With Cisco IOS, several link-efficiency mechanisms are available. Link fragmentation and interleaving (LFI), Multilink PPP (MLP), and Real-Time Transport Protocol (RTP) header compression can provide for more efficient use of bandwidth.
Table 9-8 describes Cisco IOS link-efficiency mechanisms.
Table 9-8 Link-Efficiency Mechanisms
| Mechanisms | Description |
| Link fragmentation and interleaving (LFI) | Reduces delay and jitter on slower-speed links by breaking up large packet flows and inserting smaller data packets (Telnet, VoIP) between them. |
| Multilink PPP (MLP) | Bonds multiple links between two nodes, which increases the available bandwidth. MLP can be used on analog or digital links and is based on RFC 1990. |
| Real-Time Transport Protocol (RTP) header compression | Provides increased efficiency for applications that take advantage of RTP on slow links. Compresses RTP/UDP/IP headers from 40 bytes down to 2–5 bytes. |
Window Size
The window size defines the upper limit of frames that can be transmitted without getting a return acknowledgment. Transport protocols such as TCP rely on acknowledgments to provide connection-oriented reliable transport of data segments. For example, if the TCP window size is set to 8192, the source stops sending data after 8192 bytes if no acknowledgment has been received from the destination host. In some cases, the window size might need to be modified because of unacceptable delay for larger WAN links. If the window size is not adjusted to coincide with the delay factor, retransmissions can occur, which affects throughput significantly. It is recommended that you adjust the window size to achieve better connectivity conditions.
References and Recommended Readings
RFC 1990: The PPP Multilink Protocol, https://tools.ietf.org/html/rfc1990
Cisco, “Campus QoS Design Simplified,” https://www.ciscolive.com/c/dam/r/ciscolive/emea/docs/2018/pdf/BRKCRS-2501.pdf
Cisco, “Cisco IOS Quality of Service Solutions Configuration Guide Library, Cisco IOS Release 15M&T,” http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/qos/config_library/15-mt/qos-15-mt-library.html
Cisco, “DSCP and Precedence Values,” https://www.cisco.com/c/en/us/td/docs/switches/datacenter/nexus1000/sw/4_0/qos/configuration/guide/nexus1000v_qos/qos_6dscp_val.pdf
Cisco, “Highly Available Wide Area Network Design,” https://www.ciscolive.com/c/dam/r/ciscolive/us/docs/2019/pdf/BRKRST-2042.pdf
Cisco, “Module 4: Enterprise Network Design,” Designing for Cisco Internetwork Solution Course (DESGN) v3.0
Wikipedia, “LTE: LTE (telecommunications),” en.wikipedia.org/wiki/LTE_(telecommunication)
Exam Preparation Tasks
You have a couple of choices for exam preparation: the following review sections, Chapter 13, “Final Preparation,” and the exam practice questions on the companion website.
Review All Key Topics
Review the most important topics in the chapter, noted with the Key Topic icon in the outer margin of the page. Table 9-9 lists these key topics and the page number on which each is found.
Table 9-9 Key Topics for Chapter 9
| Key Topic Element | Description | Page |
| List | WAN design methodologies | 312 |
| Table 9-2 | Key Design Principles | 313 |
| Table 9-3 | Application Requirements for Data, Voice, and Video Traffic | 313 |
| Table 9-4 | Physical Bandwidth Comparison | 314 |
| Table 9-5 | Availability Percentages | 315 |
| List | Deployment models | 316 |
| Paragraph | Single-homed MPLS WANs | 317 |
| Paragraph | Dual-homed MPLS WANs | 318 |
| Paragraph | Hybrid WANs: Layer 3 VPN with Internet tunnels | 318 |
| List | Internet for remote sites | 321 |
| List | WAN backup options | 321 |
| Paragraph | Failover | 322 |
| Paragraph | DiffServ | 323 |
| Table 9-6 | DSCP Mapping Table | 323 |
| Paragraph | IntServ | 324 |
| List | Classification technologies | 324 |
| Paragraph | Shaping | 325 |
| Paragraph | Policing | 325 |
| Paragraph | Queuing | 325 |
| Table 9-7 | QoS Options | 325 |
| Table 9-8 | Link-Efficiency Mechanisms | 327 |