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Designing Cisco Network Service Architectures (ARCH): Developing an Optimum Design for Layer 3 (CCDP)

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This chapter is from the book

After completing this chapter, you will be able to

  • Design IPv4 and IPv6 addressing solutions to support summarization
  • Design IPv6 migration schemes
  • Design routing solutions to support summarization, route filtering, and redistribution
  • Design scalable EIGRP routing solutions for the enterprise
  • Design scalable OSPF routing solutions for the enterprise
  • Design scalable BGP routing solutions for the enterprise

This chapter examines a select number of topics on both advance IP addressing and design issues with Border Gateway Protocol (BGP), Enhanced Interior Gateway Routing Protocol (EIGRP), and Open Shortest Path First (OSPF). As one would expect, advanced IP addressing and routing protocol design encompasses a large amount of detail that has already filled a number of books on routing protocols and networking best practices.

Designing Advanced IP Addressing

Designing IP addressing at a professional level involves several advanced considerations. This section reviews the importance of IP address planning and selection and the importance of IP address summarization. It also discusses some applications of summary addressing.

IP Address Planning as a Foundation

Structured and modular cabling plant and network infrastructures are ideal for a good design with low maintenance and upgrade costs. In similar fashion, a well-planned IP addressing scheme is the foundation for greater efficiency in operating and maintaining a network. Without proper advanced planning, networks may not be able to benefit from route summarization features inherent to many routing protocols.

Route summarization is important in scaling any routing protocol. However, some existing IP addressing schemes may not support summarization. It takes time and effort to properly allocate IP subnets in blocks to facilitate summarization. The benefits of summarized addresses are reduced router workload and routing traffic and faster convergence. Although modern router CPUs can handle a vastly increased workload as compared to older routers, reducing load mitigates the impact of periods of intense network instability. In general, summary routes dampen out or reduce network route churn, making the network more stable. In addition, summary routes lead to faster network convergence. Summarized networks are simpler to troubleshoot because there are fewer routes in the routing table or in routing advertisements, compared to nonsummarized networks.

Just as using the right blocks of subnets enables use of more efficient routing, care with subnet assignments can also support role-based functions within the addressing scheme structure. This in turn enables efficient and easily managed access control lists (ACL) for quality of service (QoS) and security purposes.

In addition to allocating subnets in summarized blocks, it is advantageous to choose blocks of addresses within these subnets that can be easily summarized or described using wildcard masking in access control lists (ACL). With a well-chosen addressing scheme, ACLs can become much simpler to maintain in the enterprise.

Summary Address Blocks

Summary address blocks are the key to creating and using summary routes. How do you recognize a block of addresses that can be summarized? A block of IP addresses might be able to be summarized if it contains sequential numbers in one of the octets. The sequence of numbers must fit a pattern for the binary bit pattern to be appropriate for summarization. The pattern can be described without doing binary arithmetic.

For the sequential numbers to be summarized, the block must be x numbers in a row, where x is a power of 2. In addition, the first number in the sequence must be a multiple of x. The sequence will always end before the next multiple of x.

For example, any address block that matches the following can be summarized:

  • 128 numbers in a row, starting with a multiple of 128 (0 or 128)
  • 64 numbers in a row, starting with a multiple of 64 (0, 64, 128, or 192)
  • 32 numbers in a row, starting with a multiple of 32
  • 16 numbers in a row, starting with a multiple of 16

If you examine through, there are 191 - 160 + 1 = 32 numbers in a row, in sequence in the third octet. Note that 32 is 25 power of 2. Note also that 160 is a multiple of 32 (5 * 32 = 160). Because the range meets the preceding conditions, the sequence through can be summarized.

Finding the correct octet for a subnet-style mask is fairly easy with summary address blocks. The formula is to subtract n from 256. For example, for 32 numbers in a row, the mask octet is 256 - 32 = 224. Because the numbers are in the third octet, you place the 224 in the third octet, to form the mask

A summary route expressed as either,, or as 172.169.160/19 would then describe how to reach subnets starting with through

Summarization for IPv6

Although the address format of IPv6 is different from IPv4, the same principles apply. Blocks of subsequent IPv6 /64 subnets can be summarized into larger blocks for decreased routing table size and increased routing table stability. To an extent, routing summarization for IPv6 is simpler than for IPv4, because you do not have to consider variable-length subnet masking (VLSM). Most IPv6 subnets have a prefix length of 64 bits, so again, you are looking for contiguous blocks of /64 subnets. The number of subnets in this block should be a power of 2, and the starting number should be a multiple of that same power of 2 for the block to be summarizable.

For example, examine the block 2001:0DB8:0:A480::/64 to 2001:0DB8:0:A4BF::/64. A quick analysis of the address block shows that the relevant part is in the last two hexadecimal characters, which are 0x80 for the first subnet in the range and 0xBF for the last subnet in the range. Conversion of these numbers to decimal yields 0x80 = 128 and 0xBF = 191. This is a block of 191 - 128 + 1 = 64 subnets. After verifying that 128 is a multiple of 64, you can conclude that the block of subnets is can be summarized.

To calculate the prefix length, you need to find the number of bits represented by the block of 64 addresses. 64 = 26; therefore, 6 bits need to be subtracted from the original /64 prefix length to obtain the prefix length of the summary, which is /58 (64 - 6 = 58).

As a result, a summary route of 2001:0DB8:0:A480::/58 can be used to describe how to reach subnets 2001:0DB8:0:A480::/64 to 2001:0DB8:0:A4BF::/64.

Changing IP Addressing Needs

IP address redesign is necessary to adapt to changes in how subnets are now being used. In some networks, IP subnets were initially assigned sequentially. Summary address blocks of subnets were then assigned to sites to enable route summarization.

However, newer specifications require additional subnets, as follows:

  • IP telephony: Additional subnets or address ranges are needed to support voice services. In some cases, the number of subnets double when IP telephony is implemented in an organization.
  • Videoconferencing: Immersive TelePresence applications are high bandwidth and sensitive to loss and latency. Generally, best practice is to segment these devices, creating the need for more subnets.
  • Layer 3 switching at the edge: Deploying Layer 3 switching to the network edge is another trend driving the need for more subnets. Edge Layer 3 switching can create the demand for a rapid increase in the number of smaller subnets. In some cases, there can be insufficient address space, and readdressing is required.
  • Network Admission Control (NAC): NAC is also being deployed in many organizations. Some Cisco 802.1X and NAC deployments are dynamically assigning VLANs based on user logins or user roles. In these environments, ACLs control connectivity to servers and network resources based on the source subnet, which is based on the user role.
  • Corporate requirements: Corporate governance security initiatives are also isolating groups of servers by function, sometimes called segmentation. Describing "production" and "development" subnets in an ACL can be painful unless they have been chosen wisely. These new subnets can make managing the network more complex. Maintaining ad hoc subnets for voice security and other reasons can be time-consuming. When it is possible, describing the permitted traffic in a few ACL statements is a highly desirable. Therefore, ACL-friendly addressing which can be summarized helps network administrators to efficiently manage their networks.

Planning Addresses

The first step in implementing ACL-friendly addressing is to recognize the need. In an environment with IP phones and NAC implemented, you need to support IP phone subnets and NAC role subnets in ACLs. In the case of IP phones, ACLs will probably be used for both QoS and voice-security rules. For NAC role-based subnets, ACLs will most likely be used for security purposes.

Servers in medium-to-large server farms should at least be grouped so that servers with different functions or levels of criticality are in different subnets. That saves listing individual IP addresses in lengthy ACLs. If the servers are in subnets attached to different access switches, it can be useful to assign the subnets so that there is a pattern suitable for wildcarding in ACLs.

If the addressing scheme allows simple wildcard rules to be written, those simple ACL rules can be used everywhere. This avoids maintaining per-location ACLs that need to define source or destination addresses to local subnets. ACL-friendly addressing supports maintaining one or a few global ACLs, which are applied identically at various control points in the network. This would typically be accomplished with a tool such as Cisco Security Manager.

The conclusion is that it is advantageous to build a pattern into role-based addressing and other addressing schemes so that ACL wildcards can match the pattern. This in turn supports implementing simpler ACLs.

Applications of Summary Address Blocks

Summary address blocks can be used to support several network applications:

  • Separate VLANs for voice and data, and even role-based addressing
  • Bit splitting for route summarization
  • Addressing for virtual private network (VPN) clients
  • Network Address Translation (NAT)

These features are discussed in detail in the following sections.

Implementing Role-Based Addressing

The most obvious approach to implement role-based addressing is to use network 10. This has the virtue of simplicity. A simple scheme that can be used with Layer 3 closets is to use 10.number_for_closet.VLAN.x /24 and avoid binary arithmetic. This approach uses the second octet for closets or Layer 3 switches, the third octet for VLANs, and the fourth octet for hosts.

If you have more than 256 closets or Layer 3 switches to identify in the second octet, you might use some bits from the beginning of the third octet, because you probably do not have 256 VLANs per switch.

Another approach is to use some or all of the Class B private addressing blocks. This approach will typically involve binary arithmetic. The easiest method is to allocate bits using bit splitting. An example network is 172.0001 xxxx.xxxx xxxx.xxhh hhhh. In this case, you start out with 6 bits reserved for hosts in the fourth octet, or 62 hosts per subnet (VLAN). The x bits are to be split further.

This format initially uses decimal notation to the first octet and binary notation in the second, third, and fourth octets to minimize conversion back and forth.

If you do not need to use the bits in the second octet to identify additional closets, you end up with something like 172.16.cccc cccR.RRhh hhhh:

  • The c characters indicate that 7 bits allow for 27 or 128 closet or Layer 3 switches.
  • The R characters indicate 3 bits for a role-based subnet (relative to the closet block), or 8 NAC or other roles per switch.
  • The h characters indicate 6 bits for the 62-host subnets specified.

This addressing plan is enough to cover a reasonably large enterprise network.

Another 4 bits are available to work with in the second octet if needed.

Using a role-aware or ACL-friendly addressing scheme, you can write a small number of global permit or deny statements for each role. This greatly simplifies edge ACL maintenance. It is easier to maintain one ACL for all edge VLANs or interfaces than different ACLs for every Layer 3 access or distribution switch.

Bit Splitting for Route Summarization

The previous bit-splitting technique has been around for a while. It can also be useful in coming up with summary address block for routing protocols if you cannot use simple octet boundaries. The basic idea is to start with a network prefix, such as, or a prefix in the range to, 192.168.n.0, or an assigned IP address. The remaining bits can then be thought of as available for use for the area, subnet, or host part of the address. It can be useful to write the available bits as x, then substitute a, s, or h as they are assigned. The n in an address indicates the network prefix portion of the address, which is not subject to change or assignment.

Generally, you know how large your average subnets need to be in buildings. (A subnet with 64 bits can be summarized and will cover most LAN switches.) That allows you to convert six x bits to h for host bits.

You can then determine the number of necessary WAN links and the amount you are comfortable putting into one area to decide the number of a bits you need to assign. The leftover bits are s bits. Generally, one does not need all the bits, and the remaining bits (the a versus s boundary) can be assigned to allow some room for growth.

For example, suppose is being used, with subnets of 62 hosts each. That commits the final 6 bits to host address in the fourth octet. If you need 16 or fewer areas, you might allocate 4 a bits for area number, which leaves 6 s bits for subnet. That would be 26 or 64 subnets per area, which is many.

Example: Bit Splitting for Area 1

This example illustrates how the bit-splitting approach would support the addresses in OSPF area 1. Writing 1 as four binary bits substitutes 0001 for the a bits. The area 1 addresses would be those with the bit pattern 172.16.0001 ssss.sshh hhhh. This bit pattern in the third octet supports decimal numbers 16 to 31. Addresses in the range to would fall into area 1. If you repeat this logic, area 0 would have addresses to, and area 2 would have addresses to

Subnets would consist of an appropriate third octet value for the area they are in, together with addresses in the range 0 to 63, 64 to 127, 128 to 191, or 192 to 255 in the last octet. Thus,,,,, and would be the first five subnets in area 1.

One recommendation that preserves good summarization is to take the last subnet in each area and divide it up for use as /30 or /31 subnets for WAN link addressing.

Few people enjoy working in binary. Free or inexpensive subnet calculator tools can help. For those with skill writing Microsoft Excel spreadsheet formulas, you can install Excel Toolkit functions to help with decimal-to-binary or decimal-to-hexadecimal conversion. Then, build a spreadsheet that lists all area blocks, subnets, and address assignments.

IPv6 Address Planning

Because the IPv6 address space is much larger than the IPv4 address space, addressing plans for IPv6 are in many ways simpler to create. Subnetting an IPv4 address range is always a balancing act between getting the right number of subnets, the right number of hosts per subnet, and grouping subnets in such a way that they are easily summarizable, while also leaving room for future growth. With IPv6, creating an address plan is more straightforward.

It is strongly recommended that all IPv6 subnets use a /64 prefix. With 264 hosts per subnet, a /64 prefix allows more hosts on each single subnet than a single broadcast domain could physically support. There is some concern that using /64 prefixes for every link, even point-to-point and loopback interfaces, unnecessarily wastes large chunks of IPv6 address space. For this reason, some organizations prefer to use /126 prefixes for point-to-point links and /128 prefixes for loopback interfaces.

Using a /64 prefix for any subnet that contains end hosts removes any considerations about the number of hosts per subnet from the addressing plan. The second consideration in IPv4 addressing plans is to determine the right number of subnets for each site. For IPv6, this consideration is much less problematic. Local Internet Registries (LIR) commonly assign a /48 prefix from their assigned address blocks to each customer site. With 64 bits being used for the host part of the address, this leaves 128 - 64 - 48 = 16 bits to number the subnets within the site. This translates to 216 = 65,536 possible subnets per site, which should be sufficient for all but the largest sites. If a single /48 prefix is insufficient, additional /48 prefixes can be obtained from the LIR.

Effectively, the 16 bits that are available for subnet allocation can be used freely to implement summarizable address plans or role-based addressing.

Bit Splitting for IPv6

The 16 bits that are available for subnetting can be split in many different ways. Like IPv4, the IPv6 address plan is an integral part of the overall network design and should be synchronized with other design choices that are made. In an existing network, consider mapping the IPv6 address scheme to known numbers, such as VLANs or IPv4 addresses. This mapping eases network management and troubleshooting tasks, because network operators can relate the structure of the IPv6 addresses to existing address structures.

The following are examples of IPv6 addressing schemes that split the 16 subnet bits in different ways to support different design requirements:

  • Split by area: If the site is split into areas, such as OSPF areas, the address structure should reflect this to support summarization between the areas. For example, the first 4 of the 16 bits could be used to represent the area, while the VLAN is coded into the last 12 bits. This scheme can support 24= 16 areas and 212 = 4096 subnets per area. A small range of VLAN numbers should be set aside to support point-to-point links and loopback interfaces within the area.
  • IPv4 mapping: If the current IPv4 address structure is based on network and all subnets are using /24 or shorter prefixes, the middle 16 bits in the IPv4 address can be mapped to the IPv6 address. For example, if a subnet has IPv4 prefix, the middle two octets 123.10 can be converted to hexadecimal: 123 = 0x7B and 10 = 0x0A. If the LIR-assigned prefix is 2001:0DB8:1234::/48, appending the 16 bits that are derived from the IPv4 address yields 2001:0DB8:1234:7B0A::/64 as the IPv6 prefix for the subnet. This method is convenient because it establishes a one-to-one mapping between the well known IPv4 addresses and the new IPv6 addresses. However, to use this method, the IPv4 address scheme needs to meet certain conditions, such as not using more than 16 bits for subnetting.
  • Role-based addressing: For easier access list and firewall rule definition, it can be useful to code roles (for example, voice, office data, and guest users) into the address scheme. For example, the first 4 bits could be used to represent the role, the next 4 bits to represent the area, and the final 8 bits to represent the VLAN. This results in 24 = 16 different roles that can be defined, 24 = 16 areas within the site, and 28 = 256 VLANs per area and per role. Using the first 4 bits for area makes it extremely easy to configure access lists or firewall rules, because all subnets for a specific role fall within a /52 address block. Summarization is slightly less efficient than in a scheme that is purely based on areas. Instead of one summarized address block per area, there is now a summarized block per role.

The methods that are shown here are just examples. When creating an address plan as part of a network design, carefully consider other address or network elements to define an address plan that matches and supports these elements.

Addressing for VPN Clients

Focusing some attention on IP addressing for VPN clients can also provide benefits. As role-based security is deployed, there is a need for different groupings of VPN clients. These might correspond to administrators, employees, different groups of contractors or consultants, external support organizations, guests, and so on. You can use different VPN groups for different VPN client pools.

Role-based access can be controlled via the group password mechanism for the Cisco VPN client. Each group can be assigned VPN endpoint addresses from a different pool.

Traffic from the user PC has a VPN endpoint address as its source address.

The different subnets or blocks of VPN endpoint addresses can then be used in ACLs to control access across the network to resources, as discussed earlier for NAC roles. If the pools are subnets of a summary address block, routing traffic back to clients can be done in a simple way.

NAT in the Enterprise

NAT is a powerful tool for working with IP addresses. It has the potential for being very useful in the enterprise to allow private internal addressing to map to publicly assigned addresses at the Internet connection point. However, if it is overused, it can be harmful.

NAT and Port Address Translation (PAT) are common tools for firewalls. A common approach to supporting content load-balancing devices is to perform destination NAT. A recommended approach to supporting content load-balancing devices is to perform source NAT. As long as NAT is done in a controlled, disciplined fashion, it can be useful.

Avoid using internal NAT or PAT to map private-to-private addresses internally. Internal NAT can make network troubleshooting confusing and difficult. For example, it would be difficult to determine which network 10 in an organization a user is currently connected to.

Internal NAT or PAT is sometimes required for interconnection of networks after a corporate merger or acquisition. Many organizations are now using network internally, resulting in a "two network" problem after a merger. This is a severely suboptimal situation and can make troubleshooting and documentation very difficult. Re-addressing should be planned as soon as possible. It is also a recommended practice to isolate any servers reached through content devices using source NAT or destination NAT. These servers are typically isolated because the packets with NAT addresses are not useful elsewhere in the network. NAT can also be utilized in the data center to support small out-of-band (OOB) management VLANs on devices that cannot route or define a default gateway for the management VLAN, thereby avoiding one management VLAN that spans the entire data center.

NAT with External Partners

NAT also proves useful when a company or organization has more than a couple of external business partners. Some companies exchange dynamic routing information with external business partners. Exchanges require trust. The drawback to this approach is that a static route from a partner to your network might somehow get advertised back to you. This advertisement, if accepted, can result in part of your network becoming unreachable. One way to control this situation is to implement two-way filtering of routes to partners: Advertise only subnets that the partner needs to reach, and only accept routes to subnets or prefixes that your staff or servers need to reach at the partner.

Some organizations prefer to use static routing to reach partners in a tightly controlled way. The next hop is sometimes a virtual Hot Standby Router Protocol (HSRP) or Gateway Load Balancing Protocol (GLBP) address on a pair of routers controlled by the partner.

When the partner is huge, such as a large bank, static routing is too labor intensive. Importing thousands of external routes into the internal routing protocol for each of several large partners causes the routing table to become bloated.

Another approach is to terminate all routing from a partner at an edge router, preferably receiving only summary routes from the partner. NAT can then be used to change all partner addresses on traffic into a range of locally assigned addresses. Different NAT blocks are used for different partners. This approach converts a wide range of partner addresses into a tightly controlled set of addresses and simplifies troubleshooting. It can also avoid potential issues when multiple organizations are using the network.

If the NAT blocks are chosen out of a larger block that can be summarized, a redistributed static route for the larger block easily makes all partners reachable on the enterprise network. Internal routing then have one route that in effect says "this way to partner networks."

A partner block approach to NAT supports faster internal routing convergence by keeping partner subnets out of the enterprise routing table.

A disadvantage to this approach is that it is more difficult to trace the source of IP packets. However, if it is required, you can backtrack and get the source information through the NAT table.

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