Chapter 6. Routing Issues

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Chapter 6

Routing Issues

Introduction

Classless Interdomain Routing

From Millions to Thousands of Networks

ISP Address Assignment

Using CIDR Addresses Inside Your Network

Contiguous Subnets

IGRP

EIGRP

EIGRP Concepts

RIP-1 Requirements

Comparison with IGRP

Routing Update Impact

RIP-2 Requirements

OSPF

Configuring OSPF

Routing Update Impact

OSPF Implementation Recommendations

BGP Requirements

IBGP and EBGP Requirements

Loopback Interfaces

Summary

FAQs

 

This chapter will discuss the purpose of routing and the many issues that arise from routing in various network environments, from smaller networks to very large, complicated, dynamic networks such as the Internet. We will introduce the many routing protocols, such as the Routing Information Protocol (RIP), Open Shortest Path First (OSPF), and Border Gateway Protocol (BGP), and discuss the characteristics and issues involved with each. Each routing protocol has its own set of strengths and weaknesses that you will need to assess in order to understand how to implement this protocol. You will also see how these routing protocols are addressing the issue of the exhaustion of available IP addresses, the introduction of the IPv6 protocol, and the concern for growing routing tables on major routers on the Internet.

 

Solutions in this chapter:

    Introduction to routing protocols

    Supernetting with Classless Interdomain Routing (CIDR)

    Internal Routing with Interior Gateway Routing Protocol (IGRP) and Enhanced Interior Gateway Routing Protocol (EIGRP)

    Understanding the history of the Routing Information Protocol (RIP) and RIP-2

    Implementing the Open Shortest Path First (OSPF) routing protocol

    External network routing with Exterior Gateway Protocol (EGP) and Border Gateway Protocol (BGP)

Introduction

As most of you know, the rate of growth on the Internet is phenomenal, and usage has increased nearly exponentially. Networks and hosts are being added to the Internet, which threatens to eat up every available IP address unless something is done. Not only is the exhaustion of available IP addresses an important issue, we also have to deal with the tremendous amount of routing that takes place on the Internet. Routers are network devices used to route packets to different networks on the Internet. The Internet is composed of hundreds of thousands of different networks. Routers use a routing table , which is an internal table that contains routes to networks and other routers. In most routers found on the Internet, these routes are learned dynamically by the use of a dynamic routing protocol such as RIP, IGRP, OSPF, and BGP, to name a few. Routers share information with each other concerning the availability of paths and the shortest distance to a destination. In the past, the routing tables have been growing as fast as the Internet; however, technology has not been able to keep pace. The number of routes advertised has doubled every 10 months. It was estimated that there were around 2000 routes on the Internet in 1990, and two years later there were 8500 routes. In 1995 there were over 29,000 routes, which required around 10MB of memory for the router. A router requires a significant amount of RAM and CPU in order to add, modify, delete, and advertise these routing tables with other routers. The routing tables have been growing at a slower rate, and we now have about 65,000 routes.

With the advent of Classless Interdomain Routing, we have been able to limit significantly the growth of these routing tables, making them more manageable and efficient.

Classless Interdomain Routing

Classless Interdomain Routing (CIDR, pronounced as apple cider) was developed when the world was faced with the exhaustion of Class B address space and the explosion of routing between tons of Class C addresses. CIDR allows for a more efficient allocation of IP addresses than the old Class A, B, and C address scheme. This old scheme is often referred to as classful addressing, whereas CIDR is referred to as classless addressing, as illustrated in Figure 6.1.

 

Figure 6.1 The prefix length of a classless address.

 

Another term for CIDR supernetting is prefix-based addressing . As you can see in Figure 6.1, it looks very similar to custom subnet masking, where the boundary between the network ID and host ID is not fixed.

 

You will learn later in this section just how this supernetting is possible. If you are familiar with TCP/IP subnet masking, you will have no problems understanding the concept of supernetting and classless addressing. Both concepts involve masking a portion of the IP address to reveal a network address. CIDR extended the successful ideas of TCP/IP subnetting.

 

Some say that if it werent for the advent of CIDR, the Internet would not be functioning today. That is a testament to the power of CIDR, and the need of CIDR for networking supernetting. CIDR is the best hope we have for smoothing the transition from Ipv4 to Ipv6.

 

The IETF wrote the standard for CIDR in the early 1990s, and it is described in RFC 1517 through RFC 1520. CIDR has a primary requirement for using a routing protocol, such as RIP version 2, OSPF version 2, and BGP version 4.

 

CIDR helps the Internet reduce the routing overload by minimizing routing tables and making sure the most important routes are carried by most routers, making the path to sites much quicker. These routing tables are global, and contain information for routes across the planet, so you can begin to see how large these routing tables can get. The routing tables are dangerously close to a level where current software, hardware, and people can no longer effectively manage.

 

CIDR is very similar to subnetting, but actually is a more advanced method of subnetting that can combine networks into supernets ; subnetting, on the other hand, involves breaking networks into smaller, more manageable subnets . This is accomplished through the use of the subnet mask, which masks a portion of the IP address to differentiate the network ID from the host ID. With CIDR, you basically eliminate the concept of Class A, B, and C networks, and replace them with a generalized IP prefix consisting of an IP address and the mask length. For example, a single class C address would appear as 195.129.1.0/24, in which /24 refers to the number of bits of the network portion of the IP address.

 

With the traditional Class A, B, and C addressing scheme, the addresses were identified by converting the first eight bits of the address to their decimal equivalent. Table 6.1 shows the breakdown of the three address classes, and how many bits appear in the host ID and the network ID.

 

Address Class

# Network Bits

# Hosts Bits

Decimal Address Range

Class A

8 bits

24 bits

1126

Class B

16 bits

16 bits

128191

Class C

24 bits

8 bits

192223

Table 6.1 The Familiar Delineations of the IP Address Classes

 

Using the old Class A, B, and C addressing scheme, the Internet could support the following:

    126 Class A networks that could include up to 16,777,214 hosts each

    65,000 Class B networks that could include up to 65,534 hosts each

    Over 2 million Class C networks that could include up to 254 hosts each

 

As you can see, there are only three classes; every company or organization will have to choose the class that best supports their needs. Since it is nearly impossible to receive a Class A or B address, you would be stuck with a Class C address, which may or may not be suitable for your needs. If you were assigned one Class C address, and you only needed 10 addresses, you would be wasting 244 addresses. This results in what appears to be a condition of running out of addresses; however, the problem stems more from the inefficient use of the addresses. CIDR was developed to be a much more efficient method of assigning addresses.

 

A CIDR supernet consists of numerous contiguous IP addresses. An ISP can assign their customers blocks of contiguous addresses to define the supernets. Each supernet has a unique supernet address that consists of the upper bits that are shared between all IP addresses in the supernet. For example, the following group of addresses are all contiguous (198.113.0.0 through 198.113.7.0 in decimal notation).

11000110 01110001 00000   000 00000000

11000110 01110001 00000   001 00000000

11000110 01110001 00000   010 00000000

11000110 01110001 00000   011 00000000

11000110 01110001 00000   100 00000000

11000110 01110001 00000   101 00000000

11000110 01110001 00000   111 00000000

 

The supernet address for the block is 11000110 01110001 00000 (the 21 upper bits) because every address in the supernet has this in common. The complete supernet address consists of the address and the mask.

    The address is the first 32-bit address in the contiguous address block. In our case this would be 11000110 01110001 00000000 00000000 (198.113.0.0 in decimal notation).

    The mask is a 32-bit string, similar to the subnet mask, which contains a set bit in the supernet portion of the address. In our case this would be 11111111 11111111 11111000 00000000 (255.255.248.0 in decimal notation). The masked portion, however, contains the number of bits that are in the on position; in our case this would be 21.

 

The complete supernet address would be 198.113.0.0/21. The /21 indicates that the first 21 bits are used to identify the unique network, leaving the remaining bits to identify the specific host.

 

 

 

You can compare this to an office phone system where every phone number starts with a prefix such as 288 and ends with a unique four-digit combination. For example, your phone number is 288-1301, and Doug Fortune, the Human Resources supervisor, has a phone number of 288-2904. Most companies are set up so that you can dial the unique portion of the user s phone number as a means of internal dialing. To contact Doug, you would just dial 2904, which is the unique portion of his full phone number. Continuing the example, 288, the prefix of the phone number, would be the supernet address. Isnt it much easier to dial the person's four-digit extension rather than the entire seven-digit extension? Imagine if you had to dial the area code every time you made a local call. Also continuing the comparison, the area code resembles a supernet address for an area.

 

CIDR can then be used to employ a supernet address to represent multiple IP destinations. Rather than advertise a separate route for each of the members of the contiguous address space, the router can now advertise the supernet address as a single route, called an aggregate route. This aggregate route will represent all the destinations within the supernet address, thereby reducing the amount of information that needs to be contained in the routing tables of the routers. This may not seem like much of a reduction in the routing table, but multiply this by hundreds of routers on the Internet, and you can see the effect CIDR can have on the number of entries in the routing tables.

 

Table 6.2 shows how the CIDR block prefix is used to increase the number of groups of addresses that can be used, thereby offering a more efficient use of addressing than the Class A, B, or C method.

 

CIDR Block Prefix

# Equivalent Class C

# of Host Addresses

/27

1/8th of a Class C

32 hosts

/26

1/4th of a Class C

64 hosts

/25

1/2 of a Class C

128 hosts

/24

1 Class C

256 hosts

/23

2 Class C

512 hosts

/22

4 Class C

1,024 hosts

/21

8 Class C

2,048 hosts

/20

16 Class C

4,096 hosts

/19

32 Class C

8,192 hosts

/18

64 Class C

16,384 hosts

/17

128 Class C

32,768 hosts

/16

256 Class C

65,536 hosts

 

(= 1 Class B)

 

/15

512 Class C

131,072 hosts

/14

1,024 Class C

262,144 hosts

/13

2,048 Class C

524,288 hosts

Table 6.2 Characteristics of Each CIDR Block Prefix

 

At this time, the Internet is not completely CIDR-capable. Some older routers and other network devices must be upgraded to support CIDR, and compatible protocols must also be used. Non-CIDR-capable portions of the Internet can still function fine, but may be required to default towards the CIDR-capable parts of the Internet for routes that have been aggregated for nonnetwork boundaries. CIDR-capable forwarding is described as the ability of a router to maintain its forwarding table and to perform correct forwarding of IP packets without making any assumptions about the class of IP addresses.

 

The CIDR Applicability Statement composed in September of 1993 required Internet domains providing backbone and/or transit service to fully implement CIDR in order to ensure that the growth of the resources required by routers will provide Internet-wide connectivity. The Applicability Statement also recommended that all other nonbackbone and/or transit Internet domains also implement CIDR because it will reduce the amount of routing between these domains. At this time, individual domains are not required to implement CIDR. Individual domains are also not prohibited from using an addressing scheme that is not compliant with CIDR.

 

  It is very important to note that CIDR does not attempt to solve the problem of eventual exhaustion of the 32-bit IP address space. CIDR can address the short- to midterm difficulties to allow the Internet time to continue functioning effectively while progress is made on the longer term solution of IP address exhaustion. With the development of CIDR around 1993, it was given at least three years as a viable solution until the deployment of the long-term solution, IPv6 ( otherwise known as Ipng). The next generation of IP is a little behind schedule, but vendors are now making their devices compliant, and the buzz is starting to spread in the Internet community.

For IT Professionals Only

Upgrading the Routing Protocols on your Network

If you are a network engineer or administrator for a company or organization with a fairly large network, you may be faced with a dilemmamigrating your routers to another routing protocol. [mb1]   Chances are you are still using RIP, as most networks are. However, this routing protocol, as you will see in this chapter, is not the most capable protocol of the many routing protocols in existence. However, RIP may still function perfectly in your network, so you must determine whether you actually need to upgrade the routing protocol. As an IT professional in charge of your network, or contracting for another companys network, you will have to know when, if ever, to make a network protocol migration. You will have to ask yourself several questions in order to gather enough information to make an informed decision:

    How long has this routing protocol been in use in our network?

    Has our network grown significantly in the past few years?

    Has the network been suffering from degradation when communicating with remote networks?

    Do we have goals for the network that may not be met with this current routing protocol?

    Are we eventually going to segment our network into logical areas?

These questions will help you determine whether you need to investigate the possibility of migrating your routing protocols to a more modern, robust routing protocol. Do not make an important decision such as choosing a routing protocol in haste. You can severely hinder your network if you do not implement the routing protocol correctly. Spend the time, research all the available protocols, and do your homework.

From Millions to Thousands of Networks

 

For engineers , the biggest push on the Internet today is to devise a plan to limit the huge growth in available networks on the Internet. We have learned in the previous section that the addition of so many networks on the Internet has severely hindered the ability to maintain effective routing tables for all the new networks that have been added. It was becoming more difficult to route packets to their destinations because the route to the destination was sometimes not included in the large routing tables maintained by these routing domains. This threat, much like a tornado warning, was due to touch down on the Internet before the dreaded exhaustion of IP addresses.

 

Now that CIDR has come to the rescue, the problem is to implement CIDR fast enough to consolidate these networks to minimize the number of entries in the routing tables. From the millions of networks out there, CIDR is able to consolidate contiguous IP addresses, known as supernetting, into fewer numbers of networks that contain more hosts. The only caveat with CIDR is that these must be contiguous Class C addresses. The authority for assigning IP addresses has assigned large contiguous blocks of IP addresses to large Internet Service Providers. These large ISPs assign a smaller subset of contiguous addresses from their block to other ISPs or large network customers, as illustrated in Figure 6.2.

 

Figure 6.2 Maintaining contiguous CIDR blocks while assigning addresses.

 

The bottom line is that the large ISP maintains a large block of contiguous addresses that it can report to a higher authority for CIDR address aggregation. With CIDR, the large ISP does not have to report every Class C address that it owns; it has to report the prefix that every Class C address has in common. These addresses are aggregated into a single supernetted address for routing purposes. In our example, the prefix is 198.113.201, which is what all IP addresses have in common. Instead of advertising six routes, we are advertising only one. That is a decrease of 83 percent. Imagine if every ISP were able to decrease the routes they advertise by this much. This can literally bring the number of networks from millions down to thousands. Not only does this decrease the number of networks, but it is a significant reduction in the number of routing table entries. By March of 1998, the number of global routing table entries was around 50,000. Without CIDR, it is speculated that the number of global routes would have been nearly twice that number. You can always count on the standards committees behind the scenes of the Internet to deliver effective solutions when adversity stares them in the face.

ISP Address Assignment

In the near future, organizations are likely to undergo changes that will affect their IP addresses. This can result from a variety of reasons, such as a change in Internet Service Provider, structural reorganization, physically moving equipment, and new strategic relationships. An IP address renumbering plan can result in easier future IP address management.

 

When moving from one ISP to another, and CIDR is being used, it will be required to return the addresses that were allocated to the organization from the ISPs original CIDR block. These addresses belong to a single large block of address space allocated to their current ISP, which acts like an aggregator for these addresses. If your address is aggregated into your ISPs larger address block, you can then be routed under their network address.

 

What if you leave Internet Service Providers and choose to take your IP addresses with you? This is a predicament for the original ISP who can no longer advertise the addresses as part of an aggregated CIDR block, because there is now a hole in the CIDR block (resulting from the loss of the IP addresses you took with you). CIDR can address this issue by requiring routers to accept multiple matches. When a duplicate routing match is found, the router will search for the route with the longest mask, which should be the most recent route. This is referred to as an exception to a CIDR block, and is used when a block of contiguous addresses cannot be used, like the example in which we defected from one ISP to another and took our addresses with us.

 

To contain the growth of this routing information, an organization should change these addresses, which involves renumbering their subnets and hosts. If the organization does not renumber, the consequences may include limited Internet-wide IP connectivity issues. ISPs sometimes have to change to a new and larger block of addresses, and this may affect the organization that currently has addresses that were allocated to them from the original CIDR block.

 

The easiest form of renumbering is with the use of dynamic addressing, such as Dynamic Host Configuration Protocol (DHCP). However, many servers and network devices such as routers have static addresses, which will hamper the renumbering process.

 

The most important aspect of the renumbering plan is centered around routing. Routing issues have become very important, due to the large growth of the Internet and the maintenance of large routing tables that accompany this growth. Since routers are a key component to connectivity, they are a large focus of the renumbering plan.

 

If you are not aggregated into your ISPs larger address block, and you are a smaller organization, you are risking being dropped from the global routing tables. There is no governing force that has control over what addresses are added to the global routing tables; any ISP can manage their routing tables as they see fit. If you are a smaller network, you can still be included in global routing tables if your address is part of a larger CIDR address block.

Using CIDR Addresses Inside Your Network

 

The interior (intradomain) routing protocols that support CIDR are OSPF, RIP II, Integrated IS-IS, and EIGRP. If you are running one of these routing protocols in your internal network, you have the ability to use CIDR addresses inside your network. Most companies and organizations do not have internal networks large enough to require CIDR addressing. However, CIDR does provide more than just efficient addressing.

 

When implementing CIDR addressing in your internal network, you have the ability to create smaller subnets than those available with the current classful subnetting schemes. For example, in order to subnet your network using TCP/IP subnet with a custom subnet mask, the smallest subnet you have would still have 254 available hosts. With CIDR you can implement fractional aggregates , the ability to take a Class C address and assign fractions of it to customers or your internal subnets on your own network. ISPs are now using this technology to assign 64 and 32 block addresses to customers with small networks. This makes efficient use of available Class C addresses, because without CIDR, you would be wasting the remaining IP address in the Class C address that was not used. This is how you can combat IP address exhaustion within your own network, just like many people are trying to do on the Internet. Table 6.3 shows the fractional aggregates of a single Class C address.

 

 

CIDR Block Prefix

# Equivalent Class C

# of Host Addresses

/27

1/8th of a Class C

32 hosts

/26

1/4th of a Class C

64 hosts

/25

1/2 of a Class C

128 hosts

/24

1 Class C

256 hosts

Table 6.3 Fractions of a Class C Address Made Possible by CIDR

 

With CIDR we now have the ability not only to use a full Class C address, but also to assign fractions of the Class C, such as th,



IP Addressing and Subnetting, Including IPv6
IP Addressing and Subnetting, Including IPv6
ISBN: 672328704
EAN: N/A
Year: 1999
Pages: 15

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