The term edge virtual bridging (EVB) describes using a physical switch to pass layer 2 traffic between VMs running on the same host. The IEEE 802.1Qbg standard calls this reflective relay. See Chapter 10 for more information.29 B, C. Internet Key Exchange (IKE) uses User Datagram Protocol (UDP) port 500, whereas Encapsulating Security Payload (ESP) uses IP protocol 50. See Chapter 10 for more information.
30 A, C. By default, Virtual Extensible LAN (VXLAN) uses multicast to flood unknown unicasts, allowing it to perform data plane learning. See Chapter 10 for more information.
31 C. SD-Access uses VXLAN encapsulation because it can carry Ethernet frames. The others can't. See Chapter 11 for more information.
32 B. Software-defined networking in a wide area network (SD-WAN) doesn't use BGP. See Chapter 11 for more information.
33 A. When authenticating using a GET or PUT request, you should get a 200 response code if authentication succeeds. See Chapter 11 for more information.
34 A. Terminal Access Controller Access-Control System Plus (TACACS+) supports authorization, authentication, and accounting. Remote Authentication Dial-In User Service (RADIUS) doesn't support command authorization. See Chapter 12 for more information.
35 C. MAC authentication bypass is the only option that can authenticate a machine but not a user. See Chapter 12 for more information.
36 A, D. You can't use a port access control list (ACL) to block certain control plane traffic, including ARP and Spanning Tree BPDUs. You also can't use an extended IP ACL because ARP and Spanning Tree Protocol (STP) don't use IP. See Chapter 12 for more information.
Chapter 1 Networking Fundamentals
THE CCNP ENCOR EXAM OBJECTIVES COVERED IN THIS CHAPTER INCLUDE THE FOLLOWING:
Domain 1.0: Architecture✓ 1.1 Explain the different design principles used in an enterprise network✓ 1.7 Differentiate hardware and software switching mechanisms
Domain 3.0: Infrastructure✓ 3.1 Layer 2✓ 3.2 Layer 3
Networking fundamentals can at times seem abstract and even impractical. It's important to remember that networks are both logical and physical, so you need to keep a tight grip on both. If you neglect theory and just focus on typing in commands, you'll end up with a jalopy network. It might work, but not very well, and probably not for long. On the other hand, learning theory that you fail to put into practice leads to being educated but unemployed.
This chapter will give you a solid theoretical foundation on which to build practical skills. Much of the theory should already be familiar to you, and you'll likely have some “I already know this stuff” moments. But more often than not you'll gain new insights on something you already understood.
There's a lot of networking information out there, much of which is poorly explained, if not just plain wrong. Networking myths abound on forums, blogs, and even Wikipedia. Even official Cisco documentation has been known to contain the occasional errata. It's not intentional, of course. Learning networking is no different than learning any other complex topic. Some concepts are easy, whereas others just never quite click. Those harder concepts are fertile breeding ground for misconceptions that eventually get passed around until they become common knowledge, or worse, “best practices.” Almost every network professional I've encountered holds at least one glaring misconception about networking that eventually ends up stumping them (sometimes on an exam!). Chances are you, too, have been the unfortunate recipient of such information. The sooner we identify and dispel those myths, the better. That's what this chapter is all about.
The OSI Model
The origin of many networking myths can be traced back to the Open Systems Interconnection (OSI) reference model developed by Charles Bachman of Honeywell and formalized by the International Organization for Standardization (ISO). The ISO intended the OSI model to be a standard framework for data networks. It describes a set of “activities necessary for systems to interwork using communication media” (ISO/IEC 7498-4). The model organizes these activities or functions into the following seven layers:
7. Application
6. Presentation
5. Session
4. Transport
3. Network
2. Data Link
1. Physical
The seven layers are taught zealously in most introductory networking courses. You may have had them permanently drilled into your head with the help of one or two fun little mnemonics! (My favorite is “All people seem to need data processing.”) As we discuss the functions of the different layers, keep in mind that the layers of the OSI model are arbitrary. They're not written on stone tablets, nor are they the result of a rigorous scientific process that conclusively proved that the perfect network has these seven layers. The ISO arrived at each layer by attempting to group similar network functions together in a layer and then organizing the layers in a hierarchical fashion so that each layer of functions is dependent on the one below it. This led to impressive results in layers 1–4 (the lower layers) and utter confusion in layers 5–7 (the upper layers).
Table 1.1 shows what common protocols fall into each of the lower layers.
Table 1.1 The lower layers and their associated protocols
| Layer | Name | Example protocols |
| 1 | Physical | Thicknet (10BASE5) |
| Thinnet (10BASE2) | ||
| 1000BASE-T | ||
| T1/E1 | ||
| 2 | Data Link | IEEE 802.3/Ethernet II (DIX) |
| Point-to-Point Protocol (PPP) | ||
| High-Level Data Link Control (HDLC) | ||
| 3 |
Network
|
