Ch. 1 Introduction to Classless Routing

1
Ch. 1 Introduction to Classless Routing

• CCNA 3 version 3.1
• Rick Graziani
• Modified by Joanne Wagner,
• CCNP, CCAI, CCSP
• College of DuPage

2
Internet Scaling Problems

• Alternatives
• IPv6
• Subnetting
• NAT
• Private IP Addressing
• IP Unnumbered
• CIDR
• VLSM

3
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.

4
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 AddressesFirst 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. 

5
IP addressing crisis

• Address Depletion
• Internet Routing Table Explosion

6
IPv4 Addressing

• Subnet Mask
• One solution to the IP address shortage was
  thought to be the subnet mask.
• Formalized in 1985 (RFC 950), the subnet mask
  breaks a single class A, B or C network in to
  smaller pieces.

7
Subnet Example
Given the Class B address 190.52.0.0
Class B
Network
Network
Host
Host

• Using /24 subnet...
• 190.52.1.2
• 190.52.2.2
• 190.52.3.2

Internet routers still see this net as
190.52.0.0
But internal routers think all these addresses
are on different networks, called subnetworks
8
Subnet Example

• Using the 3rd octet, 190.52.0.0 was divided into
• 190.52.1.0 190.52.2.0 190.52.3.0
  190.52.4.0
• 190.52.5.0 190.52.6.0 190.52.7.0
  190.52.8.0
• 190.52.9.0 190.52.10.0 190.52.11.0
  190.52.12.0
• 190.52.13.0 190.52.14.0 190.52.15.0
  190.52.16.0
• 190.52.17.0 190.52.18.0 190.52.19.0 and so on
  ...

9
Subnet Example
Network address 190.52.0.0 with /16 network mask
Using Subnets subnet mask 255.255.255.0 or /24
Subnets
255 Subnets 28 - 1
Cannot use last subnet as it contains broadcast
address
10
Subnet Example
Subnet 0 (all 0s subnet) issue The address of
the subnet, 190.52.0.0/24 is the same address as
the major network, 190.52.0.0/16.
Subnets
255 Subnets 28 - 1
Last subnet (all 1s subnet) issue The
broadcast address for the subnet, 190.52.255.255
is the same as the broadcast address as the major
network, 190.52.255.255.
11
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/techno
  logies_tech_note09186a0080093f18.shtml

12
Long Term Solution IPv6 (coming)

• IPv6, or IPng (IP the Next Generation) uses a
  128-bit address space, yielding
• 340,282,366,920,938,463,463,374,607,431,768,2
  11,456
• 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.
• Some experts believe IPv4 will remain for more
  than 10 years.

13
Short Term Solutions IPv4 Enhancements

• CIDR (Classless Inter-Domain Routing) RFCs
  1517, 1518, 1519, 1520
• VLSM (Variable Length Subnet Mask) RFC 1009
• Private Addressing - RFC 1918
• NAT/PAT (Network Address Translation / Port
  Address Translation) RFC

14
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.
• IETFs 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 three bits) of the network
  address (or network-prefix) 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.

15
CIDR (Classless Inter-Domain Routing)

• First deployed in 1994, CIDR dramatically
  improves IPv4s 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.

16

• 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
17
CIDR (Classless Inter-Domain Routing)

• 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.

18
ISP/NAP Hierarchy - The Internet Still
hierarchical after all these years. Jeff Doyle
(Tries to be anyways!)
19
Addess Distribution - Example
20
VLSM permits route aggregation Reducing routing
table size
11.1.1.0/24 11.1.2.0/24 ... 11.1.252.0/24 11.1.254
.0/24
11.2.0.0/16 11.3.0.0/16 ... 11.252.0.0/16 11.254.0
.0/16
11.1.0.0/16
Router A
Router B
11.0.0.0/8
11.1.253.0/24
11.253.0.0/16
Router D
Router C
11.1.253.32/27 11.1.253.64/27 11.1.253.96/27 11.1.
253.128/27 11.1.253.160/27 11.1.253.192/27
11.253.32.0/19 11.253.64.0/19 ... 11.253.160.0/19
11.253.192.0/19
21
11.1.1.0/24
11.1.2.0/24
11.1.0.0/16
11.1.253.32/27
11.2.0.0/16
11.1.253.64/27
11.1.253.0/24
11.3.0.0/16
11.1.254.0/24
11.1.253.160/27
11.0.0.0/8
11.253.32.0/19
11.1.253.192/27
11.252.0.0/16
11.253.64.0/19
11.253.0.0/16
11.254.0.0/16
11.253.160.0/19
11.253.192.0/19
22
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

23 bits in common
23
Supernetting Example

• With the ISP acting as the addressing authority
  for a CIDR block of addresses, the ISPs customer
  networks, which include XYZ, can be advertised
  among Internet routers as a single supernet.

24
CIDR and the Provider
Another example of route aggregation.
25
CIDR and the provider
200.199.48.0/25
Summarization from the customer networks to
their provider.
200.199.56.0/23

• Even Better
• 200.199.48.32/27 11001000 11000111 00110000 0
  0100000
• 200.199.48.64/27 11001000 11000111 00110000 0
  1000000
• 200.199.48.96/27 11001000 11000111 00110000 0
  1100000
• 200.199.48.0/25 11001000 11000111 00110000 0
  0000000
• (As long as there are no other routes
  elsewhere within this range, well)
• 200.199.56.0/24 11001000 11000111 0011100 0
  00000000
• 200.199.57.0/24 11001000 11000111 0011100 1
  00000000
• 200.199.56.0/23 11001000 11000111 0011100 0
  00000000

26
CIDR and the provider
200.199.48.0/25
Further summarization happens with the next
upstream provider.
200.199.56.0/23

• 200.199.48.0/25 11001000 11000111 0011 0000
  00000000
• 200.199.49.0/25 11001000 11000111 0011 0001
  00000000
• 200.199.56.0/23 11001000 11000111 0011 1000
  00000000
• 200.199.48.0/20 11001000 11000111 0011 0000
  00000000
• 20 bits in common

27
CIDR Restrictions

• Dynamic routing protocols must send network
  address and mask (prefix-length) information in
  their routing updates.
• In other words, CIDR requires classless routing
  protocols for dynamic routing.
• However, you can still configure summarized
  static routes, after all, that is what a
  0.0.0.0/0 route is.

28
Example from online curriculum
29
Short Term Solutions IPv4 Enhancements

• CIDR (Classless Inter-Domain Routing) RFCs
  1517, 1518, 1519, 1520
• VLSM (Variable Length Subnet Mask) RFC 1009
• Private Addressing - RFC 1918
• NAT/PAT (Network Address Translation / Port
  Address Translation) RFC

30
VLSM (Variable Length Subnet Mask)

• Limitation of using only a single subnet mask
  across a given network-prefix (network address,
  the number of bits in the mask) was that an
  organization is locked into a fixed-number of of
  fixed-sized subnets.
• 1987, RFC 1009 specified how a subnetted network
  could use more than one subnet mask.
• VLSM is used to help alleviate the shortage of IP
  addresses.
• Allows us to use multiple subnet masks in the
  same ip address space.
• VLSM Subnetting a Subnet
• If you know how to subnet, you can do VLSM!

31
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
1
Host
Host
10.2.0.0/16
10
2
Host
Host
10.n.0.0/16
10

Host
Host
10.255.0.0/16
10
255
Host
Host

• Subnetting a /8 subnet using a /16 mask gives us
  256 subnets with 65,536 hosts per subnet.
• Lets take the 10.2.0.0/16 subnet and subnet it
  further

32
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
0
Host
10.2.1.0/24
10
2
1
Host
10.2.n.0/24
10
2

Host
10.2.255.0/24
10
2
255
Host

• Note 10.2.0.0/16 is now a summary of all of the
  10.2.0.0/24 subnets.
• Summarization coming soon!

33
VLSM Simple Example

• 10.0.0.0/8 subnetted using /16
• Subnet 1st host Last host
  Broadcast
• 10.0.0.0/16 10.0.0.1 10.0.255.254
  10.0.255.255
• 10.1.0.0/16 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 10.3.0.1 10.3.255.254
  10.0.255.255
• Etc.
• 10.255.0.0/16 10.255.0.1 10.255.255.254
  10.255.255.255

34
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.

35
207.21.24.192/30
207.21.24.204/30
207.21.24.216/30
207.21.24.128/27
207.21.24.96/27
207.21.24.64/27
207.21.24.208/30
207.21.24.212/30
207.21.24.196/30
207.21.24.200/30
207.21.24.32/27
207.21.24.0/27
207.21.24.160/27
207.21.24.224/27

• 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 .

36
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
• RouterXshow ip route
• 207.21.24.0/27 is subnetted, 4 subnets
• C 207.21.24.192 is directly connected,
  Serial0 
• C 207.21.24.196 is directly connected,
  Serial1
• C 207.21.24.200 is directly connected,
  Serial2
• C 207.21.24.204 is directly connected,
  FastEthernet0
• Routing Table with VLSM
• RouterXshow 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

Each child routes displays its own subnet mask.
Classful mask is included for the parent route.

• Parent Route shows classful mask instead of
  subnet mask of the child routes.
• Each Child Routes includes its subnet mask.
• Routing updates contain 32-bit address and subnet
  mask.

37
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.

38
Short Term Solutions IPv4 Enhancements

• CIDR (Classless Inter-Domain Routing) RFCs
  1517, 1518, 1519, 1520
• VLSM (Variable Length Subnet Mask) RFC 1009
• Private Addressing - RFC 1918
• NAT/PAT (Network Address Translation / Port
  Address Translation) RFC

39
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
• A test lab
• A home network
• Global addresses must be obtained from a provider
  or a registry at some expense.

40
Discontiguous subnets

• Mixing private addresses with globally unique
  addresses can create discontiguous subnets.
  Not the main cause however
• Discontiguous subnets, are subnets from the same
  major network that are separated by a completely
  different major network or subnet.
• Question If a classful routing protocol like
  RIPv1 or IGRP is being used, what do the routing
  updates look like between Site A router and Site
  B router?

41
Discontiguous subnets

• Classful routing protocols, notably RIPv1 and
  IGRP, cant support discontiguous subnets,
  because the subnet mask is not included in
  routing updates.
• RIPv1 and IGRP automatically summarize on
  classful boundaries.
• Site A and Site B are all sending each other the
  classful address of 207.21.24.0/24.
• A classless routing protocol (RIPv2, EIGRP, OSPF)
  would be needed
• to not summarize the classful network address and
  
• to include the subnet mask in the routing updates.

42
Discontiguous subnets

• RIPv2 and EIGRP automatically summarize on
  classful boundaries.
• When using RIPv2 and EIGRP, to disable automatic
  summarization (on both routers)
• Router(config-router)no auto-summary
• SiteB now receives 207.21.24.0/27
• SiteA now receives 207.21.24.32/27

43
Short Term Solutions IPv4 Enhancements

• CIDR (Classless Inter-Domain Routing) RFCs
  1517, 1518, 1519, 1520
• VLSM (Variable Length Subnet Mask) RFC 1009
• Private Addressing - RFC 1918
• NAT/PAT (Network Address Translation / Port
  Address Translation) RFC

44
Network Address Translation (NAT)

• NAT Network Address Translatation
• NAT, as defined by RFC 1631, is the process of
  swapping one address for another in the IP packet
  header.
• In practice, NAT is used to allow hosts that are
  privately addressed to access the Internet.

45
Network Address Translation (NAT)
2.2.2.2 TCP Source Port 1923
TCP Source Port 1026
2.2.2.2 TCP Source Port 1924
TCP Source Port 1026

• NAT translations can occur dynamically or
  statically.
• The most powerful feature of NAT routers is their
  capability to use port address translation (PAT),
  which allows multiple inside addresses to map to
  the same global address.
• This is sometimes called a many-to-one NAT.
• With PAT, or address overloading, literally
  hundreds of privately addressed nodes can access
  the Internet using only one global address.
• The NAT router keeps track of the different
  conversations by mapping TCP and UDP port numbers.

46
Classless Routing ProtocolsRIPv2
47
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.

48
Classless Routing Protocols

• The true characteristic of a classless routing
  protocol is the ability to carry subnet masks in
  their route advertisements. Jeff Doyle, Routing
  TCP/IP
• Benefits
• All-zeros and all-ones subnets
• - Although some vendors, like Cisco, can also
  handle this with classful routing protocols.
• VLSM
• Can have discontiguous subnets
• Better IP addressing allocation
• CIDR
• More control over route summarization

49
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.

50
RIP Version 2 (Joanne Wagner)

• The main disadvantages of RIP version 1
• the minimal amount of information included in
  every packet
• the large amount of unused space in the header
  of each packet
• inability to do authentication, VLSM and CIDR

51
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)
  
• -------------------------------------------
  --------------------

52
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)
  
• ----------------------------------------------
  -----------------

53
RIP v2 operation

• All of the operational procedures, timers, and
  stability functions of RIP v1 remain the same in
  RIP v2, with the exception of the broadcast
  updates.
• RIP v2 updates use reserved Class D address
  224.0.0.9.

54
Issues addressed by RIP v2

• The following four features are the most
  significant new features added to RIP v2
• Authentication of the transmitting RIP v2 node to
  other RIP v2 nodes
• Subnet Masks RIP v2 allocates a 4-octet field
  to associate a subnet mask to a destination IP
  address.
• Next Hop IP addresses A better next-hop
  address, that the advertising router, if one
  exists.
• It indicates a next-hop address, on the same
  subnet, that is metrically closer to the
  destination than the advertising router.
• If this routers interface is closest, then it is
  set to 0.0.0.0
• See Doyle, Routing TCP/IP for an example
• Multicasting RIP v2 messages Multicasting is a
  technique for simultaneously advertising routing
  information to multiple RIP or RIP v2 devices.

55
Next Hop Address (Joanne Wagner)

• The purpose of the Next Hop field is to eliminate
  packets being routed through extra hops in the
  system.
• It is particularly useful in an environment which
  uses multiple routing protocols and RIP is not
  being run on all of the routers on a network.
• For example, if RIP-2 were being run on a network
  along with another IGP, and one router ran both
  protocols, then that router could indicate to the
  other RIP-2 routers that a better next hop than
  itself exists for a given destination.

---BGP---
The Internal Routers (IR1 and IR2) are only
running RIP-2. The External Routers (XR1 and XR2)
are both running BGP, for example however, only
XR1 is running BGP and RIP-2. Since XR2 is not
running RIP-2, the IRs will not know of its
existence and will never use it as a next hop,
even if it is a better next hop than XR1. Of
course, XR1 knows this and can indicate, via the
Next Hop field, that XR2 is the better next hop
for some routes.
56
RIP v2 message format

• All the extensions to the original protocol are
  carried in the unused fields.
• The Address Family Identifier (AFI) field is set
  to two for IP. The only exception is a request
  for a full routing table of a router or host, in
  which case it will be set to zero.

57
RIP v2 message format

• The Route Tag field provides a way to
  differentiate between internal and external
  routes. (RIP itself does not use this field.)
• External routes are those that have been
  redistributed into the RIP v2.
• The Next Hop field contains the IP address of the
  next hop listed in the IP Address field.
• Metric indicates how many internetwork hops,
  between 1 and 15 for a valid route, or 16 for an
  unreachable route.

58
Compatibility with RIP v1

• RFC 1723 defines a compatibility with four
  settings, which allows versions 1 and 2 to
  interoperate
• RIP v1, in which only RIP v1 messages are
  transmitted
• RIP v1 Compatibility, which causes RIP v2 to
  broadcast its messages instead of multicast them
  so that RIP v1 may receive them
• RIP v2, in which RIP v2 messages are multicast to
  destination address 224.0.0.9
• None, in which no updates are sent
• RFC 1723 recommends that routers be configurable
  on a per-interface basis. (coming soon)

59
Authentication
Authentication is supported by modifying what
would normally be the first route entry of the
RIP message

• A security concern with any routing protocol is
  the possibility of a router accepting invalid
  routing updates.
• The Authentication Type for simple password
  authentication is two, 0x0002,
• The remaining 16 octets carry an alphanumeric
  password of up to 16 characters.
• Configuration is coming!

60
Authentication

• RFC 1723 describes only simple password
  authentication
• Cisco IOS provides the option of using MD5
  authentication instead of simple password
  authentication.
• Cisco uses the first and last route entry spaces
  for MD5 authentication purposes.
• MD5 computes a 128-bit hash value from a plain
  text message of arbitrary length and a password.

61
MD5 Authentication (FYI) http//www.cisco.com/en/U
S/tech/tk713/tk507/technologies_tech_note09186a008
00b4131.shtml
1
2
3
4
5
6
62
Same limitations of RIPv2 as with RIPv1

• Slow convergence and the need of holddown timers
  to reduce the possibility of routing loops.
• Note See CCNA 2 for review if needed.

63
Same limitations of RIPv2 as with RIPv1

• Both RIP versions use 16 hops as a metric for
  infinite distance.
• Dependent upon holddown timers.
• Triggered updates are also helpful.
• Both RIP v1 and RIP v2 use hop count.
• Note See CCNA 2 for review if needed.

64
RIP Timer Review (Joanne Wagner)

• Updates
• After startup, the router sends a Response
  message (update) out every RIP-enabled interface
  every 30 seconds, on average.
• The Response message, or update, contains the
  routers full routing table with the exception of
  entries suppressed by the split horizon rule.
• Invalid Timer
• Used to limit the amount of time a route can stay
  in a routing table without being updated.
• Initialized to 180 seconds whenever a new route
  is established and is reset to the initial value
  whenever an update is heard for that route.
• If an update for a route is not heard within that
  180 seconds (six update periods), the hop count
  for the route is changed to 16, marking the route
  as unreachable.
• Hold
• An update with a hop count higher than the metric
  recorded in the routing table will cause the
  route to go into holdown for 180 seconds (three
  update periods).
• Flush Timer
• Set to 240 seconds 60 seconds longer than the
  expiration time.
• The route will be advertised with the unreachable
  metric until the flush timer expires, at which
  time the route is removed from the routing table.

65
Basic RIPv2 configuration

• Select the routing protocol to be configured.
• Assign an IP address and subnet mask to the
  interface.
• Configure the routing protocol with the new
  network address using the network command (the
  network command specifies which interfaces will
  exchange RIP updates).

These three steps apply to both RIP v1 and RIP v2
(as well as IGRP, EIGRP, etc.)
66
Basic RIPv2 configuration

• Other
• For RIP and IGRP, the passive interface command
  stops the router from sending updates to a
  particular neighbor, but the router continues to
  listen and use routing updates from that
  neighbor. (More later.)
• Router(config-router) passive-interface
  interface
• Default behavior of version 1 restored
• Router(config-router) no version

67
Compatibility with RIP v1

• NewYork
• interface fastethernet0/0
• ip address 192.168.50.129 255.255.255.192
• ip rip send version 1
• ip rip receive version 1
• interface fastethernet0/1
• ip address 172.25.150.193 255.255.255.240
• ip rip send version 1 2
• interface fastethernet0/2
• ip address 172.25.150.225 225.255.255.240
• router rip
• version 2
• network 172.25.0.0
• network 192.168.50.0

RIPv2

• Interface FastEthernet0/0 is configured to send
  and receive RIP v1 updates.
• FastEthernet0/1 is configured to send both
  version 1 and 2 updates.
• FastEthernet0/2 has no special configuration and
  therefore sends and receives version 2 by default.

68
Discontiguous subnets and classless routing

• router ripversion 2no auto-summary

• RIP v1 always uses automatic summarization.
• The default behavior of RIP v2 is to summarize at
  network boundaries the same as RIP v1.

69
Configuring authentication (EXTRA)

• Router(config)key chain Romeo
• Router(config-keychain)key 1
• Router(config-keychain-key)key-string Juliet
• The password must be the same on both
  routers (Juliet), but the name of the key (Romeo)
  can be different.
• Router(config)interface fastethernet 0/0
• Router(config-if)ip rip authentication key-chain
  Romeo
• Router(config-if)ip rip authentication mode md5
• If the command ip rip authentication mode md5 is
  not added, the interface will use the default
  clear text authentication. Although clear text
  authentication may be necessary to communicate
  with some RIP v2 implementations, for security
  concerns use the more secure MD5 authentication
  whenever possible.

70
Show commands
71
Show commands
72
Debug commands
73
RIPv2 Summary