CCNA2#4

Accidentally using the clock rate command on a DTE interface generates a

%Error: This command applies only to DCE interface message.

By default, command history is enabled and the system captures the last

10 command lines in its history buffer. Use the show history privileged EXEC command to display the contents of the buffer.

An addressing table

A table that captures device names, interfaces, IPv4 addresses, subnet masks, and default gateway addresses.

Directly connected interfaces

Added to the routing table when an interface is configured and active.

Static routes

Added when a route is manually configured and the exit interface is active.

Local Route interfaces

Added when an interface is configured and active. This entry is only displayed in IOS 15 or newer for IPv4 routes and all IOS releases for IPv6 routes.

Dynamic routing protocol

Added when routing protocols that dynamically learn about the network, such as EIGRP or OSPF, are implemented and networks are identified.

Process switching

An older packet forwarding mechanism still available for Cisco routers. When a packet arrives on an interface, it is forwarded to the control plane where the CPU matches the destination address with an entry in its routing table, and then determines the exit interface and forwards the packet. It is important to understand that the router does this for every packet, even if the destination is the same for a stream of packets. This process-switching mechanism is very slow and rarely implemented in modern networks.

Availability

Availability is a measure of the probability that the network is available for use when it is required.

Enhanced Interior Gateway Routing Protocol (EIGRP)

Bandwidth, delay, load, reliability

Before the interface state is considered up/up and added to the IPv4 routing table, the interface must:

Be assigned a valid IPv4 or IPv6 address Be activated with the no shutdown command Receive a carrier signal from another device (router, switch, host, etc.)

Cisco Express Forwarding (CEF)

CEF is the most recent and preferred Cisco IOS packet-forwarding mechanism. Like fast switching, CEF builds a Forwarding Information Base (FIB), and an adjacency table. However, the table entries are not packet-triggered like fast switching but change-triggered such as when something changes in the network topology. Therefore, when a network has converged, the FIB and adjacency tables contain all the information a router would have to consider when forwarding a packet. The FIB contains pre-computed reverse lookups, next hop information for routes including the interface and Layer 2 information. Cisco Express Forwarding is the fastest forwarding mechanism and the preferred choice on Cisco routers.

Most network capable devices (i.e., computers, tablets, and smartphones) require the following components to operate

Central processing unit (CPU) Operating system (OS) Memory and storage (RAM, ROM, NVRAM, Flash, hard drive)

Open Shortest Path First (OSPF)

Cisco's cost based on cumulative bandwidth from source to destination

An IPv6 interface must be:

Configured with IPv6 address and subnet mask - Use the ipv6 address ipv6-address/prefix-length [link-local | eui-64] interface configuration command. Activated - The interface must be activated using the no shutdown command.

ipv6 address ipv6-address / prefix-length eui-64

Configures a global unicast IPv6 address with an interface identifier (ID) in the low-order 64 bits of the IPv6 address using the EUI-64 process.

ipv6 address ipv6-address / prefix-length link-local

Configures a static link-local address on the interface that is used instead of the link-local address that is automatically configured when the global unicast IPv6 address is assigned to the interface or enabled using the ipv6 enable interface command. Recall, the ipv6 enable interface command is used to automatically create an IPv6 link-local address whether or not an IPv6 global unicast address has been assigned.

Console access requires:

Console cable - RJ-45-to-DB-9 console cable Terminal emulation software - Tera Term, PuTTY, HyperTerminal

The Branch site devices connect as follows:

Corporate resources (i.e., file servers and printers) connect to Layer 2 switches using Ethernet cables. Desktop PCs and voice over IP (VoIP) phones connect to Layer 2 switches using Ethernet cables. Laptops and smartphones connect wirelessly to wireless access points (WAPs). The WAPs connect to switches using Ethernet cables. Layer 2 switches connect to an Ethernet interface on the edge router using Ethernet cables. An edge router is a device that sits at the edge or boundary of a network and routes between that network and another, such as between a LAN and a WAN. The edge router connects to a WAN service provider (SP). The edge router also connects to an ISP for backup purposes.

Cost

Cost indicates the general expense for purchasing of network components, and installation and maintenance of the network.

ipv6 address ipv6-address / prefix-length

Creates a global unicast IPv6 address as specified.

The Central site devices connect as follows:

Desktop PCs and VoIP phones connect to Layer 2 switches using Ethernet cables. Layer 2 switches connect redundantly to multilayer Layer 3 switches using Ethernet fiber-optic cables (orange connections). Layer 3 multilayer switches connect to an Ethernet interface on the edge router using Ethernet cables. The corporate website server is connected using an Ethernet cable to the edge router interface. The edge router connects to a WAN SP. The edge router also connects to an ISP for backup purposes.

The primary functions of a router are to

Determine the best path to send packets Forward packets toward their destination

When designing a new network or mapping an existing network, document the network. At a minimum, the documentation should identify:

Device names Interfaces used in the design IP addresses and subnet masks Default gateway addresses

show ip interface brief

Displays a summary for all interfaces including the IPv4 address of the interface and current operational status.

show running-config interface interface-id

Displays the commands configured on the specified interface.

show ip route

Displays the contents of the IPv4 routing table stored in RAM. In Cisco IOS 15, active interfaces should appear in the routing table with two related entries identified by the code 'C' (Connected) or 'L' (Local). In previous IOS versions, only a single entry with the code 'C' will appear.

exclude

Excludes all output lines that match the filtering expression

A router is also usually configured with its own default gateway. This is sometimes known as the

Gateway of Last Resort.

Routing Information Protocol (RIP)

Hop count

To enable network access, devices must be configured with IP address information to identify the appropriate:

IP address Subnet mask Default gateway

Dynamically

IP address information is provided by a server using the Dynamic Host Configuration Protocol (DHCP). The DHCP server provides a valid IP address, subnet mask, and default gateway for end devices. Other information may be provided by the server.

C -

Identifies a directly connected network

D -

Identifies a dynamically learned network from another router using EIGRP.

O -

Identifies a dynamically learned network from another router using the OSPF routing protocol.

S -

Identifies a static route created to reach a specific network.

IP address

Identifies a unique host on a local network.

Route timestamp

Identifies how much time has passed since the route was learned.

Route source

Identifies how the route was learned.

Next-hop

Identifies the IPv4 address of the next router to forward the packet to.

L -

Identifies the address assigned to a router's interface. This allows the router to efficiently determine when it receives a packet for the interface instead of being forwarded.

Destination network

Identifies the address of the remote network.

Outgoing interface

Identifies the exit interface to use to forward a packet toward the final destination.

Default gateway

Identifies the router to send a packet to when the destination is not on the same local network subnet.

Administrative distance

Identifies the trustworthiness of the route source. Lower values indicate preferred route source.

Metric

Identifies the value assigned to reach the remote network. Lower values indicate preferred routes.

Subnet mask

Identifies with which network subnet the host can communicate.

Directly connected network

If the destination IP address of the packet belongs to a device on a network that is directly connected to one of the interfaces of the router, that packet is forwarded directly to the destination device. This means that the destination IP address of the packet is a host address on the same network as the interface of the router.

Remote network

If the destination IP address of the packet belongs to a remote network, then the packet is forwarded to another router. Remote networks can only be reached by forwarding packets to another router.

No route determined

If the destination IP address of the packet does not belong to either a connected or remote network, the router determines if there is a Gateway of Last Resort available. A Gateway of Last Resort is set when a default route is configured on a router. If there is a default route, the packet is forwarded to the Gateway of Last Resort. If the router does not have a default route, then the packet is discarded.

To be available, an interface must be:

If using IPv4, configured with an address and a subnet mask - Use the ip address ip-address subnet-mask interface configuration command. Activated - By default, LAN and WAN interfaces are not activated (shutdown). To enable an interface, it must be activated using the no shutdown command. (This is similar to powering on the interface.) The interface must also be connected to another device (a hub, a switch, or another router) for the physical layer to be active.

include

Includes all output lines that match the filtering expression

Home office devices can connect as follows:

Laptops and tablets connect wirelessly to a home router. A network printer connects using an Ethernet cable to the switch port on the home router. The home router connects to the service provider cable modem using an Ethernet cable. The cable modem connects to the Internet service provider (ISP) network.

One distinguishing feature between switches and routers is the type of interfaces supported by each. For example

Layer 2 switches support LANs and, therefore, have multiple FastEthernet or Gigabit Ethernet ports.

When configuring a Cisco switch or router, the following basic tasks should be performed first:

Name the device - Distinguishes it from other routers. Secure management access - Secures privileged EXEC, user EXEC, and Telnet access, and encrypts passwords to their highest level. Configure a banner - Provides legal notification of unauthorized access.

A common analogy used to describe the three packet-forwarding mechanisms is as follows:

Process switching solves a problem by doing math long hand, even if it is the identical problem. Fast switching solves a problem by doing math long hand one time and remembering the answer for subsequent identical problems. CEF solves every possible problem ahead of time in a spreadsheet.

Routers support three packet-forwarding mechanisms:

Process switching, Fast switching, Cisco Express Forwarding (CEF)

Topology diagram

Provides a visual reference that indicates the physical connectivity and logical Layer 3 addressing. Often created using software, such as Microsoft Visio.

Read-Only Memory (ROM)

Provides permanent storage for bootup instructions, basic diagnostic software and a limited IOS in case the router cannot load the full featured IOS. ROM is firmware and referred to as non-volatile because it does not lose its contents when power is turned off.

Flash

Provides permanent storage for the IOS and other system-related files. The IOS is copied from flash into RAM during the bootup process. Flash is non-volatile and does not lose its contents when power is turned off.

Non-Volatile Random Access Memory (NVRAM)

Provides permanent storage for the startup configuration file (startup-config). NVRAM is non-volatile and does not lose its contents when power is turned off.

Random Access Memory (RAM)

Provides temporary storage for various applications and processes including the running IOS, the running configuration file, various tables (i.e., IP routing table, Ethernet ARP table) and buffers for packet processing. RAM is referred to as volatile because it loses its contents when power is turned off.

Routers store data using:

Random Access Memory (RAM) , Read-Only Memory (ROM), Non-Volatile Random Access Memory (NVRAM)

Reliability

Reliability indicates the dependability of the components that make up the network, such as the routers, switches, PCs, and servers. Reliability is often measured as a probability of failure or as the mean time between failures (MTBF).

Enabling and assigning a loopback address is simple:

Router(config)# interface loopback number Router(config-if)# ip address ip-address subnet-mask Router(config-if)# exit

Scalability

Scalability indicates how easily the network can accommodate more users and data transmission requirements. If a network design is optimized to only meet current requirements, it can be very difficult and expensive to meet new needs when the network grows.

In a production environment, infrastructure devices are commonly accessed remotely using

Secure Shell (SSH) or HyperText Transfer Protocol Secure (HTTPS). Console access is really only required when initially configuring a device, or if remote access fails.

Security

Security indicates how protected the network is, including the information that is transmitted over the network. The subject of security is important, and techniques and practices are constantly evolving. Consider security whenever actions are taken that affect the network.

begin

Shows all the output lines from a certain point, starting with the line that matches the filtering expression

section

Shows entire section that starts with the filtering expression

Speed

Speed is a measure of the data rate in bits per second (b/s) of a given link in the network. While the term "speed" is commonly used when referring to the network bandwidth, it is not technically accurate. The actual speed that the bits are transmitted does not vary over the same medium. The difference in bandwidth is due to the number of bits transmitted per second, not how fast they travel over wire or wireless medium.

There are two common types of static routes in the routing table:

Static route to a specific network Default static route

What does a router do with a packet received from one network and destined for another network?

Step 1. De-encapsulates the Layer 2 frame header and trailer to expose the Layer 3 packet. Step 2. Examines the destination IP address of the IP packet to find the best path in the routing table. Step 3. If the router finds a path to the destination, it encapsulates the Layer 3 packet into a new Layer 2 frame and forwards the frame out the exit interface.

Statically

The host is manually assigned the correct IP address, subnet mask, and default gateway. The DNS server IP address can also be configured.

Topology

There are physical and logical topologies. The physical topology is the arrangement of the cables, network devices, and end systems. It describes how the network devices are actually interconnected with wires and cables. The logical topology is the path over which the data is transferred in a network. It describes how the network devices appear connected to network users.

Remote routes

These are remote networks connected to other routers. Routes to these networks can either be statically configured or dynamically configured using dynamic routing protocols.

Directly connected routes

These routes come from the active router interfaces. Routers add a directly connected route when an interface is configured with an IP address and is activated.

Fast switching

This is a common packet forwarding mechanism which uses a fast-switching cache to store next-hop information. When a packet arrives on an interface, it is forwarded to the control plane where the CPU searches for a match in the fast-switching cache. If it is not there, it is process-switched and forwarded to the exit interface. The flow information for the packet is also stored in the fast-switching cache. If another packet going to the same destination arrives on an interface, the next-hop information in the cache is re-used without CPU intervention.

When the router is configured using the ipv6 unicast-routing global configuration command, the router

begins sending ICMPv6 Router Advertisement messages out the interface. This enables a PC connected to the interface to automatically configure an IPv6 address and to set a default gateway without needing the services of a DHCPv6 server. Alternatively, a PC connected to the IPv6 network can get its IPv6 address statically assigned

When a host sends a packet to a device on a different IP network, then the packet is forwarded to the

default gateway, because a host device cannot communicate directly with devices outside of the local network. The default gateway is the destination that routes traffic from the local network to devices on remote networks. It is often used to connect a local network to the Internet.

Another very useful feature that improves the user experience in the command-line interface (CLI) is the filtering of show output. Filtering commands can be used to

display specific sections of output. To enable the filtering command, enter a pipe (|) character after the show command and then enter a filtering parameter and a filtering expression.

On a Cisco IOS router, the show ip route command can be used to

display the IPv4 routing table of a router. A router provides additional route information, including how the route was learned, how long the route has been in the table, and which specific interface to use to get to a predefined destination.

When a host sends a packet to a device that is on the same IP network, the packet is

forwarded out of the host interface to the destination device.

When a router has two or more paths to a destination with equal cost metrics, then the router

forwards the packets using both paths equally. This is called equal cost load balancing. The routing table contains the single destination network, but has multiple exit interfaces, one for each equal cost path. The router forwards packets using the multiple exit interfaces listed in the routing table.

The ping command for IPv6 is

identical to the command used with IPv4 except that an IPv6 address is used.

The default gateway is usually the address of the

interface on the router connected to the local network. The router maintains routing table entries of all connected networks as well as entries of remote networks, and determines the best path to reach those destinations.

An interface can generate its own IPv6 link-local address without having a global unicast address by using the

ipv6 enable interface configuration command.

Routers use static routes and dynamic routing protocols to

learn about remote networks and build their routing tables. Routers use tables to determine the best path to send packets. Routers encapsulate the packet and forward it to the interface indicated in routing table.

The loopback interface is a

logical interface internal to the router. It is not assigned to a physical port and can therefore never be connected to any other device. It is considered a software interface that is automatically placed in an UP state, as long as the router is functioning. The loopback interface is useful in testing and managing a Cisco IOS device because it ensures that at least one interface will always be available. For example, it can be used for testing purposes, such as testing internal routing processes, by emulating networks behind the router.

If the link light is not on, then there

may be a problem with either the network cable or the network itself. The switch port where the connection terminates would also have an LED indicator lit. If one or both ends are not lit, try a different network cable.

Similarly, network infrastructure devices commonly use

multiple LED indicators to provide a quick status view. For example, a Cisco Catalyst 2960 switch has several status LEDs to help monitor system activity and performance. These LEDs are generally lit green when the switch is functioning normally and lit amber when there is a malfunction.

A router connects

multiple networks, which means that it has multiple interfaces that each belong to a different IP network. When a router receives an IP packet on one interface, it determines which interface to use to forward the packet to the destination. The interface that the router uses to forward the packet may be the final destination, or it may be a network connected to another router that is used to reach the destination network.

Host computers connect to a wired network using a

network interface and RJ-45 Ethernet cable. Most network interfaces have one or two LED link indicators next to the interface. Typically, a green LED means a good connection while a blinking green LED indicates network activity.

Each network that a router connects to typically requires a

separate interface. These interfaces are used to connect a combination of both local-area networks (LANs) and wide-area networks (WANs). LANs are commonly Ethernet networks that contain devices, such as PCs, printers, and servers. WANs are used to connect networks over a large geographical area. For example, a WAN connection is commonly used to connect a LAN to the Internet service provider (ISP) network.

A router is essentially a

specialized computer. It requires a CPU and memory to temporarily and permanently store data to execute operating system instructions, such as system initialization, routing functions, and switching functions.

Statically assigned addresses are commonly used to identify

specific network resources, such as network servers and printers. They can also be used in smaller networks with few hosts. However, most host devices acquire their IPv4 address information by accessing a DHCP server. In large enterprises, dedicated DHCP servers providing services to many LANs are implemented. In a smaller branch or small office setting, DHCP services can be provided by a Cisco Catalyst switch or a Cisco ISR.

A primary function of a router is to forward packets toward their destination. This is accomplished by using a

switching function, which is the process used by a router to accept a packet on one interface and forward it out of another interface. A key responsibility of the switching function is to encapsulate packets in the appropriate data link frame type for the outgoing data link. In this context, the term "switching" literally means moving packets from source to destination and should not be confused with the function of a Layer 2 switch.

When a packet arrives on a router interface, the router

uses its routing table to determine how to reach the destination network. The destination of the IP packet might be a web server in another country or an email server on the local area network. It is the responsibility of routers to deliver those packets efficiently. The effectiveness of internetwork communications depends, to a large degree, on the ability of routers to forward packets in the most efficient way possible.

Unlike a computer, a router does not have

video adapters or sound card adapters. Instead, routers have specialized ports and network interface cards to interconnect devices to other networks.