Chapter 1's Review Questions

1: The Transport Layer is the host-to-host layer in the OSI model and the TCP/IP suite. It is in-between the upper and lower layers and depending on the protocol is responsible for delivery, error detection, and correction. Describe the upper layers of the OSI model and include examples.
A1: Answer: The upper layers of the OSI model include the following:

  • L7 Application— Service use and advertisement (file and print services, e-mail)

  • L6 Presentation— Translation, encryption, and compression (character codes, public/private key, ASCII, JPEG)

  • L5 Session— Dialog, session administration, connection establishment, and data transfer (NetBIOS, Sockets, drive mappings)

2: Describe the lower layers of the OSI model and include examples.
A2: Answer: The lower layers of the OSI model include the following:

  • L3 Network— Logical addressing and routing (IP, ARP, RARP, ICMP, and routers)

  • L2 Data Link— Physical addressing, media access, and frame formats (Ethernet, Token Ring, Frame Relay, and switches)

  • L1 Physical— 0s and 1s, cabling, and signaling (Category 5, RJ-45, HSSI, coax, fiber, and hubs)

3: Draw a picture showing the differences between OSI layers and TCP/IP layers.
A3: Answer: Refer to Figure 1-16, Figure 1-17, and Figure 1-18 for pictures illustrating the differences between the OSI seven-layer model and the DoD five-layer TCP/IP suite.
4: Explain encapsulation using the appropriate protocol data unit terminology.
A4: Answer: For a TCP/IP-based application, data gets encapsulated in a TCP segment, which gets encapsulated in an IP packet, which gets encapsulated in an Ethernet frame in order to get to the Physical Layer bits for transmission across the medium.
5: Explain de-encapsulation, including how Layer 2 hands off to Layer 3, how Layer 3 hands off to Layer 4, and so on.
A5: Answer: De-encapsulation is like opening envelopes or presents. Each layer reads and carries out the instructions from its peer layer, discards the header, and sends the packets up the stack for further processing. The Physical Layer passes bits in frames to the Data Link Layer. The Data Link Layer uses a type code or SAP to determine which Layer 3 protocol to hand off to. The Network Layer uses a protocol number to pass to the Layer 4 protocol. The Transport Layer uses a port number to send to an upper-layer application.
6: What is the difference between a hub, switch, and router?
A6: Answer: A hub is a Layer 1 device that does absolutely no filtering (it spits bits). A switch is a Layer 2 device that can assist with collisions and make filtering decisions based on physical addresses. A router is a Layer 3 device that can assist with collisions and broadcasts and can make filtering decisions based on logical addresses. A Layer 3 switch is really a router.
7: What is the difference between routed and routing protocols? Give examples of each.
A7: Answer: Routing protocols exchange routes with other routers. Examples include OSPF, BGP, RIP, and EIGRP. Routed protocols deliver packets; they send user data. Examples include IP and IPX.
8: Describe packet flows through routers.
A8: Answer: Packet flow is an important concept. Routers route to the destination network address. They buffer and switch packets from the inbound interface to the outbound interface within the router. Performance is definitely affected by the switching type. Fast switching refers to when a router does a route table lookup for the first packet toward a destination and caches it so that it doesn't have to perform a route table lookup on each and every packet. (Imagine the overhead if a router actually performs a route table lookup on each and every packet, which is called process switching and is used when you perform such tasks as debug commands.) Newer devices offer Cisco Express Forwarding (CEF) as a switching type, whereby even the first packet gets cached. Remember these important points: Routers route hop-to-hop, and routers switch from the inbound interface to the outbound interface of the router at Layer 3.
9: How can the OSI model assist in troubleshooting?
A9: Answer: The OSI model helps you take a layered, systematic approach to troubleshooting. The OSI model provides other benefits as well (such as inter-operability, standardization, and it enables you to subdivide developer tasks without having to alter other layers). For example, those making network interface cards (NICs) really don't want to be concerned with what upper-layer applications and protocols run over the hardware. However, NIC vendors must be concerned with LAN technologies such as Ethernet and Token Ring and what physical specifications (cable and connectors) to follow.
10: List the seven steps of the Cisco troubleshooting model?
A10: Answer: The Cisco troubleshooting model is as follows:

  1. Define the problem.

  2. Gather the facts.

  3. Consider possibilities (based on facts).

  4. Create an action plan.

  5. Implement action plan.

  6. Observe results.

  7. Document the solution.

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