© 2011 itt educational services inc. nt-2640 advanced networking: unit 7: slide 1 unit 7 advanced...

© 2011 ITT Educational Services Inc. NT-2640 Advanced Networking: Unit 7: Slide 1

Unit 7

Advanced IP Routing

Chapter 19 to 21

Class Agenda 10/31/15

• Learning Objectives• Unit 7: Discussions and Video• Lab Activities will be done in class.• Assignments will be given in class.• Break Times. 10 Minutes break in every 1 Hour.• Note: Submit all Assignment and labs due

today.

Finding All Subnet IDs

• Given a single class A, B, or C network, and the single subnet mask to use for all subnets, what are all the subnet IDs?

• When learning how to answer this question, you can think about the problem in either binary or in decimal.

• The decimal process begins by identifying the first, or numerically lowest, subnet ID. After that, the process identifies a pattern in all subnet IDs for a given subnet mask, so that you can find each successive subnet ID through simple addition.

© 2011 ITT Educational Services Inc. NT-2640 Wan Technologies: Unit 6: Slide 3

The First Subnet ID: The Zero Subnet

• The first step in finding all subnet IDs of one network is incredibly simple: copy the network ID.

• That is, take the class A, B, or C network ID – in other words, the classful network ID – and write it down as the first subnet ID.

• No matter what class A, B, or C network you use, and no matter what subnet mask you use, the first (numerically lowest) subnet ID is equal to the network ID.

• For example, if you begin with classful network 172.20.0.0, no matter what the mask is, the first subnet ID is 172.20.0.0.


Finding the Pattern Using the Magic Number

• Subnet IDs follow a predictable pattern, at least when using our assumption of a single subnet mask for all subnets of a network.

• The pattern is equal to the magic number.• To review, the magic number is 256, minus the mask’s decimal value, in a

particular octet that the book refers to as the interesting octet.


Generic Subnets Chart

Table 18-1 Generic List-All-Subnets Chart

Octet 1 2 3 4

Mask

Magic Number

Network Number/Zero Subnet

Next Subnet

Next Subnet

Last Subnet

Broadcast Subnet

Out of Range (Used by Process)


Finding the Magic Number• Step 1. Write down the subnet mask, in decimal, in the first

empty row of the table.• Step 2. Identify the interesting octet, which is the one octet of

the mask with a value other than 255 or 0. Draw a rectangle around the column of the interesting octet.

• Step 3. Calculate and write down the magic number by subtracting the subnet mask’s interesting octet from 256.

• Step 4. Write down the classful network number, which is the same number as the zero subnet, in the next empty row of the list-all-subnets chart.


Finding the Magic Number

• Step 5. To find each successive subnet number: a. For the three uninteresting octets, copy the previous subnet number’s values. b. For the interesting octet, add the magic number to the previous subnet

number’s interesting octet.

• Step 6. Once the sum calculated in Step 5b reaches 256, stop the process. The number with the 256 in it is out of range, and the previous subnet number is the broadcast subnet.

Results of First Four Steps: 172.16.0.0, 255.255.240.0


Results on 172.16.0.0, 255.255.240.0

• Step 1. Record mask 255.255.240.0, which was given as part of the problem statement.

• Step 2. The mask’s third octet is neither 0 nor 255, making the third octet interesting.

• Step 3. Because the mask’s value in the third octet is 240, the magic number = 256 – 240 = 16.

• Step 4. Because the network ID is 172.16.0.0, the first subnet ID, the zero subnet, is also 172.16.0.0.


List of Subnet IDs: 172.16.0.0, 255.255.240.0


Results of First Four Steps: 192.168.1.0, 255.255.255.224


List of Subnet IDs: 192.168.1.0, 255.255.255.224


© 2011 ITT Educational Services Inc. NT-2640 Advanced Networking: : Unit 1: Slide 14

Check Point

1. Describe the magic number process

2. What is subnet ID or zero subnet?

3. What is the subnet zero for IP address 192.168.131.29/26?

© 2011 ITT Educational Services Inc. NT-2640 Advanced Networking: Unit 7: Slide 15

Objectives

In this unit, students will demonstrate an: •Understanding of VLSM including Design and Subnetting Concepts

•Understanding of Route Summarization Concepts

•Understanding of the Host Networking Utilities for Troubleshooting IP Networking Issues including “ping” and “traceroute”

•Understanding of Host & Router IP Routing Troubleshooting including Commons Steps

•Understanding of OSPF including Neighbor Discovery and Establishment, Topology Exchange and Route Selection Processes

•Understanding of OSPF Areas, Router Types, and Convergence

•Understanding of OSPF Configuration and Verification Steps

VLSM

• VLSM occurs when an internetwork uses more than one mask in different subnets of a single Class A, B, or C network.

• VLSM allows engineers to reduce the number of wasted IP addresses in each subnet, allowing more subnets and avoiding having to obtain another registered IP network number from regional IP address assignment authorities.

• , even when using private IP networks (as defined in RFC 1918), large corporations might still need to conserve the address space, again creating a need to use VLSM.


Classless and Classful Routing Protocols

• For a routing protocol to support VLSM, the routing protocol must advertise not only the subnet number but also the subnet mask when advertising routes.

• A routing protocol must include subnet masks in its routing updates to support manual route summarization.

• Each IP routing protocol is considered to be either classless or classful , based on whether the routing protocol does (classless) or does not (classful) send the mask in routing updates.

• Each routing protocol is either classless or classful by its very nature; no commands exist to enable or disable whether a particular routing protocol is a classless or classful routing protocol.


Classless and Classful Interior IP Routing Protocols

Routing Protocol

Is It Classless?

Sends Mask in Updates

Supports VLSM

Supports Manual Route Summarization

RIP-1 No No No No

IGRP No No No No

RIP-2 Yes Yes Yes Yes

EIGRP Yes Yes Yes Yes

OSPF Yes Yes Yes Yes


Manual Route Summarization• Small networks might have only a few dozen routes in their routers’ routing

tables. • The larger the network, the larger the number of routes. • In fact, Internet routers have more than 100,000 routes in some cases.• The routing table might become too large in large IP networks. As routing

tables grow, they consume more memory in a router. • Also, each router can take more time to route a packet, because the router

has to match a route in the routing table, and searching a larger table generally takes more time.

• And with a large routing table, it takes more time to troubleshoot problems, because the engineers working on the network need to sift through more information.

• Route summarization reduces the size of routing tables while maintaining routes to all the destinations in the network.


Route Summarization Concepts

• Engineers use route summarization to reduce the size of the routing tables in the network.

• Route summarization causes some number of more-specific routes to be replaced with a single route that includes all the IP addresses covered by the subnets in the original routes.

• Summary routes, which replace multiple routes, must be configured by a network engineer.

• Route summarization works much better when the network was designed with route summarization in mind.


Route Summarization Configuration

• Route summarization configuration differs with different routing protocols; Enhanced IGRP (EIGRP) is used.

• The summary routes for EIGRP are created by the ip summary-address interface subcommands on Yosemite and Seville in this case.


Autosummarization And Manual summarization

• Manual route summarization can improve routing efficiency, reduce memory consumption, and improve convergence by reducing the length of routing tables.

• Automatic summarization of routes at the boundaries of classful networks, uses a feature called autosummarization.


Autosummarization Support and Configuration

• Classful routing protocols must use autosummarization. Some classless routing protocols support autosummarization, defaulting to use it, but with the ability to disable it with the no auto-summary router subcommand.

• Below summarizes the facts about autosummarization on Cisco routers.


* Autosummarization Support and Defaults

Routing Protocol Classless?

Supports Autosummarization?

Defaults to Use Autosummarization?1

Can Disable Autosummarization?

RIP-1 No Yes Yes No

RIP-2 Yes Yes Yes Yes

EIGRP Yes Yes Yes Yes

OSPF Yes No — —

Chapter 20

“Troubleshooting II”


The ping and traceroute Commands

• Internet Control Message Protocol (ICMP)• TCP/IP includes ICMP, a protocol designed to help manage and control the

operation of a TCP/IP network. • The ICMP protocol provides a wide variety of information about a network’s

health and operational status. • Control Message is the most descriptive part of the name. ICMP helps

control and manage IP’s work by defining a set of messages and procedures about the operation of IP.

• Therefore, ICMP is considered part of TCP/IP’s network layer. Because ICMP helps control IP, it can provide useful troubleshooting information.

• In fact, the ICMP messages sit inside an IP packet, with no transport layer header, so ICMP is truly an extension of the TCP/IP network layer.


• The ping Command and the ICMP Echo Request and Echo Reply• The ping command uses the ICMP Echo Request and Echo Reply

messages. • When people say they sent a ping packet, they really mean that they sent an

ICMP Echo Request. • These two messages are somewhat self-explanatory. • The Echo Request simply means that the host to which it is addressed

should reply to the packet. • The Echo Reply is the ICMP message type that should be used in the reply. • The Echo Request includes some data that can be specified by the ping

command; whatever data is sent in the Echo Request is sent back in the Echo Reply.


The Ping Command

• The ping command lists various responses that in some cases imply that an unreachable message was received.


ping Command Code Description

! ICMP Echo Reply received

. Nothing was received before the ping command timed out

U ICMP unreachable (destination) received

N ICMP unreachable (network/subnet) received

M ICMP Can’t Fragment message received

? Unknown packet received

The ICMP Time Exceeded Message

• The ICMP Time Exceeded message notifies a host when a packet it sent has been discarded because it was “out of time.”

• Packets are not actually timed, but to prevent them from being forwarded forever when there is a routing loop, each IP header uses a Time to Live (TTL) field.

• Routers decrement the TTL by 1 every time they forward a packet; if a router decrements the TTL to 0, it throws away the packet.

• This prevents packets from rotating forever. Figure 7-3 shows the basic process.


TTL Decremented to 0


The traceroute Command• The ping command is a powerful troubleshooting tool that can be used to

answer the question “Does the route from here to there work?” • The traceroute command provides an arguably better troubleshooting tool

because not only can it determine if the route works, but it can supply the IP address of each router in the route.

If the route is not working, traceroute can identify the best places to start troubleshooting the problem.

The IOS traceroute command uses the Time Exceeded message and the IP TTL field to identify each successive router in a route.

• The traceroute command sends a set of messages with increasing TTL values, starting with 1.

The traceroute command expects these messages to be discarded when routers decrement the TTL to 0, returning Time Exceeded messages to the traceroute command. The source IP addresses of the Time Exceeded messages identify the routers that discarded the messages, which can then be displayed by the traceroute command.


show ip route Reference

• The show ip route command plays a huge role in troubleshooting IP routing and IP routing protocol problems.

• Below shows the output of the show ip route command.

show ip route Command Output Reference


Lab Activities.

• Complete 7 Lab in class.

• All answers to overdue labs should be submitted in the next class.


Assignment

• Unit 7 assignment will be given in class.

• Reading Assignment: Read chapter 22 and 23


© 2011 itt educational services inc. nt-2640 advanced networking: unit 7: slide 1 unit 7 advanced...

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