Chapter 6: Network layer

Which key combination allows a user to abort setup mode?

Ctrl-C

The router connected to the local network segment is referred to as the ________________________________.

Default Gateway

During the boot process, where will the router bootstrap program look for the IOS image by default? NVRAM, RAM, ROM or Flash

Flash. The IOS image is typically stored in flash memory. If the image is not in flash memory when the router boots, the router bootstrap program can look for it on a TFTP server.

A router may have to fragment a packet when forwarding it from one medium to another medium that has a smaller ______________________.

MTU - Maximum Transmission Unit C: 6.1.3

If there are two or more possible routes to the same destination, the _______________________ is used to determine which route is used in the routing table.

Metric. Refer to curriculum topic: 6.2.2 If there are two or more possible routes to the same destination, the metric is used to decide which route appears in the routing table.

Which value, that is contained in an IPv4 header field, is decremented by each router that receives a packet?

TTL - Time to Live. When a router receives a packet, the router will decrement the Time-to-live (TTL) field by one. When the field reaches zero, the receiving router will discard the packet and will send an ICMP Time Exceeded message to the sender.

What is the network layer PDU?

The IP Packet

What is encapsulation on the Network Later?

The network layer encapsulates the protocol data unit (PDU) from the transport layer into a packet. The encapsulation process adds IP header information, such as the IP address of the source (sending) and destination (receiving) hosts.

What role does routing play in the Network Layer?

The network layer provides services to direct packets to a destination host on another network. To travel to other networks, the packet must be processed by a router. *The role of the router is to select the best path and direct packets toward the destination host in a process known as routing.* A packet may cross intermediary devices before reaching the destination host. Each router a packet crosses to reach the destination host is called a hop.

What are the 4 router interface configuration tasks?

1. *interface* type-and-no 2. *description* description text 3. *ip address* IPv4-address subnet-mask 4. *no shutdown*

What are the 3 major limitations of IPv4?

1. IP address depletion 2. Internet routing table expansion 3. Lack of end-to-end connectivity (Network Address Translation)

Which IPv4 address can a host use to ping the loopback interface?

A host can ping the loopback interface by sending a packet to a special IPv4 address within the network 127.0.0.0/8

Does a Switch need a default gateway address?

Not directly. But the SVI does, in order to remotely access the switch from another network using SSH or Telnet, the switch must have an SVI with an IPv4 address, subnet mask, and default gateway address configured.

What do we call communication between networks?

Routing.

Is the router on the same network as the host? Is there a general way to find out what the default gateway address is?

The default gateway is only used when the host wants to send a packet to a device on another network. The default gateway address is generally the router interface address attached to the local network of the host. The IP address of the host device and the router interface address must be in the same network.

Routing is the process of identifying the ____________ ____________ to a destination.

best path

What are the 8 basic steps to configure a Switch? Hints: 1. Configure _______ name 2. Secure user EXEC mode 3. Secure remote Telnet / SSH access 4. Secure privileged EXEC mode 5. Secure all passwords in the config file 6. Legal notification 7. Configure the management of SVI 8. Save the Configuration

*1. Configure the device name* hostname name *2. Secure user EXEC mode* line console 0 password password login *3. Secure remote Telnet / SSH access* line vty 0 15 password password login *4. Secure privileged EXEC mode* enable secret password *5. Secure all passwords in the config file* service password-encryption * 6. MOTD Provide legal notification* banner motd delimiter message delimiter *7. Configure the management SVI* interface vlan 1 ip address ip-address subnet-mask no shutdown *8. Save the configuration* copy running-config startup-config

What is the command used to save configuration?

*copy running-config startup-config*

How do you configure the default gateway of a switch?

*ip default-gateway* global config command

What command can be used to display the host routing table on a windows PC?

*route print* *netstart -r*

What command can be used to verify interface configuration?

*show ip interface brief* The output generated displays all interfaces, their IPv4 address, and their current status. The configured and connected interfaces should display a Status of "up" and Protocol of "up". Anything else would indicate a problem with either the configuration or the cabling.

What are the 4 basic processes used by the network layer to accomplish end-to-end transport?

1) Addressing end devices - must be configured with *Unique IP addresses* for identification on the network. 2) Encapsulation 3) Routing 4) De-encapsulation

What are the 3 most common ways to access a Cisco router?

1. Console - physical management port that provides out-of-band access to a cisco device. 2. Secure Shell (SSH) - remotely establishing a secure CLI connection through a virtual interface. 3. Telnet - as above, but insecure. Does not provide an encrypted connection.

Another role of the network layer is to direct packets between hosts. A host can send a packet to.....

1. Itself 2. Local Host 3. Remote Host

Describe the Router Boot process.

1. Perform POST (hardware check - performed by built-in ROM chip) 2. Load Bootstrap (Copied from ROM to RAM - locate the IOS) 3. Load the IOS (operating system file for the router - loaded into RAM after Bootstrap finds the IOS file to be used) 4. Load the config file from FLASH (NVRAM), a TFTP Server OR go into Setup Mode (to create a configuration file)

What are the 3 major phases of the router bootup process?

1. Perform the Post-On Self Test (POST) and load bootstrap program 2. Locate and load the Cisco IOS software 3. Locate and Load the startup config file or enter setup mode.

What 4 types of memory does Cisco router use?

1. RAM - volatile memory toe store applications, processes, and data needed to be executed by the CPU. 2. ROM - Non-volatile memort used to store crucial operational instructions + a limited IOS. 3. NVRAM - Non-volatile memory used as permanent storage for the startup configuration file 4. Flash - Non-volatile computer memory used as permanent storage for the IOS and other system related files.

What are the 3 functions of a Default Gateway?

1. Routes traffic to other networks. 2. Has a local IP address in the same range as the other hosts on the network. 3. Can take data in and forward data out.

Name the 6 significant fields in the IPv4 header, think also about their functions.

1. Version > always set to 0100 for IPv4 2. Differentiated Services (DS) > identifies the priority of each packet 3. Time-to-live (TTL) > commonly referred to as hop count 4. Protocol > identifies the upper-layer protocol to be used next 5. Source IPv4 Address > identifies the IP address of the sending host 6. Destination IPv4 address > identifies the IP address of the recipient host

What are 2 functions that are provided by the network layer?

1. providing end devices with a unique network identifier 2. Directing data packets to destination hosts on other networks The network layer is primarily concerned with passing data from a source to a destination on another network. IP addresses supply unique identifiers for the source and destination. The network layer provides connectionless, best-effort delivery. Devices rely on higher layers to supply services to processes.

The basic configuration of a Router is nearly exactly the same as a switch, except for one item. Pick 1: 1. Configure _______ name 2. Secure user EXEC mode 3. Secure remote Telnet / SSH access 4. Secure privileged EXEC mode 5. Secure all passwords in the config file 6. Legal notification 7. Configure the management of SVI 8. Save the Configuration

7. Configure the management of the SWITCH Virtual Interface.

Why do we say that that the IP protocol is unreliable in its best-effort delivery characteristic?

> Ip does not guarantee that all packets are delivered or received > IP does not have the capability to manage and recover from undelivered or corrupt packets > Packets may arrive at the destination corrupted, out of sequence, or not at all.

If out -of-order packets are delivered, or packets are missing, how is this resolved?

Applications using the date - or upper layer services - must resolve these issues. This allows IP to function very efficiently. *In the TCP/IP protocol suite, reliability is the role of the transport layer*

What does the *show interfaces* command display?

Displays statistics for all interfaces on the device

What does the *show ip interface* command display?

Displays the IPv4 statistics for all interfaces on a router

What does the *show ip route* command display?

Displays the contents of the IPv4 routing table stored in RAM

When transporting data from real-time applications, such as streaming audio and video, which field in the IPv6 header can be used to inform the routers and switches to maintain the same path for the packets in the same conversation? Nest Header, Flow Label, Traffic Class, or Differentiated services?

Flow Label. The Flow Label in IPv6 header is a 20-bit field that provides a special service for real-time applications. This field can be used to inform routers and switches to maintain the same path for the packet flow so that packets will not be reordered.

What is the purpose of a console port?

For the initial config and command-line-interface management access. 2 ports are available; the commonly used regular RJ-45 port and a new USB Type-B (mini-B USB) connector. Only 1 can be accessed at a time.

What happens when a router receives a packet and no routing entry exits for the given destination IP address?

If no routing entry exists, the router may forward the packet to its own default route if one is configured, or it will drop the packet.

What is the Default Gateway?

It is the network device that can route traffic to other networks. It is the router that can route traffic out of the local network.

The acronym _______________ is used to define the process that allows multiple devices to share a single routable IP address.

NAT Network Access Translation is the process of modifying the IP packet header information on packets going outside the corporate network. Corporate networks typically use private addresses on the inside LAN and need a public address to be able to communicate through the WAN.

Which portion of the network layer address does a router use to forward packets? Broadcast address, gateway address, host or network portion.

Network Portion (curriculum 6.2.2)

What do we mean by "IP is connectionless"?

No dedicated end-to-end connection is created before data is sent. > IP requires no initial exchange of control information to establish an end-to-end connection before packets are forwarded. > IP requires no additional fields in the header to maintain an established connection > Senders are unaware whether destination devices are present and functional when sending packets, nor are they aware if the destination received the packet, or if they are able to access and read the packet.

What is the purpose of the network layer - or OSI Layer 3?

Provides services to allow end devices to exchange data across the network.

What happens to the data portion of the packet - the encapsulated transport layer PDU - during the network layer processes?

Remains unchanged. The routing performed by these intermediate devices only considers the contents of the network layer packet header.

IP was designed as a protocol woth low overhead. It provides only the functions that are necessary to deliver a packet from a sourec tp a destination over an interconneceted system of networks. *It was not designed to track and manage the flow of packets*. At what layers, if required, do these functions primarily occur?

TCP @ LAyer 4.

How does the network layer use the MTU value?

The MTU is passed to the network layer by the data link layer. The data link layer indicates to the network layer the MTU for the medium that is being used. The network layer uses that information to determine how large the packet can be when it is forwarded. When packets are received on one medium and forwarded on a medium with a smaller MTU, the network layer device can fragment the packet to accommodate the smaller size.

What network devices need to be configured with a default gateway?

The default gateway address is typically configured on all devices that wish to communicate beyond just their local network. Switch - SVI Router

Within a production network, what is the purpose of configuring a switch with a default gateway address?

The default gateway address is used to forward packets originating from the switch to remote networks. Refer to curriculum topic: 6.4.3 A default gateway address allows a switch to forward packets that originate on the switch to remote networks. A default gateway address on a switch does not provide Layer 3 routing for PCs that are connected on that switch. A switch can still be accessible from Telnet as long as the source of the Telnet connection is on the local network.

How does the host know what the IPv4 address is of the default gateway?

The host receives the IPv4 address of the default gateway either dynamically from Dynamic Host Configuration Protocol (DHCP) or configured manually.

What are some of the benefits of encapsulating data layer by layer?

The process of encapsulating data layer by layer enables the services at the different layers to develop and *scale without affecting the other layers*. This means the transport layer segments can be readily packaged by IPv4 or IPv6 or by any new protocol that might be developed in the future

What does the transport layer (OSI Layer 4) do in comparison to the network layer protocols?

Transport > manages data transport between the processes running on each host Network layer protocols > specify the packet structure and processing used to carry the data from one host to another host. Operating without regard to the data carried in each packet allows the network layer to carry packets for multiple types of communications between multiple hosts.

what is de-encapsulation on the network layer?

When the packet arrives at the network layer of the destination host, the host checks the IP header of the packet. If the destination IP address within the header matches its own IP address, the IP header is removed from the packet. After the packet is de-encapsulated by the network layer, the resulting Layer 4 PDU is passed up to the appropriate service at the transport layer.

What are 3 characteristics of the IP Protocol?

[1] Connectionless [2] Best Effort [3] Media Independent (note: will adjust the size of the packet sent depending on what type of network access will be used)

What are the improvements that IPv6 provides?

[1] increased address space. 128 bit hierarchical addressing, IPv4 has 32 bits [2] Improved packet handling [3] Eliminated the need for NAT. Simplified header format for efficient packet handling Larger payload for increased throughput and transport efficiency Hierarchical network architecture for routing efficiency Autoconfiguration for addresses Elimination of need for network address translation (NAT) between private and public addresses

If you use the analogy that a network is like a room, then the default gateway is like a __________________. If you want to get to another room or network you need to find the _________________. (clue: word repeats)

doorway. To continue the analogy, a PC or computer that does not know the IP address of the default gateway is like a person, in a room, that does not know where the doorway is. They can talk to other people in the room or network, *but if they do not know the default gateway address, or there is no default gateway, then there is no way out*.

Devices that are beyond the local network segment are known as _____________ ______________.

remote hosts.

When a source device sends a packet to a remote destination device, then the help of __________________ and _________________ is needed.

routers and routing