compliant routers, address class is meaningless.
– The network portion of the address is determined by the network
subnet mask or prefix-length (/8, /19, etc.)
– The first octet (first two bits) of the network address (or networkprefix) is NOT used to determine the network and host portion of the
network address.
CIDR helped reduced the Internet routing table explosion with
supernetting and reallocation of IPv4 address space.
Rick Graziani graziani@cabrillo.edu
19
Active BGP entries
Date of Graph: 22 Aug 2006
For information on BGP visit: http://www.potaroo.net/ispcol/2006-05/bgp.html
Rick Graziani graziani@cabrillo.edu
20
CIDR (Classless Inter-Domain Routing)
• First deployed in 1994, CIDR dramatically improves IPv4’s scalability
•
and efficiency by providing the following:
– Eliminates traditional Class A, B, C addresses allowing for more
efficient allocation of IPv4 address space.
– Supporting route aggregation (summarization), also known as
supernetting, where thousands of routes could be represented by a
single route in the routing table.
• Route aggregation also helps prevent route flapping on
Internet routers using BGP. Flapping routes can be a serious
concern with Internet core routers.
CIDR allows routers to aggregate, or summarize, routing information
and thus shrink the size of their routing tables.
– Just one address and mask combination can represent the routes to
multiple networks.
– Used by IGP routers within an AS and EGP routers between AS.
Rick Graziani graziani@cabrillo.edu
21
Without CIDR, a
router must
maintain
individual
routing table
entries for these
class B
networks.
With CIDR, a
router can
summarize
these routes
using a single
network
address by
using a 13-bit
prefix:
172.24.0.0 /13
Steps:
1. Count the number of left-most matching bits, /13 (255.248.0.0)
2. Add all zeros after the last matching bit:
172.24.0.0 = 10101100 00011000 00000000 00000000
Rick Graziani graziani@cabrillo.edu
22
CIDR (FYI)
• By using a prefix address to summarizes routes, administrators can
•
•
•
•
keep routing table entries manageable, which means the following
– More efficient routing
– A reduced number of CPU cycles when recalculating a routing
table, or when sorting through the routing table entries to find a
match
– Reduced router memory requirements
Route summarization is also known as:
– Route aggregation
– Supernetting
Supernetting is essentially the inverse of subnetting.
CIDR moves the responsibility of allocation addresses away from a
centralized authority (InterNIC).
Instead, ISPs can be assigned blocks of address space, which they
can then parcel out to customers.
Rick Graziani graziani@cabrillo.edu
23
ISP/NAP Hierarchy - “The Internet: Still hierarchical after all
these years.” Jeff Doyle (Tries to be anyways!)
NAP (Network Access Point)
Network
Service
Provider
Regional
Service
Provider
ISP
Subscribers
ISP
Subscribers
Rick Graziani graziani@cabrillo.edu
ISP
Subscribers
Network
Service
Provider
Regional
Service
Provider
Regional
Service
Provider
ISP
ISP
Subscribers
Subscribers
Regional
Service
Provider
ISP
Subscribers
ISP
Subscribers
ISP
Subscribers
24
Supernetting Example
• Company XYZ needs to address 400 hosts.
• Its ISP gives them two contiguous Class C addresses:
•
•
– 207.21.54.0/24
– 207.21.55.0/24
Company XYZ can use a prefix of 207.21.54.0 /23 to supernet
these two contiguous networks. (Yielding 510 hosts)
207.21.54.0 /23
– 207.21.54.0/24
– 207.21.55.0/24
Rick Graziani graziani@cabrillo.edu
23 bits in common
25
Supernetting Example
•
With the ISP acting as the addressing authority for a CIDR block of
addresses, the ISP’s customer networks, which include XYZ, can be
advertised among Internet routers as a single supernet.
Rick Graziani graziani@cabrillo.edu
26
CIDR and the Provider
Another example of route aggregation.
Rick Graziani graziani@cabrillo.edu
27
Summarization from the
customer networks to their
provider.
?
?
1. Count the number of left-most matching bits
2. Add all zeros after the last matching bit
200.199.48.32/27 11001000 11000111 00110000 00100000
200.199.48.64/27 11001000 11000111 00110000 01000000
200.199.48.96/27 11001000 11000111 00110000 01100000
?
?
200.199.56.0/24
200.199.57.0/24
?
Rick Graziani graziani@cabrillo.edu
11001000 11000111 0011100 0 00000000
11001000 11000111 0011100 1 00000000
?
28
Summarization from the
customer networks to their
provider.
200.199.48.0/25
200.199.56.0/23
1. Count the number of left-most matching bits
2. Add all zeros after the last matching bit
200.199.48.32/27
200.199.48.64/27
200.199.48.96/27
200.199.48.0/25
11001000 11000111 00110000 0 0100000
11001000 11000111 00110000 0 1000000
11001000 11000111 00110000 0 1100000
11001000 11000111 00110000 0 0000000
200.199.56.0/24
200.199.57.0/24
200.199.56.0/23
11001000 11000111 0011100 0 00000000
11001000 11000111 0011100 1 00000000
11001000 11000111 0011100 0 00000000
Rick Graziani graziani@cabrillo.edu
29
CIDR and the provider
200.199.48.0/25
200.199.56.0/23
Further summarization
happens with the next
upstream provider.
200.199.48.0/25
200.199.49.0/25
200.199.56.0/23
11001000 11000111 0011 0000 00000000
11001000 11000111 0011 0001 00000000
11001000 11000111 0011 1000 00000000
200.199.48.0/20
11001000 11000111 0011 0000 00000000
20 bits in common
Rick Graziani graziani@cabrillo.edu
30
CIDR Restrictions
• CIDR requires classless routing protocols for dynamic routing.
• Dynamic routing protocols must send network address and mask
•
(prefix-length) information in their routing updates.
However, you can still configure summarized static routes, after all, that
is what a 0.0.0.0/0 route is.
Rick Graziani graziani@cabrillo.edu
31
Summarized and Specific Routes: Longest-bit Match
(More Specific Match or Longest Match Wins)
Merida
Summarized Update
Specific Route Update
172.16.0.0/16
172.16.1.0/24
172.16.5.0/24
172.16.5.0/24
Quito
Cartago
172.16.2.0/24 172.16.10.0/24
• Merida receives a summarized /16 update from Quito and a more
•
•
•
specific /24 update from Cartago.
Merida will include both routes in the routing table.
Merida will forward all packets matching at least the first 24 bits of
172.16.5.0 to Cartago (172/16/5/0/24), longest-bit match.
Merida will forward all other packets matching at least the first 16 bits
to Quito (172.16.0.0/16).
Rick Graziani graziani@cabrillo.edu
32
Route flapping
•
•
•
•
•
•
Route flapping occurs when a router interface alternates rapidly between the
up and down states.
Route flapping can cripple a router with excessive updates and recalculations.
However, the summarization configuration prevents the RTC route flapping
from affecting any other routers.
The loss of one network does not invalidate the route to the supernet.
While RTC may be kept busy dealing with its own route flap, RTZ, and all
upstream routers, are unaware of any downstream problem.
Summarization effectively insulates the other routers from the problem of route
flapping.
Rick Graziani graziani@cabrillo.edu
33
IPv4 Enhancements
• Private Addressing - RFC 1918
• NAT/PAT (Network Address Translation / Port Address
•
•
Translation) – RFC
CIDR (Classless Inter-Domain Routing) – RFCs 1517,
1518, 1519, 1520
VLSM (Variable Length Subnet Mask) – RFC 1009
Rick Graziani graziani@cabrillo.edu
34
VLSM (Variable Length Subnet Mask)
• Limitation of using only a single
•
•
subnet mask across a given networkprefix (network address, the number
of bits in the mask) was that an
organization is locked into a fixednumber of fixed-sized subnets.
1987, RFC 1009 specified how a
subnetted network could use more
than one subnet mask.
VLSM = Subnetting a Subnet
– “If you know how to subnet, you
can do VLSM!”
Rick Graziani graziani@cabrillo.edu
Subnets
10.0.0.0/16
10.1.0.0/16
10.2.0.0/16
10.2.0.0/24
10.2.1.0/24
10.2.2.0/24
Etc.
10.2.255.0/24
10.3.0.0/16
Etc.
10.255.0.0/16
35
VLSM – Simple Example
1st octet
2nd octet
3rd octet
4th octet
10.0.0.0/8
10
Host
Host
Host
10.0.0.0/16
10
Subnet
Host
Host
10.0.0.0/16
10
0
Host
Host
10.1.0.0/16
10.2.0.0/16
10.n.0.0/16
10.255.0.0/16
10
10
10
10
1
2
…
255
Host
Host
Host
Host
Host
Host
Host
Host
• Subnetting a /8 subnet using a /16 mask gives us 256 subnets with
•
65,536 hosts per subnet.
Let’s take the 10.2.0.0/16 subnet and subnet it further…
Rick Graziani graziani@cabrillo.edu
36
VLSM – Simple Example
Network
Subnet
Host
Host
10.2.0.0/16
10
2
Host
Host
10.2.0.0/24
10
2
Subnet
Host
10.2.0.0/24
10.2.1.0/24
10
10
2
2
0
1
Host
Host
10.2.n.0/24
10.2.255.0/24
10
10
2
2
…
255
Host
Host
• Note: 10.2.0.0/16 is now a summary of all of the 10.2.0.0/24 subnets.
Rick Graziani graziani@cabrillo.edu
37
VLSM – Simple Example
10.0.0.0/8
Subnet
10.0.0.0/16
10.1.0.0/16
“subnetted using /16”
1st host
Last host
Broadcast
10.0.0.1
10.0.255.254
10.0.255.255
10.1.0.1
10.1.255.254
10.1.255.255
10.2.0.0/16 “sub-subnetted using /24”
–Subnet
1st host
Last host
Broadcast
–10.2.0.0/24
10.2.0.1
10.2.0.254
10.2.0.255
–10.2.1.0/24
10.2.1.1
10.2.1.254
10.2.1.255
–10.2.2.0/24
10.2.2.1
10.2.2.254
10.2.2.255
– Etc.
–10.2.255.0/24 10.2.255.1 10.2.255.254 10.2.255.255
10.3.0.0/16
Etc.
10.255.0.0/16
10.3.0.1
10.3.255.254
10.0.255.255
10.255.0.1 10.255.255.254 10.255.255.255
Rick Graziani graziani@cabrillo.edu
38
VLSM – Simple Example
Subnets
10.0.0.0/16
10.1.0.0/16
10.2.0.0/16
10.2.0.0/24
10.2.1.0/24
10.2.2.0/24
Etc.
10.2.255.0/24
10.3.0.0/16
Etc.
10.255.0.0/16
•
•
•
An example of VLSM, NOT of good network design.
10.1.0.0/16
10.2.0.0/24
10.2.1.0/24
10.7.0.0/16
10.3.0.0/16
10.2.6.0/24
10.4.0.0/16
10.2.3.0/24
10.2.8.0/24
10.5.0.0/16 10.8.0.0/16
10.2.5.0/24
10.6.0.0/16
10.2.4.0/24
Your network can now have 255 /16 subnets with 65,534 hosts each AND 256
/24 subnets with 254 hosts each.
All you need to make it work is a classless routing protocol that passes the
subnet mask with the network address in the routing updates.
Classless routing protocols: RIPv2, EIGRP, OSPF, IS-IS, BGPv4 (coming)
Rick Graziani graziani@cabrillo.edu
39
Another VLSM Example using /30 subnets
207.21.24.0/24 network subnetted into eight /27 (255.255.255.224)
subnets
207.21.24.192/27 subnet, subnetted into eight /30
(255.255.255.252) subnets
•
•
This network has seven /27 subnets with 30 hosts each
AND eight /30 subnets with 2 hosts each.
/30 subnets are very useful for serial networks.
Rick Graziani graziani@cabrillo.edu
40
207.21.24.192/27
0
1
2
3
4
5
6
7
207.21.24.192/30
207.21.24.196/30
207.21.24.200/30
207.21.24.204/30
207.21.24.208/30
207.21.24.212/30
207.21.24.216/30
207.21.24.220/30
Rick Graziani graziani@cabrillo.edu
207.21.24. 11000000
/27 /30
207.21.24. 110 00000
207.21.24. 110 00100
207.21.24. 110 01000
207.21.24. 110 01100
207.21.24. 110 10000
207.21.24. 110 10100
207.21.24. 110 11000
207.21.24. 110 11100
Hosts Bcast
01 10 11
01 10 11
01 10 11
01 10 11
01 10 11
01 10 11
01 10 11
01 10 11
2 Hosts
.193 & .194
.197 & .198
.201 & .202
.205 & .206
.209 & .210
.213 & .214
.217 & .218
.221 & .222
41
207.21.24.192/30
207.21.24.204/30
207.21.24.216/30
207.21.24.96/27
207.21.24.128/27
207.21.24.64/27
207.21.24.196/30
207.21.24.160/27
207.21.24.208/30
207.21.24.200/30
207.21.24.224/27
207.21.24.32/27
207.21.24.212/30
207.21.24.0/27
• This network has seven /27 subnets with 30 hosts each AND seven /30
•
subnets with 2 hosts each (one left over).
/30 subnets with 2 hosts per subnet do not waste host addresses on
serial networks .
Rick Graziani graziani@cabrillo.edu
42
VLSM and the Routing Table
Displays one subnet mask for all child routes.
Classful mask is assumed for the parent route.
Routing Table without VLSM
RouterX#show ip route
207.21.24.0/27 is subnetted,
C
207.21.24.192 is directly
C
207.21.24.196 is directly
C
207.21.24.200 is directly
C
207.21.24.204 is directly
4 subnets
connected,
connected,
connected,
connected,
Serial0
Serial1
Serial2
FastEthernet0
Each child routes displays its own subnet mask.
Classful mask is included for the parent route.
Routing Table with VLSM
RouterX#show ip route
207.21.24.0/24 is variably subnetted, 4 subnets, 2 masks
C
207.21.24.192 /30 is directly connected, Serial0
C
207.21.24.196 /30 is directly connected, Serial1
C
207.21.24.200 /30 is directly connected, Serial2
C
207.21.24.96 /27 is directly connected, FastEthernet0
• Parent Route shows classful mask instead of subnet mask of the child
routes.
• Each Child Routes includes its subnet mask.
Rick Graziani graziani@cabrillo.edu
43
Final Notes on VLSM
• Whenever possible it is best to group contiguous routes together so
•
•
they can be summarized (aggregated) by upstream routers. (coming
soon!)
– Even if not all of the contiguous routes are together, routing tables
use the longest-bit match which allows the router to choose the
more specific route over a summarized route.
– Coming soon!
You can keep on sub-subnetting as many times and as “deep” as you
want to go.
You can have various sizes of subnets with VLSM.
Rick Graziani graziani@cabrillo.edu
44
Classful Routing Protocols
•
•
•
Classful Routing Protocols
– RIPv1
– IGRP
Classful routing protocols carry the network address in the
routing update, but do not carry the subnet mask.
Questions:
– When receiving an routing update, how does the router
know what mask to associate with the network address?
– What if network address being sent in the update is
subnet address, how does it determine the mask?
Rick Graziani graziani@cabrillo.edu
45
Classful Routing Protocols
Sending Routing Updates
• Routing update includes only the:
– Network address
– Metric
• If the routing update is being sent out of an interface with a different
major network, then the update is summarized to the classful address.
• There is no mask sent in routing updates from routers using classful
routing protocols.
Receiving Routing Updates
• If routing update belongs to the same major network as the interface it
is being received on, the subnet mask of the interface is applied to the
network in the routing update.
• If routing update belongs to a different major network than the interface
it is being received on, the classful subnet mask of the network is
applied to the network in the routing update.
Rick Graziani graziani@cabrillo.edu
46
Classful Routing Protocols
Sending/Receiving Subnet Routes
• If routing update belongs to the
same major network as the interface
it is being received on, the subnet
mask of the interface is applied to
the network in the routing update.
• SanJose1 sends out the routing
update 172.30.3.0.
• SanJose1 does not summarize this
route to its classful address
(172.30.0.0).
• SanJose1 sends out the actual
subnet address 172.30.3.0 because
it is being sent out an interface
Serial0, 172.30.2.2, which belongs
to the same major network address
as the update (172.30.0.0).
Rick Graziani graziani@cabrillo.edu
172.30.3.0
47
Classful Routing Protocols
Sending/Receiving Subnet Routes
• If routing update belongs to the same
major network as the interface it is
being received on, the subnet mask of
the interface is applied to the network
in the routing update.
• SanJose2 receives the routing update
with the network address 172.30.3.0
on its Serial0 interface which has the
address 172.30.3.1.
• Because the update belongs to the
same major network (172.30.0.0) as
the receiving interface, SanJose2
applies the mask of the interface that
it received the update on, /24.
• The routing table process adds the
network address 172.30.3.0, the /24
mask (255.255.255.0), and the
appropriate metric to the routing table.
Rick Graziani graziani@cabrillo.edu
172.30.3.0
New Route Added:
172.30.3.0 255.255.255.0
48
Classful Routing Protocols
Sending/Receiving Routes over different
Major Network
• If routing update belongs to a different
major network than the interface it is
being received on, the classful subnet
mask of the network is applied to the
network in the routing update by the
receiving router.
• SanJose1 sends out the routing update
172.30.0.0.
• SanJose1 summarizes the 172.30.3.0
route and any other subnets in its routing
table that belong to the 172.30.0.0
network (172.30.1.0 and 172.30.2.0), to
their major classful address (172.30.0.0).
• SanJose1 sends out the summarized
classful address 172.30.0.0 because it is
being sent out an interface Serial1,
192.168.4.9, which belongs to a different
major network address (192.168.4.8)
than the update (172.30.0.0).
Rick Graziani graziani@cabrillo.edu
172.30.0.0
49
Classful Routing Protocols
Sending/Receiving Routes over different
Major Network
• If routing update belongs to a different
major network than the interface it is
being received on, the classful subnet
mask of the network is applied to the
network in the routing update by the
receiving router.
•
•
•
Baypointe receives the routing update with the
network address 172.30.0.0 on its Serial0
interface which has the address 192.168.4.10.
Because the update (172.30.0.0) belongs to a
different major network than the receiving
interface (192.168.4.8), Baypointe applies the
classful mask of the network address in the
update, /16.
The routing table process adds the network
address 172.30.0.0, the /16 mask
(255.255.0.0), and the appropriate metric to
the routing table.
Rick Graziani graziani@cabrillo.edu
172.30.0.0
New Route Added:
172.30.0.0 255.255.0.0
50
Classful Routing Protocols
Issues with Classful Routing Protocols
• Because the mask is derived from
either the receiving interface (subnets
within the same major network) or
assumed as the classful mask
(summarized networks between major
networks), this limits addressing
schemes that can be configured on
these networks.
Rick Graziani graziani@cabrillo.edu
172.30.3.0
172.30.0.0
51
Classful Routing Protocols
Issues with Classful Routing Protocols
• Because the network mask is not
included with the network address in
the routing update, networks that use
Classful routing protocols cannot
support:
– VLSM
– CIDR
– Discontiguous Subnets
• All of these require the receiving
router to know the correct subnet
mask.
• CIDR
– This is a summarized route, which
also needs to have the mask
included in the update.
Rick Graziani graziani@cabrillo.edu
172.30.3.0
172.30.0.0
52
Classful Routing Protocols – No VLSM
Subnets
10.0.0.0/16
10.1.0.0/16
10.2.0.0/16
10.2.0.0/24
10.2.1.0/24
10.2.2.0/24
Etc.
10.2.255.0/24
10.3.0.0/16
Etc.
10.255.0.0/16
An example of VLSM, NOT of good network design.
10.1.0.0/16
10.2.0.0/24
10.2.1.0/24
10.7.0.0/16
10.3.0.0/16
10.2.6.0/24
10.4.0.0/16
10.2.3.0/24
10.2.8.0/24
10.5.0.0/16 10.8.0.0/16
10.2.5.0/24
10.6.0.0/16
10.2.4.0/24
VLSM
• For the same major network, there may be multiple masks.
• Routers can no longer derive the mask from its own receiving interface.
Rick Graziani graziani@cabrillo.edu
53
Classful Routing Protocols – No CIDR
200.199.48.0/25
200.199.56.0/23
CIDR
These are summarized
routes, which also need to
have their masks included in
the update.
200.199.48.0/25
200.199.49.0/25
200.199.56.0/23
11001000 11000111 0011 0000 00000000
11001000 11000111 0011 0001 00000000
11001000 11000111 0011 1000 00000000
200.199.48.0/20
11001000 11000111 0011 0000 00000000
20 bits in common
Rick Graziani graziani@cabrillo.edu
54
Discontiguous subnets
•
•
•
•
•
Classful routing protocols,
cannot support discontiguous
subnets, because the subnet
mask is not included in routing
updates.
Classful routing protocols
automatically summarize on
classful boundaries.
SantaCruz1 and SantaCruz2 are
both sending ISP the classful
major network address of
172.30.0.0.
ISP will apply the classful mask
of /16, 255.255.0.0 to both
routes.
Both equal cost routes will be
entered in the routing table and
routing with unexpected and
many times incorrect result will
occur.
Rick Graziani graziani@cabrillo.edu
Internet
10.0.0.0/8
.1 e0
ISP
.25
s0
s1 .21
192.168.4.24/30
172.30.2.0/24
.26 s0
Lo0
.1 SantaCruz1
.1 e0
172.30.1.0/24
192.168.4.20/30
s0
.22
Lo0
SantaCruz2 .1
172.30.110.0/24
.1 e0
172.30.100.0/24
55
Classful and Classless Routing
Protocols
RIPv1 and RIPv2
Classless routing protocols
• The true defining characteristic of classless routing protocols is the
•
capability to carry subnet masks in their route advertisements.
“One benefit of having a mask associated with each route is that the
all-zeros and all-ones subnets are now available for use.”
– Cisco allows the all-zeros and all-ones subnets to be used with
classful routing protocols.
Rick Graziani graziani@cabrillo.edu
57
Classless Routing Protocols
Classless Routing Protocols:
• RIPv2
• EIGRP
• OSPF
• IS-IS
• BGPv4
Note: Remember classful/classless routing protocols is different than
classful/classless routing behavior. Classlful/classless routing protocols
(RIPv1, RIPv2, IGRP, EIGRP, OSPF, etc.) has to do with how routes get into
the routing table; how the routing table gets built. Classful/classless routing
behavior (no ip classless or ip classless) has to do with the lookup process of
routes in the routing table (after the routing table has been built). It is possible
to have a classful routing protocol and classless routing behavior or visa
versa. It is also possible to have both a classful routing protocol and classful
routing behavior; or both a classless routing protocol and classless routing
behavior.
Rick Graziani graziani@cabrillo.edu
RIP version 1
•
•
•
•
Classful Routing Protocol, sent over UDP port 520
Does not include the subnet mask in the routing updates.
Automatic summarization done at major network boundaries.
Updates sent as broadcasts unless the neighbor command is uses
which sends them as unicasts.
0
1
2
3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| command (1)
| version (1)
|
must be zero (2)
|
+---------------+---------------+-------------------------------+
| address family identifier (2) |
must be zero (2)
|
+-------------------------------+-------------------------------+
|
IP address (4)
|
+---------------------------------------------------------------+
|
must be zero (4)
|
+---------------------------------------------------------------+
|
must be zero (4)
|
+---------------------------------------------------------------+
|
metric (4)
|
+---------------------------------------------------------------+
Rick Graziani graziani@cabrillo.edu
RIP version 2
•
•
•
•
Classless Routing Protocol, sent over UDP port 520
Includes the subnet mask in the routing updates.
Automatic summarization at major network boundaries can be disabled.
Updates sent as multicasts unless the neighbor command is uses which
sends them as unicasts.
0
1
2
3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| command (1)
| version (1)
|
must be zero (2)
|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address Family Identifier (2) |
Route Tag (2)
|
+-------------------------------+-------------------------------+
|
IP Address (4)
|
+---------------------------------------------------------------+
|
Subnet Mask (4)
|
+---------------------------------------------------------------+
|
Next Hop (4)
|
+---------------------------------------------------------------+
|
Metric (4)
|
+---------------------------------------------------------------+
Rick Graziani graziani@cabrillo.edu
For more information on RIPv2
•
•
We will not discuss RIPv2 in detail except as an
introduction into Classless Routing Protocols.
We will do a lab regarding RIPv2, but for more information
regarding RIPv2 see my PowerPoint presentation on my
CCNP 1 web site:
– Chapter 4 - RIP version 2
Rick Graziani graziani@cabrillo.edu
61
Ch. 1 – Introduction to
Classless Routing
CCNA 3 version 3.0
By
Rick Graziani