Chapter 11: Network Performance Considerations: Coexistence of IPv4 and IPv6

Cisco Press

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Regardless of the IP protocol type, forwarding performance testing is best performed in a black-box environment. The stimulus and the measurements are independent of the device tested and its architecture. RFC 2544 provides general guidelines and requirements for throughput testing:

  • Throughput, as defined in RFC 1242, is measured as nondrop rate (NDR), the maximum traffic rate with no packet drop.

  • The NDR should be determined in steps of 60 or fewer seconds and then verified by forwarding traffic for a minimum of a 60-second time interval at the determined NDR.

  • Frame sizes tested should cover the set recommended for the various media types. In the case of Ethernet, for example, 64, 128, 256, 512, 1024, 1280, and 1518 bytes.

  • Traffic should be bidirectional, unless otherwise specified.

These recommendations are made to evaluate the performance of forwarding unicast traffic. However, it is also important to evaluate the forwarding performance of multicast traffic, too. This type of traffic will most likely be present in the IPv6 deployments. With multicast, the test options are multiple because there are various ways in which the router can replicate traffic. For this reason, it is best for the evaluation to be performed based on traffic patterns and requirements specific to the network role for which the router is evaluated.

The larger IPv6 addresses require more intense lookups, and that can impact the router performance, as mentioned in the previous section. For this reason, it is more important to evaluate a router's forwarding performance for prefixes of various lengths in the /16 and /64 range in IPv6 than in IPv4.

All major test tool vendors provide RFC 2544–based test suites that can be used to measure the NDR of devices under test (DUT). These suites can be executed with both IPv4 and IPv6 traffic. They are well suited to black-box testing, and they offer a certain level of consistency for this type of measurement.

The test tool suites offer multiple tuning parameters, so it is important to be aware of their settings and ensure they meet the requirements of RFC 2544.

The test tools that form the shell around the DUT should be complemented with a few probes that acquire data from the DUT itself. Relevant data that should be collected during test includes memory utilization and integrity, CPU values at box or linecard level, and general system messages. Although throughput data in itself is important as far as a standalone router is concerned, sometimes NDR is obtained at 100 percent use of the CPU. From a network operation perspective, this is unacceptable because the router might have to totally neglect its control-plane to meet the measured NDR.

Considering all the parameters that are being measured during the evaluation, it is always a good practice to define a baseline for the test environment that is being used.

The advantage of the black-box testing approach is that it allows for a consistent evaluation of forwarding performance of raw traffic as well as complex traffic that includes higher-layer content or extension headers. It also provides a good way to evaluate the impact on performance of advanced features (access lists, for example) enabled on the DUT. A black-box approach to testing allows for a clear one-to-one comparison of the results obtained in each of these cases. It also allows for meaningful comparisons between IPv4 and IPv6 throughput performance data.

Note that the minimum packet size for IPv6 is larger than IPv4 (IPv6 header: 40 bytes; IPv4 header: 20 bytes). This is important when considering the low-packet-size performance data.

It is important to mention that there are also two different ways to look at the throughput performance of a router:

  • Interface-to-interface throughput refers to measuring the NDR by sending bidirectional traffic through two same-type interfaces of the DUT.

  • System throughput refers to measuring the NDR by sending bidirectional traffic through all interfaces of a router that is fully populated in terms of linecards and interfaces.

Both tests are conceptually similar, and they should observe the RFC 2544 recommendations.

It is generally expected that a router's IPv6 forwarding performance is similar to its IPv4 forwarding performance and as close as possible to the line rate of the tested interface.

The Right Router for the Job

When choosing a router for a certain role in a network, performance is not the only factor considered. Others are equally important, such as the following:

  • Feature richness and versatility

  • Price

  • Scalability

All these factors reflect certain aspects of a router's design. Previous sections highlighted some of the IPv6-specific challenges faced by a router's control and forwarding planes. Ultimately, a router's performance is determined by its implementation of the control and forwarding functions as well as its integration of the IPv6 protocol. For this reason, it is important to have an idea of the overall design of the evaluated router when analyzing its performance data.

Router Architecture Overview

Routers evolved from mere specialized computers where all processing is software based to sophisticated devices where functionality is shared between software running on powerful CPUs and highly specialized hardware. Routers are becoming more powerful, more reliable, and more scalable; but all this comes at a cost. It is therefore important to build the right router for the right market segment. This explains the multitude of router types available from which to choose.

Software Versus Hardware Forwarding

The control-plane functions of a router are always performed in software. On the other hand, packet forwarding along with some advanced features can be performed by dedicated hardware resources. Based on the implementation of the forwarding plane, routers can be classified as follows:

  • Software forwarding router—A device using its main CPU for basic and enhanced packet forwarding; no hardware assistance is available.

  • Hardware Forwarding Router—A device that has hardware assistance for basic or enhanced packet forwarding.


Note - A packet that cannot be handled by the hardware is usually punted to software processing by default. This is not true for all router vendors.


Hardware-assisted forwarding often provides the best forwarding performance. This advan-tage comes at the expense of versatility. The dedicated hardware is designed to support a certain set of features, so additional features require its redesign. For this reason, hardware-forwarding-based platforms are generally well positioned in or close to the network core and edge. There the interfaces are high speed, and the focus is on forwarding performance rather than feature richness. Software-forwarding-based platforms are more suited in the access layer, where the interfaces are lower speed, and various features are being used.

Both types of routers are present in the Cisco product line:

  • Software forwarding routers—Cisco 800, 1700, 1800, 2600, 2800, 3600, 3700, 3800, 7200, and 7500 series

  • Hardware forwarding routers—Cisco 7600, 10000, 10720, 12000 series and the Cisco Carrier Routing System (CRS-1); layer 3 switches: Catalyst 6500, 3560 and 3750 series

Centralized Versus Distributed Forwarding

A router can take all its forwarding decisions in a centralized manner or it can distribute the function among multiple intelligent subsystems. This design choice separates routers in two categories:

  • Centralized forwarding router—Every packet-forwarding decision is made by a central forwarding engine.

  • Distributed forwarding router—Forwarding decisions are made on different forwarding engines that can control a linecard, a port, a section of a chassis, and so on.

All forwarding engines involved in the decision-making process have to support IPv6. If they do not, the router defaults to a centralized mode of operation.

The distributed architectures are particularly important for larger, modular routers that have to scale well with additional linecards. When the forwarding decision making is distributed to these intelligent cards, the router performance is not impacted by an increase in the number of interfaces and modules. This type of router is prevalent at the core and the edge of the network.

Examples of distributed, IPv6-capable routers from the Cisco family include the following:

  • Cisco 7500 series router

  • Cisco 7600 series router with distributed CEF 720 linecards

  • Cisco 12000 series Internet router

  • CRS-1 router


Note - A distributed architecture also allows software forwarding platforms to have a performance close to line rate and that scales linearly as cards are added. This is the case of the 7500 Cisco routers.


The concepts presented in this section represent a high-level overview of router architecture. These concepts can help you classify routers and have certain performance expectations from them based on their design. However, today's routers are complex systems, and there is a lot more to a complete and thorough discussion of their architecture than what is covered in this brief discussion. For more detail on this topic, refer to the book Inside Cisco IOS Software Architecture (CCIE Professional Development) by Vijay Bollapragada, et al.

IPv6 Forwarding Performance of Cisco Routers

Armed with an understanding of the various router architectures and the methodology to test their performance, it is time to see the differences between their IPv4 and IPv6 performance. This section presents forwarding performance examples for the two protocol types on Cisco routers that target various segments of a network.

Low-End Routers

The low-end routers are typically deployed in the access layer of the network. They generally have low speed and few interfaces. Because they are software-based routers, they are easily enabled to support IPv6. The Cisco product line from the 830 series to the Cisco 3800 series can be easily enabled for IPv6 when it is upgraded to one of the supported Cisco IOS software release, such as 12.2T, 12.3, 12.4, 12.3T, and 12.4T. Low-end routers have a centralized architecture.


Note - CEF is available for IPv6 (Cisco Expressing Forwarding v6 and distributed Cisco Expressing Forwarding v6) starting with Cisco IOS Release 12.2(13)T.


Despite being software platforms, many of the low-end routers use powerful CPUs that enable them to achieve line-rate packet forwarding on their interfaces. To provide encryp-tion services, which are particularly CPU intensive, hardware assistance might be needed to sustain the same performances as the other services.

Table 11-1 presents an example of how IPv6 compares to IPv4 performance on a low-end router from the Cisco 3700 series. The throughput was determined between two Fast Ethernet interfaces, with bidirectional traffic and no advanced features enabled. The theoretical maximum throughput for the interface type analyzed is also listed for reference. Figure 11-3 is a graphical representation of the forwarding performance in percentage of the targeted line rate.

Table 11-1 IPv6 Basic Forwarding Between Two Fast Ethernet Interfaces, Bidirectional, No ACL on Cisco 3725

Packet Size (Ethernet II)

IPv4 (pps*)

IPv6 (pps)

Maximum (pps)

64 bytes

63,918.5

48,064

148,810

128 bytes

63,431

49,867

84,449

256 bytes

45,290

45,290

45,290

512 bytes

23,492

23,492

23,497

1024 bytes

11,973

11,973

11,973

1518 bytes

8127

8127

8128

IMIX

33,515

33,515

33,515

*Packets per second


Note - IMIX is a 7:4:1 distribution of Ethernet-encapsulated packets of sizes 64, 570, and 1518 bytes. This leads to a 353-byte packet-size average.

Sometimes the performance numbers are multiplied by a factor of two when bidirectional traffic is used during testing. For this reason, it is important to fully qualify the test methodology used.


Figure 11-3

Figure 11-3

Example of IPv4 Versus IPv6 Forwarding Performance of a Low-End Router (Cisco 3725 - FastE)

It is worth noting that line-rate forwarding is obtained before the IMIX packet size, which represents a likely packet-size distribution in an operational network.

Mid-Range Routers

In the case of mid-range routers, finding the balance between cost, features, and perfor-mance becomes even more important. Routers in this market segment can be positioned in different roles and have to perform multiple functions from access to distribution/aggregation and even core at times. The versatility required of the mid-range platforms is reflected in the multitude of router architectures applied to them. Software and hardware forwarding, as well as centralized and distributed architectures, are all present.

Leveraging powerful CPUs allows routers with low density of ports to deliver competitive forwarding performance while maintaining the edge in terms of feature richness. Table 11-2 shows the performance data of a Cisco-software-based, centralized forwarding mid-range platform. The performance is measured with bidirectional traffic between two Gigabit Ethernet interfaces on a 7304 router with an NPE-G100 processor. Figure 11-4 is a graphical representation of the information in Table 11-2. It shows IPv4 versus IPv6 throughput performance in percentage of targeted line rate.

Table 11-2 Cisco 7304 NPE-G100 Performance Between 2 Gigabit Ethernet Interfaces, Bidirectional, No ACL

Packet Size (Ethernet II)

GE–IPv4 (pps)

GE–IPv6 (pps)

Maximum (pps)

64 bytes

569,103

330,213

1,488,095

128 bytes

579,586

330,213

844,595

256 bytes

452,898

332,877

452,898

512 bytes

234,962

234,962

234,962

1024 bytes

119,731

119,731

119,731

1518 bytes

81,274

81,274

81,274

IMIX

334,224

334,224

334,224

Figure 11-4

Figure 11-4

Example of IPv4 Versus IPv6 Forwarding Performance of a Mid-Range Router (Cisco 7304 - GigE)

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