CCNA3 3.1-01 Routing

CCNA v3.0 RetoolingWhat 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).
CCNA v3.0 Semester 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.
Routing Protocols Supporting VLSM
207.21.24.0 /24
How many hosts are needed for the largest LAN?
How many bits need to be borrowed to address this network?
Site A
Site B
Site C
25 users
25 users
10 users
8 users
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.
Site A
Site B
Site C
25 users
25 users
10 users
8 users
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
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
Subnetting in a Box
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
255
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
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
Borrowing 3 bits would break each of these 4 networks in half again.
Subnet mask: 255.255.255.224.
0
255
Borrowing 4 bits would break each of these 8 networks in half again.
Subnet mask: 255.255.255.240.
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.
Sheet1
Classless Routing Protocols (do share subnet info)
RIP v1
RIP v2
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)
Site A
Site B
Site C
25 users
25 users
10 users
8 users
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
Site A
Site B
Site C
25 users
25 users
10 users
8 users
Site A
Through applying VLSM, the topology was able to be addressed and still have two complete subnets available for future growth.
207.21.24.32 /27
207.21.24.64 /27
207.21.24.160 /28
207.21.24.176 /28
207.21.24.192 /30
207.21.24.196 /30
Site A
Site B
Site C
25 users
25 users
10 users
8 users
Exercise 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)
Headquarters
Subnet according to the largest subnet needed. (Headquarters 60 hosts)
0
255
Borrow 2 bits or /26. This would give you 4 networks with 64 host addresses on each subnet.
128
127
64
192
63
191
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
(255.255.255.192)
0
64
128
192
The 5 Branch offices need 12 hosts each.
The next address block available is the .128 - .191 block. Use VLSM.
Headquarters
(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.
0
64
128
192
160
144
176
To obtain a block for Branch 5, we will need to subnet the .192 - .255 block using a /28 mask.
Headquarters
(255.255.255.192)
Now connect the 5 WAN links to the Branch offices.
These are point-to-point connections and only require 2 addresses.
0
64
128
192
Here we will use a /30 mask to further subnet the subnets.
Headquarters
(255.255.255.192)
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
64
128
192
Headquarters
(255.255.255.192)
Address provided by ISP
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
Class
Number of Hosts
32
/26
64
/25
128
/24
65,536
/15
131,072
/14
262,144
/13
524,288
/12
1,048,576
/11
2,097,152
/10
4,194,304
/9
8,388,608
/8
65,536 Class C or 256 Class B or 1 Class A
16,777,216
Sheet3
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.
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
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.
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
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.
Number of Hosts
32
/26
64
/25
128
/24
256
/23
512
/22
1,024
/21
2,048
/20
4,096
/19
8,192
/18
16,384
/17
32,768
/16
65,536
/15
131,072
/14
262,144
/13
524,288
/12
1,048,576
/11
2,097,152
/10
4,194,304
/9
8,388,608
/8
65,536 Class C or 256 Class B or 1 Class A
16,777,216
Sheet3
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
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
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
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).
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.
300 users
100 users
100 users
100 users
How many /24 networks do we have?
How many host addresses do we have?
What is the largest LAN requirement?
300 users
100 users
100 users
100 users
Host required - 300, 100, 100, 100, and 3 WAN links
200.32.108.0
0
0
0
0
255
255
255
255
0
0
0
0
255
255
255
255
0
0
0
0
255
255
255
255
0
0
0
0
255
255
255
255
Two /24s
300 users
100 users
100 users
100 users
200.32.108.0 /23
200.32.110.0 /25
200.32.110.128 /25
200.32.111.0 /25
200.32.111.240 /30
200.32.111.248 /30
200.32.111.244 /30
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
(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.
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.
Authentication of routing updates
External route tags
Multicast route updates
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.
RIP v2 Packet Format
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.
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
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
Configuring RIP v1
To configure RIP v1 on a router, enter the following commands:
Router# config t
Router(config)# router rip
Router(config-router)# 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.
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)# version 2
NOTE - The default behavior can be restored by entering the command no version in the config-router mode.
Router(config-router)# no version
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
You can override the default behavior of RIP by configuring a particular interface to behave differently.
Overriding Default Behavior of RIP
Interface e0 sends and receives version 1 updates only.
RIP v2 configured on the router.
Router(config-router)# version 2
Router(config-if)# ip rip send version 1
Router(config-if)# ip rip receive version 1
You can override the default behavior of RIP by configuring a particular interface to behave differently.
Interface e2 has no special configuration and therefore sends and receives version 2 by default.
Overriding Default Behavior of RIP
Interface e1 sends and receives both version 1 and 2 updates.
Router(config)# int e1
Router(config-if)# ip rip send version 1 2
Router(config-if)# ip rip receive version 1 2
Router(config)# int e2
Administrative distance of 0 - default
outgoing interface
Administrative distance of 1 - default
Next hop interface
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.
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
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?
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]
Classless Routing Protocols
Class B
Class CSmall Networks24 bits8 bits 192 - 2232,097,152254
The Class System
/271/8th of a Class C32
/261/4th of a Class C64
/251/2 of a Class C128
/241 Class C or 1 /24256
/232 Class C or 2 /24s512
/224 Class C or 4 /24s1,024
/218 Class C or 8 /24s2,048
/2016 Class C or 16 /24s4,096
/1932 Class C or 32 /24s8,192
/1864 Class C or 64 /24s16,384
/17128 Class C or 128 /24s32,768
/16256 Class C or 1 Class B65,536
/141,024 Class C or 4 Class B262,144
/124096 Class C or 16 Class B1,048,576
/118192 Class C or 32 Class B2,097,152
/1016384 Class C or 64 Class B4,194,304
/932768 Class C or 128 Class B8,388,608
/865,536 Class C or 256 Class B or 1 Class A16,777,216
Prefix Equivalents

Date post:	22-Sep-2015
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CCNA3 3.1-01 Routing

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