Выдержки из Chapter 4 книги Cisco Press "Interconnecting Cisco Network Devices, Part 1 (ICND1): CCNA Exam 640-802 and ICND1 Exam 640-822" для повторения перед экзаменом.■ Static routing: This type of routing requires that you manually enter route information into a routing table.
■ Dynamic routing: This type of routing builds a routing table dynamically, using routing information that is obtained from routing protocols.
■ Default routing: This type of routing replaces the need to hold an explicit route to every network. The default route entry can be either statically configured or learned from a dynamic routing protocol.
The routing tables can be populated by the following methods:
■ Directly connected networks: This entry comes from having router interfaces directly attached to network segments and is the most certain method of populating a routing table. If the interface fails or is administratively shut down, the entry for that network will be removed from the routing table. The administrative distance is 0 and, therefore, will preempt all other entries for that destination network, because the entry with the lowest administrative distance is the best, most trusted source.
■ Static routes: Static routes are manually entered directly into the configuration of a router by a system administrator. The default administrative distance for a static route is 1; therefore, the static routes will be included in the routing table unless there is a direct connection to that network. Static routes can be an effective method for small, simple networks that do not change frequently.
■ Dynamic routes: Dynamic routes are learned by the router, and the information is responsive to changes in the network so that it is constantly being updated. There is, however, always a lag between the time that a network changes and when all the routers become aware of the change. The time delay for a router to match a network change is called convergence time. The shorter the convergence time, the better, and different routing protocols perform differently in this regard. Larger networks require the dynamic routing method because there are usually many addresses and constant changes, which, if not acted upon immediately, would result in loss of connectivity.
■ Default route: A default route is used when no explicit path to a destination is found in the routing table. The default route can be manually inserted or populated from a dynamic routing protocol.
Metrics can be based on either a single characteristic or several characteristics of a path.
The metrics that are most commonly used by routing protocols are as follows:
■ Bandwidth: The data capacity of a link (the connection between two network devices)
■ Delay: The length of time required to move a packet along each link from source to destination—depends on the bandwidth of intermediate links, port queues at each router, network congestion, and physical distance
■ Hop count: The number of routers that a packet must travel through before reaching its destination (In Figure 4-4, the hop count from host A to host B would be 1 or 2 depending on the path.)
■ Cost: An arbitrary value assigned by a network administrator or operating system, usually based on bandwidth, administrator preference, or other measurement
Summary of Exploring the Functions of Routing
The following list summarizes the key points that were discussed in the previous sections:
■ Routers have certain components that are also found in computers and switches. These components include the CPU, motherboard, RAM, and ROM.
■ Routers have two primary functions in the IP packet delivery process: maintaining routing tables and determining the best path to use to forward packets.
■ Routers determine the optimal path for forwarding IP packets between networks. Routers can use different types of routes to reach the destination networks, including static, dynamic, directly connected, and default routes.
■ Routing tables provide an ordered list of best paths to known networks and include information such as destination, next-hop associations, and routing metrics.
■ Routing algorithms process the received updates and populate the routing table with the best route.
■ Commonly used routing metrics include bandwidth, delay, hop count, and cost.
■ Distance vector routing protocols build and update routing tables automatically by sending all or some portion of their routing table to neighbors. The distance vector routing approach determines the direction (vector) and distance to any network in the internetwork.
■ Link-state routing protocols build and update routing tables automatically, running the shortest path first (SPF) algorithms against the link-state database to determine the best paths, and flood routing information about their own links to all the routers in the network.
■ Cisco developed EIGRP, which combines the best features of distance vector and linkstate routing protocols.
Least Significant Bit and Most Significant Bit
In a binary number, the digit on the rightmost side is the least significant bit (LSB), and the digit on the leftmost side is the most significant bit (MSB). The significance of any digits in between these sides is based on their proximity to either the LSB or the MSB.
Summary of Understanding Binary Numbering
The following list summarizes the key points that were discussed in the previous sections:
■ All computers operate using a binary system.
■ Binary systems (base 2) use only the numerals 0 and 1.
■ Decimal systems (base 10) use the numerals 0 through 9.
■ Using the powers of 2, a binary number can be converted into a decimal number.
■ Using the powers of 2, a decimal number can be converted into a binary number.
The disadvantages of a flat network are as follows:
■ All devices share the same bandwidth.
■ All devices share the same Layer 2 broadcast domain.
■ It is difficult to apply security policies because there are no boundaries between devices.
The advantages of subnetting a network are as follows:
■ Smaller networks are easier to manage and map to geographical or functional requirements.
■ Overall network traffic is reduced, which can improve performance.
■ You can more easily apply network security measures at the interconnections between subnets than throughout the entire network.
The following list summarizes the key points that were discussed in the previous sections:
■ Networks, particularly large ones, are often divided into smaller subnetworks, or subnets. Subnets can improve network performance and control.
■ A subnet address extends the network portion and is created by borrowing bits from the original host portion and designating them as the subnet field.
■ Determining the optimal number of subnets and hosts depends on the type of network and the number of host addresses required.
■ The algorithm for computing a number of subnets is 2s, where s is the number of subnet bits.
■ The subnet mask is the tool that the router uses to determine which bits are routing (network and subnet) bits and which bits are host bits.
■ End systems use subnet masks to compare the network portion of the local network addresses with the destination addresses of the packets to be sent.
■ Routers use subnet masks to determine whether the network portion of an IP address is on the corresponding routing table or whether the packet needs to be sent to the next router.
■ Determining the subnetwork and host addresses using a subnet mask is accomplished through this procedure:
— Write the octet being split in binary.
— Write the mask in binary, and draw a line to delineate the significant bits.
— Place a line under the mask so that you can view the significant bits.
— Copy the subnet bits four times.
— Define the network address by placing all 0s in the host bits.
— Define the broadcast address by placing all 1s in the host bits.
— Define the first and last host numbers.
— Increment the subnet bits by 1.
Initial Startup of a Cisco Router
■ Runs the power-on self test (POST) to test the hardware
■ Finds and loads the Cisco IOS Software that the router uses for its operating system
■ Finds and applies the configuration statements about router-specific attributes, protocol functions, and interface addresses
Summary of Starting a Cisco Router
The following list summarizes the key points that were discussed in the previous sections:
■ The router startup sequence is similar to the startup sequence of the Catalyst switch. The router first performs the POST, and then the router finds and loads the Cisco IOS image. Finally, it finds and loads the device configuration file.
■ Use the enable command to access the privileged EXEC mode from the user EXEC mode.
■ After logging in to a Cisco router, the initial startup status of a router can be verified using the router status commands show version, show running-config, and show.
Based on the output of the show interfaces command, possible problems can be fixed as follows:
■ If the interface is up and the line protocol is down, a problem exists. Some possible causes include the following:
— No keepalives
— Mismatch in encapsulation type
■ If both the line protocol and the interface are down, a cable might never have been attached when the router was powered up, or some other interface problem must exist. For example, in a back-to-back connection, the other end of the connection might be administratively down.
■ If the interface is administratively down, it has been manually disabled (the shutdown command has been issued) in the active configuration.
Summary of Configuring a Cisco Router
The following list summarizes the key points that were discussed in the previous sections:
■ From the privileged EXEC mode, the global configuration mode can be entered, providing access to other configuration modes such as the interface configuration mode or line configuration mode.
■ The main function of a router is to relay packets from one network device to another. To do this, the characteristics of the interfaces through which the packets are received and sent must be defined. Interface characteristics such as the IP address and bandwidth are configured using the interface configuration mode.
■ In a TCP/IP environment, end stations communicate seamlessly with servers or other end stations. This communication occurs because each node using the TCP/IP protocol suite has a unique 32-bit logical IP address.
■ When the router interface configuration has been completed, it can be verified by using show commands.
Summary of Exploring the Packet Delivery Process
The following list summarizes the key points that were discussed in the previous sections:
■ If the hosts are not on the same segment, the frame is sent to the default gateway.
■ Packets sent to the default gateway will have the local host source and remote host destination IP address.
■ Frames sent to the default gateway will have the local host source and the default gateway MAC address.
■ A router will change the Layer 2 address as needed but will not change the Layer 3 address.
■ The show ip arp command displays the mapping between network addresses and MAC addresses that the router has learned.
■ IOS connectivity tools:
— Ping
— Traceroute
Physical and Environmental Threats
There are four classes of insecure installations or physical access threats, as follows:
■ Hardware threats: Threats of physical damage to the router or router hardware
■ Environmental threats: Threats such as temperature extremes (too hot or too cold) or humidity extremes (too wet or too dry)
■ Electrical threats: Threats such as voltage spikes, insufficient supply voltage (brownouts), unconditioned power (noise), and total power loss
■ Maintenance threats: Threats such as poor handling of key electrical components (electrostatic discharge), lack of critical spare parts, poor cabling, poor labeling, and so on
Summary of Understanding Cisco Router Security
The following list summarizes the key points that were discussed in the previous sections:
■ Passwords can be used to restrict access.
■ The first level of security is physical.
■ The login banner can be used to display a message before the user is prompted for a username.
■ Remote access can be configured using Telnet or SSH.
Summary of Using the Cisco SDM
The following list summarizes the key points that were discussed in the previous sections:
■ Cisco SDM is a useful tool for configuring Cisco access routers.
■ Cisco SDM contains several easy-to-use wizards for efficient configuration of Cisco access routers.
■ Cisco SDM allows you to customize Cisco access router configurations using advanced features.
DHCP supports these three mechanisms for IP address allocation:
■ Automatic allocation: DHCP assigns a permanent IP address to a client.
■ Dynamic allocation: DHCP assigns an IP address to a client for a limited period of time (or until the client explicitly relinquishes the address).
■ Manual allocation: A client IP address is assigned by the network administrator, and DHCP is used simply to convey the assigned address to the client.
DHCPDISCOVER
When a DHCP client boots up for the first time, it transmits a DHCPDISCOVER message on its local physical subnet. Because the client has no way of knowing the subnet to which it belongs, the DHCPDISCOVER is an all-subnets (all-hosts) broadcast (destination IP address of 255.255.255.255). The client does not have a configured IP address; therefore, the source IP address of 0.0.0.0 is used.
DHCPOFFER
A DHCP server that receives a DHCPDISCOVER message can respond with a DHCPOFFER message, which contains initial configuration information for the client. For example, the DHCP server provides the requested IP address. The subnet mask and default gateway are specified in the options field, subnet mask, and router options, respectively. Other common options in the DHCPOFFER message include IP address lease time, renewal time, domain name server, and NetBIOS Name Service (Microsoft Windows Internet Name Service [Microsoft WINS]).
DHCPREQUEST
After the client receives a DHCPOFFER message, it responds with a DHCPREQUEST message, indicating its intent to accept the parameters in the DHCPOFFER.
DHCPACK
After the DHCP server receives the DHCPREQUEST message, it acknowledges the request with a DHCPACK message, thus completing the initialization process.
Summary Using a Cisco Router as a DHCP Server
The following list summarizes the key points that were discussed in the previous sections:
■ DHCP is built on a client-server model.
■ DHCP server hosts allocate network addresses and deliver configuration parameters.
■ Cisco IOS Software includes Dynamic Host Configuration Protocol (DHCP) server.
■ SDM can be used to configure DHCP server on the router.
■ Required configuration items are as follows:
— Pool name
— Pool network and subnet
— Starting and ending addresses
■ SDM can be used to monitor DHCP server on the router.
■ The show ip dhcp conflict command can be used to find conflicts.
Summary of Accessing Remote Devices
The following list summarizes the key points that were discussed in the previous sections:
■ After being connected to a remote device, network staff might want to access a local device without terminating the Telnet session. Telnet allows temporary suspension and then resumption of a remote session.
■ Ending a Telnet session on a Cisco device uses the exit, logout, disconnect, or clear command.
■ The ping and trace commands provide information about the connectivity with and path to remote devices.
Chapter Summary
The following list summarizes the key points that were discussed in this chapter:
■ Routers operate at Layer 3, and their function is path determination.
■ Binary numbers are based on the “powers of 2.”
■ IP addressing:
— Dotted decimal representation of a binary string
— Identifies the network, subnet, and host
■ Routers have a startup process where they test the hardware and load the operating system and configuration.
■ Basic router configuration is usually done through the console port using CLI and consists of the following:
— Host address
— Interface IP addressing
■ Router have the similar hardware, environmental, electrical, and maintenance-related security threats as switches.
■ Basic router security consists of the following:
— Login banner
— Telnet versus SSH
■ The Cisco IOS DHCP server is a full DHCP server that can be configured using SDM.
■ Cisco IOS provides a set of tools for remote accessing and testing:
— Telnet
— SSH
— ping
— traceroute
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