CCNA v3.0 Module 1

1
CCNA v3.0Module 1

• Introduction to Classless Routing

2
What is VLSM?

• A Variable Length Subnet Mask (VLSM) is a means
  of allocating IP addressing resources to subnets
  according to their individual need rather than
  some general network-wide rule.
• VLSM allows an organization to use more than one
  subnet mask within the same network address
  space. It is often referred to as subnetting a
  subnet, and can be used to maximize addressing
  efficiency.
• Large subnets are created for addressing LANs and
  small subnets are created for WAN links (a 30 bit
  mask is used to create subnets with only two
  host).

3
Subnetting vs. VLSM

• Subnetting allows you to divide big networks into
  smaller, equal-sized slices.
• VLSM allows you to divide big networks into
  smaller, different-sized slices. This enables you
  to make maximum use of your valuable IP address
  space.
• So basically, you are now utilizing subnet masks
  in the same IP address space.

4
Routing Protocols Supporting VLSM

• RIP v2
• EIGRP
• OSPF

5
Addressing a Network with Standard Subnetting

• Site A has two Ethernet networks
• Site B had one Ethernet network
• Site C had one Ethernet network
• 207.21.24.0 /24
• How many network addresses are needed?
• How many hosts are needed for the largest LAN?
• How many bits need to be borrowed to address this
  network?

6
Addressing a Network with Standard Subnetting

• Site A has two Ethernet networks
• Site B had one Ethernet network
• Site C had one Ethernet network

If we borrow 3 bits from a class C address, that
will give us eight networks, but we can only use
six of them. Each network will have 30 usable
addresses. It will take four network addresses
to accommodate the Ethernet networks at each
site. That leaves us with two extra networks.
There is also a point-to-point WAN connection
between each site. These two connections will
take up the remaining two networks.
7
Addressing a Network with Standard Subnetting

• Borrowing 3 bits will meet the current needs of
  the company, but it leaves little room for
  growth.
• Each network will have 30 usable addresses,
  including the point-to-point WAN links (which
  only require two addresses).

207.21.24.0
8
We can use subnet 0
To enable subnet 0 on a Cisco router (if not
already enabled), it is necessary to use the
global configuration command ip
subnet-zero. Router configure terminal
(config t) Router(config) ip subnet-zero To
disable subnet 0, use the no form of this
command. Router(config) no ip subnet-zero
9
Subnetting in a Box
In a class C network there are 256 addresses.
Provides 1 network with 256 addresses. When we
subnet the address, we break it down in to
smaller units or subnets. Subnet mask
255.255.255.0
0
256 addresses
255
10
Subnetting in a Box
Borrowing 1 bit would break the 256 addresses in
to two parts (networks) Providing 2 networks
each with 128 addresses. Subnet mask
255.255.255.128.
0
255
11
Subnetting in a Box
Borrowing 2 bits would break each of the 2
networks in half again. Providing 4 networks,
each with 64 addresses. Subnet mask
255.255.255.192.
0
255
12
Subnetting in a Box
Borrowing 3 bits would break each of these 4
networks in half again. Providing 8 networks,
each with 32 addresses. Subnet mask
255.255.255.224.
0
255
13
Subnetting in a Box
Borrowing 4 bits would break each of these 8
networks in half again. Providing 16 networks,
each with 16 addresses. Subnet mask
255.255.255.240.
0
32
160
159
31
96
224
223
255
95
14
Addressing a Network Using VLSM

• When using VLSM to subnet an address, not all of
  the subnets have to be the same size.
• A different subnet mask may be applied to some of
  the subnets to further subnet the address.
• In order to take advantage of VLSM, the proper
  routing protocol must be selected.
• Not all routing protocols share subnetting
  information in their routing table updates.

15
Addressing a Network Using VLSM

• To subnet using VLSM, identify the LAN with the
  largest number of hosts. Subnet the address
  207.21.24.0 /24 based on this information.
• Site A has two Ethernet networks (25 hosts each)
• Site B had one Ethernet network (10 hosts)
• Site C had one Ethernet network (8 hosts)

16
Addressing a Network Using VLSM

• Subnet 1 2 to address Site A Ethernet
  networks.
• Subnet 5 to accommodate Site B C Ethernet
  networks.
• Subnet 6 can be subnetted to accommodate the WAN
  links.

Free Addresses
17
Addressing a Network Using VLSM

• Through applying VLSM, the topology was able to
  be addressed and still have two complete subnets
  available for future growth.

207.21.24.192 /30
207.21.24.196 /30
207.21.24.32 /27
207.21.24.64 /27
207.21.24.160 /28
207.21.24.176 /28
18
Addressing a Network Using VLSMExercise 1

• Your company IP network is 195.39.71.0 /24.
• Headquarters is connected to five branch offices
  by a WAN link, and to an ISP.
• Determine an appropriate IP addressing scheme.
• (the ISP owns the addresses on its link)

19
195.39.71.0 /24 Subnet according to the largest
subnet needed. (Headquarters 60 hosts)
0
Borrow 2 bits or /26. This would give you 4
networks with 64 host addresses on each subnet.
255
20
Playing it safe, we will not use the first subnet
(subnet 0).
We will start addressing with 195.39.71.64 /26.
Headquarters needs 60 hosts, so we will assign
them .64 - .127.
Headquarters 60 hosts 26 bit mask or
/26 (255.255.255.192)
21
The 5 Branch offices need 12 hosts each.
The next address block available is the .128 -
.191 block. Use VLSM.
Headquarters 60 hosts 26 bit mask or
/26 (255.255.255.192)
Using a /28 mask will give us 16 hosts at each
location. This will take care of 4 of the Branch
offices.
22
To obtain a block for Branch 5, we will need to
subnet the .192 - .255 block using a /28 mask.
Branch 512 hosts/28(255.255.255.240)
Headquarters 60 hosts 26 bit mask or
/26 (255.255.255.192)
23
Now connect the 5 WAN links to the Branch
offices. These are point-to-point connections and
only require 2 addresses.
0
128
Branch 112 hosts/28(255.255.255.240)
Branch 312 hosts/28(255.255.255.240)
Branch 412 hosts/28(255.255.255.240)
Branch 212 hosts/28(255.255.255.240)
64
192
224
Branch 512 hosts/28(255.255.255.240)
Here we will use a /30 mask to further subnet the
subnets.
Headquarters 60 hosts 26 bit mask or
/26 (255.255.255.192)
208
WAN 5
24
Any remaining networks could be used for future
growth of either LANs or WANs. Subnet 0 could
also be further subnetted according to the needs
of the network.
0
128
Branch 112 hosts/28(255.255.255.240)
Branch 312 hosts/28(255.255.255.240)
Branch 412 hosts/28(255.255.255.240)
Branch 212 hosts/28(255.255.255.240)
64
192
224
Branch 512 hosts/28(255.255.255.240)
Headquarters 60 hosts 26 bit mask or
/26 (255.255.255.192)
208
WAN 5
25
Applying the Addresses to the Topology
26
Classful Addressing

• The IPv4 address architecture uses (a/n)
• 8 bit network number for Class A addresses
• 16 bit network number for Class B addresses
• 24 bit network number for Class C addresses

1 - 126
128 - 191
192 - 223
27
Classful Addressing

• Classful addressing (A, B, C) is obsolete.

28
Classless Interdomain Routing

• CIDR (pronounced cider) ignores class.
• Using CIDR, a router views a bit mask to
  determine the network and host portions of an
  address.
• This allows CIDR to craft network address spaces
  according to the size of a network instead of
  force-fitting networks into pre-sized network
  address spaces.

29
Classless Interdomain Routing

• CIDR sounds a lot like VLSM
• CIDR is usually discussed in general Internet
  context (ISPs)
• Uses custom length prefixes to reduce workload in
  key Internet routers
• VLSM is usually discussed in enterprise context
• Uses custom length prefixes to have better usage
  of enterprise address space

30
Classless Interdomain Routing

• Routers use the network-prefix, rather than the
  first 3 bits of the IP address, to determine the
  dividing point between the network number and the
  host number.
• In the CIDR model, each piece of routing
  information is advertised with a bit mask or
  prefix-length ( /x ). The prefix-length is a way
  of specifying the number bits in the
  network-portion of each routing table entry.

31
Classless Interdomain Routing

• For example, a network with 20 bits of
  network-number and 12 bits of host-number would
  be advertised with a 20 bit prefix (/20).
• The clever thing is that the IP address
  advertised with the /20 prefix could be a former
  Class A, Class B, or Class C.
• All addresses with a /20 prefix represent the
  same amount of address space (212 or 4,096 host
  addresses).
• 20 bits network 12 bits host

32
Classless Interdomain Routing

• Address space can now be assigned in chunks
  that fit the need.
• If an organization needs 254 host addresses, what
  difference does it make whether they are given
• a Class C (200.23.76.0 /24)
• 1/256th of a Class B (145.38.20.0 /24)
• 1/65,536th of a Class A (91.187.7.0 /24)
• Using a /24 prefix, each of these specifies eight
  host bits which will support 254 hosts.

33
(No Transcript)
34
Route Aggregation w/ CIDR or (Summarization)

• You need 500 addresses.
• Given two consecutive /24 addresses
• (200.201.202.0 /24 and 200.201.203.0 /24)
• This address space could be advertised to the
  rest of the Internet as 200.201.202.0 /23.
• Why? (the two /24s have the first 23 bits in
  common).
• 11001000.11001001.11001010.00000000
• 11001000.11001001.11001011.00000000

23 bits network prefix
35
CIDR Scenario continued

• If the ISP owns all of the 200.201.0.0 networks
  (256 /24s), why should it advertise all of them
  separately?
• Instead, it could simply advertise 200.201.0.0
  /16 (which would be 200.201.0.0 /24 through
  200.201.255.0 /24).
• This would reduce the size of the routing tables
  on the router to which the routes are advertised.
• 11001000.11001001.00000000.00000000
• 11001000.11001001.11111111.00000000

.0.0
.255.0
16 bits network prefix
36
CIDR Scenario continued

• The summary of route 200.201.202.0 /23 is called
  a CIDR block or a supernet.
• Because we are dealing with binary, the block
  size is always a power of two (2, 4, 8, 16, 32,
  etc.). The starting point of the block must be a
  multiple of the power of two that is being used
  (21 2, 4, 6, 8, etc.).
• 200.201.202.0
• 200.201.204.0
• 200.201.206.0
• 200.201.208.0
• 200.201.210.0

Examples of starting addresses
37
Network Prefixes
23 bits

• 200.201.200.0 11001000.11001001.11001000.00000000
• 200.201.201.0 11001000.11001001.11001001.00000000
• 200.201.202.0 11001000.11001001.11001010.00000000
• 200.201.203.0 11001000.11001001.11001011.00000000
  
• 200.201.204.0 11001000.11001001.11001100.00000000
• 200.201.205.0 11001000.11001001.11001101.00000000
• 200.201.206.0 11001000.11001001.11001110.00000000
• 200.201.207.0 11001000.11001001.11001111.00000000
• 200.201.208.0 11001000.11001001.11010000.00000000
• 200.201.209.0 11001000.11001001.11010001.00000000
• 200.201.210.0 11001000.11001001.11010010.00000000
• 200.201.211.0 11001000.11001001.11010011.00000000

38
Network Prefixes
22 bits

• 200.201.200.0 11001000.11001001.11001000.00000000
• 200.201.201.0 11001000.11001001.11001001.00000000
• 200.201.202.0 11001000.11001001.11001010.00000000
• 200.201.203.0 11001000.11001001.11001011.00000000
  
• 200.201.204.0 11001000.11001001.11001100.00000000
• 200.201.205.0 11001000.11001001.11001101.00000000
• 200.201.206.0 11001000.11001001.11001110.00000000
• 200.201.207.0 11001000.11001001.11001111.00000000
• 200.201.208.0 11001000.11001001.11010000.00000000
• 200.201.209.0 11001000.11001001.11010001.00000000
• 200.201.210.0 11001000.11001001.11010010.00000000
• 200.201.211.0 11001000.11001001.11010011.00000000

200.201.200.0/22
200.201.204.0/22
200.201.208.0/22
39
Network Prefixes
21 bits

• 200.201.200.0 11001000.11001001.11001000.00000000
• 200.201.201.0 11001000.11001001.11001001.00000000
• 200.201.202.0 11001000.11001001.11001010.00000000
• 200.201.203.0 11001000.11001001.11001011.00000000
  
• 200.201.204.0 11001000.11001001.11001100.00000000
• 200.201.205.0 11001000.11001001.11001101.00000000
• 200.201.206.0 11001000.11001001.11001110.00000000
• 200.201.207.0 11001000.11001001.11001111.00000000
• 200.201.208.0 11001000.11001001.11010000.00000000
• 200.201.209.0 11001000.11001001.11010001.00000000
• 200.201.210.0 11001000.11001001.11010010.00000000
• 200.201.211.0 11001000.11001001.11010011.00000000

200.201.200.0/21
40
CIDR in a Nutshell

• Hand out pieces of classful networks (to avoid
  wasting addresses)
• Identify the network portion of an address with a
  network prefix ( /x)
• Advertise blocks of networks (to reduce the size
  of routing tables).

41
CIDR Example

• Objective
• Create an addressing scheme using VLSM.
• Scenario
• You are assigned the CIDR address 200.32.108.0
  /22 and you must support the network shown in the
  diagram. Create an addressing scheme that will
  meet the diagram requirements.

42
Dissect the problem

• Given the CIDR address 200.32.108.0 /22
• How many /24 networks do we have?
• How many host addresses do we have?
• What is the largest LAN requirement?

43

• Address given - 200.32.108.0 /22
• Host required - 300, 100, 100, 100, and 3 WAN
  links

0
0
200.32. 110.0
200.32.108.0
255
255
0
0
200.32. 109.0
200.32. 111.0
255
255
44

• Address given - 200.32.108.0 /22
• Host required - 300, 100, 100, 100, and 3 WAN
  links

0
0
200.32. 110.0
200.32.108.0
300 hosts 200.32.108.0 /23
255
255
0
0
200.32. 109.0
200.32. 111.0
255
255
45

• Address given - 200.32.108.0 /22
• Host required - 300, 100, 100, 100, and 3 WAN
  links

0
0
200.32. 110.0
200.32.108.0
300 hosts 200.32.108.0 /23
255
255
0
0
200.32. 109.0
200.32. 111.0
255
255
46

• Address given - 200.32.108.0 /22
• Host required - 300, 100, 100, 100, and 3 WAN
  links

0
0
128
100 hosts 200.32.110.128 /25
100 hosts 200.32.110.0 /25
200.32. 110.0
200.32.108.0
300 hosts 200.32.108.0 /23
255
127
255
0
0
100 hosts 200.32.111.0 /25
200.32. 109.0
200.32. 111.0
255
255
47

• Address given - 200.32.108.0 /22
• Host required - 300, 100, 100, 100, and 3 WAN
  links

0
0
128
100 hosts 200.32.110.128 /25
100 hosts 200.32.110.0 /25
200.32. 110.0
200.32.108.0
300 hosts 200.32.108.0 /23
255
127
255
0
0
128
100 hosts 200.32.111.0 /25
200.32. 109.0
200.32. 111.0
255
127
255
48
CIDR Result

• Given the CIDR address 200.32.108.0 /22

Two /24s
49
Classless Interdomain Routing

• For the router to operate in a classless manner
  and match destination IP addresses to a CIDR
  network address,
• The global command ip classless must be
  configured.
• Router(config) ip classless

50
Routing Information Protocol(RIP)

• RIP is a relatively old, but still commonly used
  interior gateway protocol (IGP).
• It was created for use in small homogeneous
  networks.
• It is a distance-vector protocol that is used
  with classful IP addressing only.
• RIP v1 sends routing update messages at regular
  intervals (30 seconds) and when the network
  topology changes.
• RIP uses hop count as its only metric and
  maintains only the best route to a destination.

51
RIP Version 2

• Known as RIP V2
• In RIP v2 all of the operation procedures,
  timers, and stability functions of RIP v1 remain
  the same in version 2, with the exception of the
  broadcast updates.
• RIP v2 has become the standard version of RIP
  used in networks today.

52
RIP V2 is RIP V1 with extensions

• Subnet masks carried with each route entry
• Authentication of routing updates
• Next-hop addresses carried with each route entry
• External route tags
• Multicast route updates

53
RIP v2
The most important of these extensions is the
addition of a Subnet Mask field This enables the
use of variable-length subnet masks (VLSMs) and
qualifies RIP v2 as a classless routing protocol.
54
RIP v2

• RIP v2 allocated a 4-octet field to associate a
  subnet mask to a destination IP address.
• When used in tandem, the IP address and its
  subnet mask enable RIP v2 to specifically
  identify the type of destination that the route
  leads to.
• This allows RIP v2 to route specific subnets,
  regardless of whether the subnet mask is fixed or
  of variable length.

55
RIP v2

• RIP v2 differs from RIP v1 in the way update are
• sent out.
• RIP v1 sends updates as a broadcast (all stations
  receive the broadcast message)
• RIP v1 does not send subnet mask information in
  its updates.
• RIP v2 sends updates as a multi-cast.
  Multi-casting is a technique for simultaneously
  advertising routing information to multiple RIP
  devices via the class D address 224.0.0.9

56
RIP v1 RIP v2 comparisons

• Both use hop count as a metric
• Both have the same metric value for infinite
  distance (16)
• Both use split horizon to prevent routing loops.
• RIP v1 broadcasts routing table updates, while
  RIP v2 multicasts its updates

57
Configuring RIP v1
To configure RIP v1 on a router, enter the
following commands Router config
t Router(config) router rip Router(config-route
r) network 192.168.12.0 NOTE - If no version is
specified in the configuration, version 1 will be
used. The router will listen for version 1 and 2
updates but send only version 1.
58
Configuring RIP v2
To take advantage of version 2s features, it is
necessary to turn off version 1 support and
enable version 2 updates with the following
commands Router(config) router
rip Router(config-router) version 2
Router(config-router) network
192.168.12.0 NOTE - The default behavior can be
restored by entering the command no version in
the config-router mode. Router(config) router
rip Router(config-router) no version
59
Verifying Troubleshooting RIP

• show ip route to make sure routers have learned
  all networks dynamically
• show ip protocols to see information about the
  routing protocols used.
• debug ip RIP to see live routing updates

60
Overriding Default Behavior of RIP
You can override the default behavior of RIP by
configuring a particular interface to behave
differently.
Router(config) router ripRouter(config-router)
version 2Router(config-router) network
192.168.12.0Router(config-router) exit
RIP v2 configured on the router.
Router(config) int e0Router(config-if) ip
address 192.168.12.33 255.255.255.224Router(confi
g-if) ip rip send version 1Router(config-if)
ip rip receive version 1
Interface e0 sends and receives version 1 updates
only.
61
Overriding Default Behavior of RIP
You can override the default behavior of RIP by
configuring a particular interface to behave
differently.
Router(config) int e1Router(config-if) ip
address 192.168.12.65 255.255.255.224
Router(config-if) ip rip send version 1 2
Router(config-if) ip rip receive version 1 2
Interface e1 sends and receives both version 1
and 2 updates.
Interface e2 has no special configuration and
therefore sends and receives version 2 by default.
Router(config) int e2Router(config-if) ip
address 192.168.12.97 255.255.255.224
62
Review of Static Default Routing
63
Configuring static routes w/ outgoing interface
outgoing interface
Administrative distance of 0 - default
64
Configuring static routes w/ next-hop IP address
Next hop interface
Administrative distance of 1 - default
65
Configuring Static Routes

• Remember, an administrator actually enters these
  routes into the routing table.
• That makes them static route entries because
  the router is not discovering those routes.
• If for some reason that outgoing interface goes
  down or is not available for some reason, then at
  that time the route will be removed from the
  routing table.
• Show ip route shows the routing table.
• The route would still be in the configuration
  (because it was entered globally), but that route
  could now no longer be used by the router because
  the interface it refers to is down for some
  reason.

66
Administrative Distance

• What is the default for a outgoing interface?
• What is the default for the next-hop address?
• Defaults can always be changed!!!
• Just make it higher if you want it to be a
  backup route.
• ip route 192.168.2.0 255.255.255.0 192.188.4.1 120

67
S0 192.168.2.1/24
S0 192.168.4.1/24
Router A
Router B
Router C
S1 192.168.2.2/24
S1 192.168.4.2/24
192.168.1.0/24
192.168.3.0/24
192.168.5.0/24
What would you enter to configure a static route
from Router C to the LAN on Router A using
outgoing interface? The LAN on Router B from
Router A using next-hop?
68
The static default route

• A router should be configured with a special type
  of static route a default route.
• This default route routes packets with
  destinations that do not match any of the other
  routes in the routing table
• It is a gateway of last resort that allows the
  router to forward destination unknown packets
  out a particular interface
• ip route 0.0.0.0 0.0.0.0 next-hop-address
  outgoing interface

69
Default Route on non-directly connected networks
70
Default Route on non-directly connected networks
71
CCNA v3.0Module 1

• Introduction to Classless Routing