CCNP Routing Study Guide- P5 pps

Take a look at the way a Cisco 2500 series router asks you to put the subnet mask in the tion when using the Setup mode: configura-Configuring interface Ethernet0: Is this interface in

Let’s now take a look at how Cisco handles CIDR

Cisco and CIDR

Cisco has not always followed the CIDR standard Take a look at the way

a Cisco 2500 series router asks you to put the subnet mask in the tion when using the Setup mode:

configura-Configuring interface Ethernet0:

Is this interface in use? [yes]: return

Configure IP on this interface? [yes]: return

IP address for this interface: 1.1.1.1

Number of bits in subnet field [0]: 8

Class A network is 1.0.0.0, 8 subnet bits; mask is /16Notice that the router asks for the number of bits used only for subnet-ting, which does not include the default mask When dealing with these ques-tions, remember that your answers involve the number of bits used for creating subnets, not the number of bits in the subnet mask The industry standard is that you count all bits used in the subnet mask and then display that number as a CIDR, for example, /25 is 25 bits

The newer IOS that runs on Cisco routers, however, runs a Setup script that no longer asks you to enter the number of bits used only for subnetting Here is an example of a new 1700 series router in Setup mode:

Configure IP on this interface? [no]: y

IP address for this interface: 1.1.1.1

Subnet mask for this interface [255.0.0.0]: 255.255.0.0Class A network is 1.0.0.0, 16 subnet bits; mask is /16Notice that the Setup mode asks you to enter the subnet mask address It then displays the mask using the slash notation format Much better

indi-In many cases, the router doesn’t even need specific routes to each and every subnet (e.g., 172.16.1.0/24) It would be just as happy if it knew how

to get to the major network (e.g., 172.16.0.0/16) and let another router take

it from there A router’s ability to take a group of subnetworks and

summa-rize them as one network (i.e., one advertisement) is called route

summari-zation, as shown in Figure 3.5.

In some of the literature, you may find route summarization referred to as

route aggregation.

F I G U R E 3 5 Route summarization

Besides reducing the number of routing entries that a router must keep track of, route summarization can also help protect an external router from making multiple changes to its routing table, due to instability within a par-ticular subnet For example, let’s say that we were working on a router that connected to 172.16.2.0/24 As we were working on the router, we rebooted

it several times If we were not summarizing our routes, an external router would see each time 172.16.2.0/24 went away and came back Each time, it would have to modify its own routing table However, if our external router

I am the way to get to network 172.16.0.0/16

172.16.2.0/24 172.16.1.0/24

172.16.3.0/24

net-Route Summarization Example 1

We have the following networks that we want to advertise as a single mary route:

sum-172.16.100.0/24172.16.101.0/24172.16.102.0/24172.16.103.0/24172.16.104.0/24172.16.105.0/24172.16.106.0/24

To determine what the summary route would be for these networks, we can follow a simple two-step process

1. Write out each of the numbers in binary, as shown in Table 3.14

2. Examine the table to determine the maximum number of bits (starting from the left) that all of the addresses have in common (where they stop lining up; we bolded them to make them easier for you to see) The number of common bits is the subnet mask for the summarized address (/20).

In our example, we can see from the table that all of the addresses have the first 20 bits in common The decimal equivalent of these first 20 bits is 172.16.96.0 So, we can write our new summarized address as 172.16.96.0/20

If we were to later add a network 172.16.98.0, it would need to come off the router summarizing this address space If we didn’t, it could cause problems Okay, this is confusing, we know This is why we’re going to give you three more examples

Route Summarization Example 2

In this example, we will summarize 10.1.0.0 through 10.7.0.0 First, put everything into binary, and then follow the bits, starting on the left and stop-ping when the bits do not line up Notice where we stopped boldfacing the following:

Now, create the network number using only the boldfaced bits Do not count the bits that are not in boldface The second octet has no bits on (bits

in the bolded section), so we get this:

Extending IP Addresses 87

To come up with the summary mask, count all the bolded bits as ones Because eight bits are boldface in the first octet and five bits in the second, we’ll get this:

255.248.0.0

Route Summarization Example 3

This example will show you how to summarize 172.16.16.0 through 172.16.31.0 First, let’s put the network addresses into binary and then line

Notice where the bits stop lining up (in boldface) Count only the bits that are on (ones) to get the network address:

172.16.0.0Now, create the summary mask by counting all the bits that are in bold-face up to the point where they stop lining up We have eight bits in the first octet, eight bits in the second octet, and four bits in the third octet That is

a /20 or255.255.240.0Boy, that sure seems like a pain in the pencil, huh? Try this shortcut Take the first number and the very last number, and put them into binary:

Can you see that we actually came up with the same numbers? It is a lot easier than writing out possibly dozens of addresses Let’s do another exam-ple, but let’s use our shortcut

Route Summarization Example 4

In this example, we will show you how to summarize 192.168.32.0 through 192.168.63.0 By using only the first network number and the last, we’ll save

a lot of time and come up with the same network address and subnet mask: First number: 192.168.32.0 =

11000000.10101000.00100000.00000000

Last number: 192.168.63.0 =

11000000.10101000.00111111.00000000

Network address: 192.168.32.0Subnet mask: 255.255.224.0172.16.16.0 10101100.0001000.00010000.00000000

172.16.31.0 10101100.0001000.00011111.00000000

Extending IP Addresses 89

Design Considerations for Route Summarization

Keep the following information in mind when designing your network marization points:

sum- Only classless routing protocols support route summarization ples of classless routing protocols include RIPv2, EIGRP, and OSPF Therefore, if you are working in a RIPv1 or IGRP environment, route summarization is not going to work for you

Exam-Classless and classful protocols were discussed in Chapter 2, “Routing Principles.”

 Route summarization is most effective when the addresses have been organized in a hierarchy (i.e., “hierarchical addressing”) When we speak of addresses being hierarchical, we mean that the IP subnets at the “bottom of the tree” (i.e., the ones with the longest subnet masks) are subsets of the subnets at the “top of the tree” (i.e., the ones with the shortest subnet masks) Figure 3.6 will be used to illustrate hierar-chical versus non-hierarchical addressing

F I G U R E 3 6 Discontiguous networking example

In the VLSM section of this chapter, we discussed how route tion in discontiguous networks could cause some hosts to become unreach-able, as we saw in Figure 3.4 If both RouterA and RouterB are sending out advertisements to the WAN cloud advertising that they are the path to net-work 172.16.0.0/16, then devices in the WAN cloud will not know which advertisement to believe

summariza-Hierarchical Adressing

10.1.0.0/16

10.1.2.8/30 10.1.2.4/30

10.1.1.8/30 10.1.1.4/30

Non-Hierarchical Adressing

10.1.0.0/16

10.1.2.8/30 10.1.2.4/30

10.3.0.0/16 172.16.2.0/24

Extending IP Addresses 91

Remember that you can avoid this situation by proper address planning ahead of time However, you may find yourself in a situation where you are dealing with a legacy installation, and you need to overcome this issue of dis-contiguous networks

One solution is to turn off route summarization on the routers To keep routing protocols such as RIPv2 and EIGRP from automatically summariz-ing routes, we can explicitly disable route summarization in the Cisco IOS Following are examples of IOS configurations, where we are disabling auto-matic route summarization As the OSPF chapters will show, OSPF does not automatically summarize

To turn off auto-summarization for RIP version 2 routed networks, use the following router configuration:

Another way to allow discontiguous networks to be interconnected over

a serial link is to use Cisco’s IOS feature called IP unnumbered We’ll look

at this next

IP Unnumbered

With IP unnumbered, a serial interface is not on a separate network, as all router interfaces tend to be Instead, the serial port “borrows” an IP address from another interface In the following router configuration example, inter-face Serial 0 is using a borrowed IP address from interface Ethernet 0:interface serial 0

ip unnumbered ethernet 0

Therefore, by using IP unnumbered, the apparently discontiguous subnets, shown in Figure 3.4, are actually supported Understand that both sides of the network must be the same address class In other words, you can’t bor-row an IP address on one side from a 10.0.0.0 network and then from 172.16.0.0 on the other side of the point-to-point link

There are a few things to be aware of before using IP unnumbered interfaces For example, IP unnumbered is not supported on X.25 or SMDS networks Also, since the serial interface has no IP number, you will not be able to ping the interface to see if it is up, although you can determine the interface status with SNMP In addition, IP security options are not supported on an IP unnum- bered interface.

Decimal-to-Binary Conversion Chart

For your convenience, Table 3.15 provides a decimal-to-binary chart

to help you with your IP addressing

Decimal-to-Binary Conversion Chart 93

T A B L E 3 1 5 Decimal-to-Binary Chart (continued)

Decimal Binary Decimal Binary Decimal Binary Decimal Binary

T A B L E 3 1 5 Decimal-to-Binary Chart (continued)

Decimal Binary Decimal Binary Decimal Binary Decimal Binary

Decimal-to-Binary Conversion Chart 95

T A B L E 3 1 5 Decimal-to-Binary Chart (continued)

Decimal Binary Decimal Binary Decimal Binary Decimal Binary

After a review of fundamental IP addressing concepts, which detailed the various classes of IP numbers in addition to the concepts of subnetting and CIDR, this chapter discussed how to preserve IP addresses by using VLSMs (Variable-Length Subnet Masks) It also examined various design considerations, such as using contiguous network addressing and using classless routing protocols (e.g., RIPv2 and EIGRP)

Next, we introduced the concept of route summarization We saw how router resources, such as memory and processor cycles, could be preserved

by representing contiguous network address space by a single route tisement We also showed how to overcome the caveat of having discontig-uous address space by using such methods as disabling automatic summarization on our routers and by using IP unnumbered

route summarizationsubnet mask

Variable-Length Subnet Mask (VLSM)

Written Lab 97

Commands Used in This Chapter

Here is the list of commands used in this chapter:

Written Lab

Given the following set of address requirements, the available Class B network address, and the topology map shown in the graphic below, use VLSM to efficiently assign addresses to each of the four network segments

no auto-summary Used to disable the automatic route

summarization performed by various classless routing protocols, such as RIPv2 and EIGRP

ip unnumbered Allows serial interfaces to borrow an IP

number from another router interface (which may or may not be specified), so that

it can join two contiguous address spaces

Point-to-Point Serial Connection

Server Farm Switch (Requires 50 IP Addresses)

Public Access Computer Lab Switch (Requires 400 IP Addresses)

Ethernet User Segment (Use Class C Subnet Mask)

Design Requirements

 You have been given the Class B address of 172.16.0.0/16 to use

 The first segment connects to a server farm requiring no more than 50

Solution to Written Exercise

Although there are multiple ways that the given address space (172.16.0.0/16) could be divided up, here is one possible solution based on the method-ology presented in this chapter

1. Create a table detailing the segments and the number of hosts required

on each segment, as shown in the following table:

2. Determine the subnet mask required to support the requirements defined in step 1, and expand the table to list the subnet masks We will use the table listed earlier in the chapter (Table 3.7), which tells the maximum number of hosts permitted by each subnet mask The

Description of Segment Number of IP Addresses Required

Server farm 50 (Because the maximum number of

servers is 50)Ethernet user segment 254 (Because a Class C subnet was

specified)Serial link 2 (Because each of the two routers needs

one IP address)Computer lab 400 (Because each PC needs its own

IP address)

Description of Segment

Number of IP Addresses Required

Subnet Mask (Number

of Bits in Subnet)

Server farm 50 (Because the

maximum number of servers is 50)

255.255.255.192 (26)

Ethernet user segment

254 (Because a Class C subnet was specified)

255.255.255.0 (24)

Description of Segment

Number of IP Addresses Required

Subnet Mask (Number

of Bits in Subnet)

Serial link 2 (Because each of the

two routers needs one

IP address)

255.255.255.252 (30)

Computer lab 400 (Because each PC

needs its own IP address)

255.255.254 (23)

128 64 32 16 8 4 2 1 128 64 32 16 8 4 2 1 (Last 16 bits in bold)Subnet mask 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 255.255.255.252Network 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 172.16.0.4

First IP in

range

0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 1 172.16.0.5

After picking the first available network number (172.16.0.4) given our 30-bit subnet mask and eliminating host IP addresses that are all ones and all zeros, we have the following range of numbers: 172.16.0.5–172.16.0.6 Each of these numbers in the range can be assigned to one side of the serial link.Next, as shown in the following table, we will calculate the range of IP addresses to use for our server farm segment, which needs 50 IP addresses

We pick the first available network address, given our 26-bit subnet mask

In this case, the first available network is 172.16.0.64

Eliminating host IP addresses that contain all ones and all zeros, as before,

we discover that our IP address range for this segment is: 172.16.0.65–172.16.0.126

We now perform the same steps for the Ethernet user segment, as shown

in the table below:

128 64 32 16 8 4 2 1 128 64 32 16 8 4 2 1 (Last 16 bits in bold)Subnet mask 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 255.255.255.248Network 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 172.16.0.64

128 64 32 16 8 4 2 1 128 64 32 16 8 4 2 1 (Last 16 bits in bold)Subnet mask 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 255.255.255.0Network 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 172.16.1.0