Configuration Example 3 Subnetting with Summarization

Summarization is a wonderful concept in networking that can give networks a variety of benefits, as discussed earlier in this chapter. It is important to provide a template that demonstrates how you might go about designing or redesigning an OSPF network with summarization in place from the beginning. This latter case is the more likely scenario, and you might have already been involved in projects to renumber and readdress networks that needed a new and improved logical look. These situations develop for a variety of reasons, some within your control but many not in your control. This example looks at taking a single public network and, through the use of subnetting with variable-length subnet masking (VLSM), allows clean summarization at the various area boundaries. Use this example as a template to follow when you are faced with such a situation.

NOTE Route summarization is desirable for a reliable and scalable OSPF internetwork. The effectiveness of route summarization, and your OSPF implementation in general, hinges on the addressing scheme that you adopt. Summarization in an OSPF internetwork occurs between each area and the backbone area. Summarization must be configured manually in OSPF.

There might be a situation where you have only one public address (a single Class B, for example) to allocate for all areas of your multi-area OSPF network. You might also want to save some address space by using VLSM so that the point-to-point serial links in each area have a subnet mask that allows two hosts per subnet, which is perfect for point-to-point networks. You have also learned that summarizing on area boundaries is very useful.

This example uses part of the address space 150.100.0.0/16. It is meant to illustrate both the concept of area masks and the breakdown of large subnets into smaller ones through the use VLSM. The following points list the assumptions that are made and describe the process used to allocate addresses:

• Determine how many areas you will have in your OSPF network. While you are considering this, evaluate the business factors that are facing your company. For example, is your company likely to expand or contract in a given area? What effect will this have on your network? This kind of business thinking is crucial. While you are a network engineer, you must also be aware of the business factors that affect your network. A value of 500 areas has been chosen for this example. This is a large number, and a 500-area OSPF network is not realistic, but it can help to illustrate the methodology used in a large fashion to make the point.

• Create an "area mask boundary" in your address space through the use of subnetting. This boundary will be where the subnets found within each area will be summarized. You have a Class B, so you must subnet how to get 500 areas, thus using the formula of 2 N-2 = X where N is the number of bits to be used for extending the subnet mask and X is the number of usable subnets. Note that 29-2 = 510 meets the requirement of 500 areas and you have ten more reserved for future use; you know just in case the unexpected happens, which is good planning! In Figure 7-5, you can see where 9 bits of subnetting have been applied to the class B address, which results in the number of subnets required for each area.

• Determine the number of subnets required in each area and the maximum number of hosts required per subnet. In this example, you require a minimum of 100 subnets with 50 hosts each and 100 subnets with 2 hosts each (the serial lines) for each area. Perhaps, this is a design for a large chain of coffee shops that needs only a few host addresses per location but has many locations. Perhaps, this is a fast-food restaurant chain, retail shops, or an auto maintenance shop. All these possibilities have the requirement of multiple clustered locations in a given area with a small number of hosts needed at each location, perhaps all in a city or single geographical area.

• Now that you understand the requirements of how many subnets you need in each area, proceed with additional subnetting. Note that you have only 7 host bits left to use because of the creation of the area summarization. In fact, the 9 bits of the area mask are part of the subnet portion of the address, but you have restricted their flexibility so that you can summarize all the subnets of an area with one range command. In taking the first subnet from the previous figure, the subnet mask has been extended to get the minimum requirements, and expansion is planned for. You can see this in Figure 7-6.

422 Chapter 7: Summarization with OSPF

Figure 7-5 Subnetting to Get Areas Properly Summarized

Figure 7-5 Subnetting to Get Areas Properly Summarized

Figure 7-6 Subnetting to Get Each Location

Network Address

r Uaa AIIZwo^ubnal | 15B . 1ÖQ. 0 ..l-^-a dlassö Network

■SilbrretBits Network Mask

255.255.255.192

Effective Hasts EffectiverSubrrets iE

"3

"3

Subnet

155,100.0.192	-
156,100.1.0
156.100.1.64
150.100.1.120
156.100 1.192
15G',100/2.0
I5B.100.2.64
156.100.2.128
156,100.2.192
156.100.3.0
156.100.3.64
15B:1O0.3.128
156,100.3.192
156,100.4.0
156 100.4.64
156.100.4 128
156-,100.4.192
LlstUmit 1	AND of Address and Network Network subnet-Mask AND of AddrBSsààti ^Lfbriet' Subnet [□ Inverted ■AND of Address and Inverted HO.StlD, QR afAddrssis.and Inverted Broadcast 100000000100000004 r Displayiri;Hbk 11111111111111111111111111□□□□□□ 1DD111DD011ÜD100 156 100 0D00D00D1D00D000 Network ID.; 156.100.0.128 0000000000000000 00000000 00000000 Network Range -Legend — Network 0000000000000000 00000000 00000000 l0oirltJDönaoioD 156 -100. 111 111 11 11000000 D000DD00 10000000 Subnet ID _tf 120 0.0.0.128 00000000 0D111111 00000000 00000000 Host ID 00000000 101 111 11 Sfctidesst Number 0 191 156.100.0.191 156.100.0.191 Subnet Hsst0#: • Choose the portion of the address space that has the 2-bit host field (subnet mask of 255.255.255.252) for the serial lines arbitrarily from one of the larger subnet fields. This method of assigning addresses for the VLSM portion of the address space is done to guarantee no address overlap. Alternatively, if the requirement had been different, you could have chosen any number of the larger subnets (with mask 255.255.255.240) and broken them up into smaller ranges with fewer hosts, or combined several of them to create subnets with more hosts. In this example, the addresses and mask boundaries were chosen simply to demonstrate the concept of summarization and the process of subnetting. Hopefully, you have noticed how intertwined they are! An alternative and possibly more realistic design could include the following items: • Approximately 20-30 areas (maximum) for the entire OSPF AS • Approximately 20-30 routers per OSPF area • One or more Class B addresses, with several Class C networks to allocate for the AS

Continue reading here: Using Private Addressing to Summarize

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