[Cisco Networking Acdaemy] Del Mar College
ITNW 1392-Beginning Router Configuration
 Instructor:Michael P. Harris
Sem2Les7

Semester 2, Lesson 7 Notes:

Displaying Router Configuration Information

1. List three 'external configuration' sources for Cisco Routers. Routers may be configured from (1) the console terminal through the console port; (2) via modem using the auxiliary port; and (3) from virtual terminals or a TFTP server once it is on a network.

List and describe the 'internally configurable' components of a router:

  1. Ram [stores routing tables, ARP cache, and running configuration file]
  2. NVRAM [stores the router's backup configuration file; retains content even when powered down]
  3. Flash [erasable, reprogrammable ROM, holds the operating system image]
  4. ROM [contains power-on diagnostics, a bootstrap program, and operating system software]
  5. Interfaces [network connections through which packets enter and exit the router]
  • Whether accessed from the console or by Telnet session through an auxiliary port, the router can be placed in several modes. Each mode provides different functions:
  • Router> (User Exec mode) - A 'look-only' mode in which the user can view some information about the router, but cannot change anything.
  • Router# (Privileged EXEC mode) - Supports the debugging and testing commands, detailed examination of the router, manipulation of configuration files, and access to configuration modes.
  • (Setup mode) - Presents an interactive prompted dialogue at the console that helps the new user create a first-time basic configuration.
  • Router (config)# (Global configuration mode) - Implements powerful one-line commands that perform simple configuration tasks.
  • Router (config-mode)# (Other configuration modes) - Provide more complicated mulitple-line configurations.
  • (RXBOOT mode) - A maintenance mode that can be used, among other things, to recover lost passwords.

3. List at least seven commands that show router statis and which configurable components about which they display information:

(1) show version [RAM-IOS];

  • Displays the configuration of the system hardware, the software version, the names and sources of configuration files, and the boot images.

(2) show processes CPU and show processes [RAM-programs];

  • Displays information about the active processes.

(3) show running-config [RAM-active configuration file];

  • ('write term' on Cisco IOS Release 10.3 or earlier) Displays the active configuration file. Displays the configured protocols. This command shows the statis of any configured Layer 3 (network) protocol.

(4.1) show mem [RAM-Tables and Buffers];

  • Shows statistics about the router's memory, including memory free pool statistics.

(4.2) show stacks [RAM-Tables and Buffers];

  • Monitors the stack use of processes and interupt routines and displays the reason for the last system reboot.

(4.3) show buffers [RAM-Tables and Buffers];

  • Provides statistics for the buffer pools on the network server.

(5) show startup-config/show config [NVRAM;

  • ('show config' on Cisco IOS Release 10.3 or earlier) Displays the backup configuration file.

(6) show flash [Flash];

  • Shows information about the Flash memory device.

(7) show interfaces [Interfaces];

  • Displays statistics for all interfaces configured on the router.
  • "show running-config" and "show starup-config" are among most used Cisco IOS software EXEC commands because they allow an administrator to see the current running configuration on the router or the image size and startup configuration commands the router will use on the next restart.
  • You will know that you are looking at the "active configuration file" (show running-config) when you see the words 'Current Configuration' at the top.
  • You will know that you are looking the 'backup configuration file' (show startup-config) when you see a messgae at the top telling you how much nonvolatile memory has been used.

The "show interface serial" command displays configurable parameters and real-time statistics related to serial interfaces.

The "show protocols" EXEC command shows the global and interface-specific status of any configured Level 3 protocols (for example, IP, DECnet, IPX, and AppleTalk).

4. Briefly describe what Cisco Discovery Protocol (CDP) "show cdp neighbors" can tell you about a network.

The command 'show cdp neighbors' works at the 'data link layer' (layer 2) to display the following information:

  1. neighbor device ID,
  2. local port type and number,
  3. decremental holdtime value in seconds,
  4. neighbor's device capability code,
  5. hardware platform of neighbor, and
  6. neighbor's remote port type and number.

Cisco Discovery Protocol (CDP) provides a single proprietary command that enables network administrators to access a summary of what the configurations look like on other directly connected routers. When a Cisco device running Cisco IOS Release 10.3 and later boots up, CDP starts up automatically. Although CDP runs by default, you must explicitly enable it on the device's interfcae using the command "cdp enable". Only directly connected neighbors exchange CDP frames.

The "show cdp interface" command displays the values of the CDP timers, the interface status, and the encapsulation used by CDP for its advertisement and discovery frame transmission.

The "show cdp entry {device name}" (or optionally "show cdp neighbors detail") command displays a single cached CDP entry. Output from this command includes all the Layer 3 addresses present in the neighbor router B; an administrator can see the IP addresses of the targeted CDP neighbor (Router B) with the single command entry on router A.

CDP was designed and implimented as a very simple, low overhead protocol. A CDP frame can be small yet retrieve a lot of useful information about neighboring routers.

Another way to learn about a remote router is to connect it with the "Telnet", a virtual terminal protocol that is part of the TCP/IP protocol suite. You can set a connection between the router and a connected device. A router can have up to five simlutaneous incoming Telnet sessions. You do not need to enter the command 'connect' or 'telnet' to establish a Telnet connection. If you prefer, you can just enter the learned host name. To end a Telnet session, use the EXEC command, 'exit, or 'logout'. Use <Ctrl><Shift> <6> to suspend a telnet 'session'. Press <Return> or (enter session number or name) to resume a session. Use the 'disconnect' command to disconnect a telnet 'session'.

5. Using the OSI model and the commands 'telnet', 'ping', 'trace', 'show IP route', and 'show interface', decsribe the basic testing of a network:

The basic testing of a network should proceed from 'layer 7' (Application) [using "telnet" to access various routers], to 'layer 3' (Network) [using "ping", "trace", and "show IP route" to learn logical address information about the network], and then to layers 3, 2, and 1 [using "show interface" to learn about specific port configurations and whether specific links are alive] and then on to layer 1 (Physical) [specific medium tests].

  • 'ping' - layer 3 question "Are protocol packets being routed?"
  • 'show IP route' - layer 3 question "Does an entry exist in the routing table?"
  • 'trace" - layer 3 question "What paths are packets taking?"
  • 'show interface serial' - layer 1 question "Is the carrier detect signal present?" and a layer 2 question - "Are keep alive messages being received?"
  • 'debug' - "What protocol messages are being sent?"

The "Telnet" application provides a virtual terminal so that administrators can use Telnet operations to connect with other hosts running TCP/IP. Test to determine whether the remote router can be accessed. If we can Telnet to one router but not to another router, it is likely that the Telnet failure is caused by specific addressing, naming, or access permission problems. These problems can exist on our router or on the router that failed as a Telnet target.

The "ping" (ICMP) command sends a special packet to the destination host and then waits for a reply packet from that host. Results from this "echo protocol" can help evaluate the path-to-host reliability, delays over the path, and whether the host can be reached or is functioning or whether protocol packets are being routed. The exclamation points (!) indicate each sucessful echo. If you instead receive one or more periods (.) on your display, the application of your router timed out waiting for a given packet echo from the ping target. The formal term for the "ping" process is "ICMP". (Internet Control Message Protocol)

The "trace" command is the ideal tool for finding where data is being sent in your network. It uses the same technology as the 'ping' command, execpt that instead of testing end-to-end connectivity, trace tests each step along the way. The trace command takes advantage of the error messages generated by routers when a packet exceeds its "Time To Live" (TTL) value. The benefit of the trace command is that it tells us which router in the path is the last one to be reached. This is called 'fault isolation'. If one of the routers on a trace path is unreachable, we would see three asterisks (*) instead of the name of the router. The 'trace' command would continue attempting to reach the next step until we escape using the "Control-Shift-6" escape sequence.

The "show IP route" command determines whether a routing table entry exists for the target network.

The "interface" has two pieces: physical (hardware) and logical (software). The hardware must make the actual connection between the devices. The software is the messages that are passed between two connected router interfaces. Whether you test the physical and data link, you ask these questions:

  1. Is there a 'Carrier Detect' signal? Is the physical link between devices good?
  2. Are the 'keepalive messages' being received? Can data packets be sent across the physical link?

One of the most important elements of the "show interface serial" command output is display of the line and data-link protocol status. The "line status" is triggered by a "Carrier Detect" signal, and refers to the "physical-layer status". The "line protocol", triggered by "keepalive frames", refers to the "data-link framing".

The 'show interfaces command" displays the router tracked statistics that provide information about the interface. The statistics reflect router operation since the last time the counters wre cleared.

The "clear counters" command resets he counters to zero. By starting from zero, you get a more clear picture of the current status of the network.

The "debug" privileged EXEC command starts the console display of the network events specified in the command parameter to aid in tracking down problems on it or on other hosts in the network. The "terminal monitor" command forwards "debug" output to your Telnet session terminal. Use the "undebug all" command to turn debugging off when you no longer need it. Substantial debugging on a busy network will slow down the network significantly.

By default, the router sends output from system error messages and the debug EXEC command to the comsole terminal. Messages can be redirected to a UNIX host or to an internal buffer. The "terminal monitor" command provides you the ability to redirect these messages to a terminal.

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