OSPF Network Design Solutions (2nd Edition)

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Also you will find our recommended Youtube and CiscoLive videos in the document. Let's Connect on Social Media! More info about me click here. This was certainly needed. Leave a Reply Cancel reply Your email address will not be published. Comment Name Email Website Notify me of follow-up comments by email. For example, assume that a router knows about the following networks that are attached to it: Subnetting essentially extends the prex to the right by making the router know a specic IP addresses; summarization, on the other hand, reduces the prex to the left, thereby enabling the router to advertise only the higherorder bits.

Figure demonstrates how subnetting and route summarization differ. Comparison of Subnetting and Route Summarization Prefix Length increased Host Subnetting increases the prefix size to enable very specific routes. Refer to Chapter 3 of Ciscos Internetwork Design Guide for an overview that describes the best practices concerning summarization.

You can also visit the following website: www. An example of route summarization is CIDR, which is discussed in detail later in this chapter. The following requirements enable route summarization to work properly:. Multiple IP addresses must share the same high-order bit to be properly summarized. Routing tables and protocols must base their routing decisions on a normal bit IP address and a variable prex length.

Remember, not all routing protocols can carry a subnet mask; those that dont are referred to as classful routing protocols. The following sections discuss classful and classless routing protocols. Classful Routing Classful routing always summarizes routes by the major network numbers. They are called classful because they always consider the network class.

This automatic summarization is always done at network boundaries. Impact of Classful Routing The use of classful routing has some considerable impact on a network. For one thing, subnets are not advertised to a different major network. In addition, noncontiguous subnets are not visible to each other.

Figure illustrates how classful routing and subnetting can affect your network. Certain techniques have been developed to assist in overcoming this problem: IP unnumbered, secondary addressing, and using OSPF. Further discussion of classful routing and the issues surrounding its use that is, discontiguous subnets are beyond the scope of this book.

How Classful Routing and Subnetting Affect the Network Each router has a subnet that it attaches to, but in a classful environment, they cannot and will not be advertised because the subnets are not on a classful boundary. Classless Routing Classless routing differs from classful routing in that the prex length is transmitted.

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This prex length is evaluated at each place it is encountered throughout the network. In other words, it can be changed to advertise routes differently at different locations within a network. This capability of classless routing enables more efcient use of IP address space and reduces routing trafc. A good example of this type of routing is OSPF.

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Classless routing has the following characteristics:. One routing entry might match a block of host, subnet, or network addresses. Routing tables can be much shorter. Switching performance can be faster unless Cisco express forwarding CEF is used. Routing protocol trafc is reduced. The trick to using this technique is ensuring that you have an adequate number of hosts allocated per subnet.

Not every protocol supports VLSM. With VLSMs, you can use different masks for the same network number on different interfaces, which enables you to conserve IP addresses for better efciency. VLSMs do this by allowing both big subnets and small subnets. As previously mentioned, you need to ensure that the number of hosts is sufcient for your needs within each subnet.

OSPF Network Design Solutions (2nd Edition)

In Example , a bit subnet mask is used, leaving 2 bits of address space reserved for serial line host addresses. There is sufcient host address space for two host endpoints on a point-to-point serial link. Example VLSM Demonstration interface ethernet 0 ip address System is configured for OSPF and assigned as the process number router ospf ! Specifies network directly connected to the system network As Example demonstrates, VLSM is efcient when used on serial lines because each line requires a distinct subnet number, even though each line has two host addresses.

This requirement wastes subnet numbers. However, if you use VLSM to address serial links in a core router, you can save space. In Figure , the regular subnet These additional subnets make 63 additional subnets available. VLSM also enables the routes within the core to be summarized as Most early networks never had their IP addresses assigned to them in a way that would enable network engineers to group them in blocks.

Instead, they had been assigned as needed, so massive renumbering projects would need to be performednot one of the most popular pastimes of anyone involved in networking. Otherwise, you might end up doing quite a lot of static routing and odd conguring just to keep your network stable. Optimal summarization occurs with contiguous blocks of addresses. If small subnets are grouped, routing information can be summarized. Group VLSM subnets so that routing information can be consolidated. Allocate VLSM by taking one regular subnet and subnetting it further.

Avoid using two different classful subnet masks inside a given network address. In conclusion, you might ask yourself why there are questions about implementing VLSM. As previously mentioned, VLSM is not supported by every protocol, although it is. So these newer protocols might have to coexist with older protocols that do not support VLSM and would have trouble routing. In addition, the use of VLSM can be difcult. If it is not properly designed, it can cause the network to not operate properly, and it increases the complexity of troubleshooting any network.

CIDR is an effective method to stem the tide of IP address allocation as well as routing table overow. Without the implementation of CIDR in and in RFC , the Internet would not be functioning today because the routing tables would have been too large for the routers to handle.

CIDR can be thought of as advanced subnetting. The subnetting mask, previously a number with special signicance, becomes an integral part of routing tables and protocols. A route is no longer just an IP address that has been interpreted according to its class with the corresponding network and host bits. This growth has caused an overwhelming utilization of Internet routers processing power and memory utilization, consequently resulting in saturation. Between and , the Internets routing tables doubled in size every 10 months.

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This growth would have resulted in about 80, routes by Routers would have required approximately 25 MB of dedicated RAM to keep track of them all, and this is just for routers with a single peer. Through the implementation of CIDR, the number of routes in was about 42, Today, the routing table is about , routes at the core of the Internet. This would shut down most common BGPspeaking routers due to memory utilization requirements, and the CPU would be degraded.

The major benet of CIDR is that it enables continuous, uninterrupted growth of large networks. CIDR enables routers to group routes to reduce the quantity of routing information that is carried by a networks routers.

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Some of the benets of using CIDR within your network are as follows:. Reduces the local administrative burden of updating external route information Saves routing table space in routers by using route aggregation. Reduces route-apping and convergence issues Reduces CPU and memory load on a router Enables the delegation of network numbers to customers or other portions of the network Increases efciency in the use of available address space.

What Do Those Slashes Mean? A former Class B address might appear as A single Class C appears as This new look to IP addresses consists of an IP address and a mask length. A mask length is often called an IP prex. The mask length species the number of left-most contiguous signicant bits in the corresponding IP address. Figure demonstrates how CIDR denes its mask. For example, the Class C address This address can also be represented in CIDR terms as Therefore, because the natural mask is 24 bits and the CIDR mask is 16 bits 16 24 , this network is referred to as a supernet.

Simply put, supernets have an IP prex that is shorter than the natural mask. This enables the more specic contiguous networkssuch as Simply put, aggregates indicate any summary route. Figure demonstrates how CIDR can be used to benet your network by reducing routing tables. The ip classless command prevents the existence of a single subnet route from blocking access through the default route to other subnets.

IP classless causes the router to forward packets that are destined for unknown subnets to the best supernet route possible, instead of dropping them.

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In other words, if a specic route is not available, a lessspecic route will be taken, provided that one exists. This is opposite to the old classful idea, in which if a specic route did not exist, the packets were dropped. As discussed earlier in this chapter, when an IP network is assigned more than one subnet mask, it is considered a network with variablelength subnet masks because the subnet masks prexes have varying lengths.

If you recall, the use of VLSM brings benets to a network and routing that allow for increased routing optimization in the form of a smaller and more concise routing table, known as route aggregation, as well as more efcient use of an organizations assigned IP address space. See Figure This is ne if the organization wants to deploy a number of large subnets, but what about the occasional small subnet that contains only 20 or 30 hosts? Because a subnetted network could have only a single mask, the network administrator was still required to assign the 20 or 30 hosts to a subnet with a bit prex.

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