Cisco Discovery Protocol
The Cisco Discovery Protocol (CDP) is an information-gathering tool used by network administrators to obtain information about directly connected Cisco devices.
CDP functionality is enabled by default on all interfaces (except for Frame Relay multipoint subinterfaces), but can be disabled at the device level.
CDP provides the following information about each neighboring device:
■ Device identifiers: For example, the configured host name of the switch
■ Address list: Up to one network layer address for each protocol supported
■ Port identifier: The name of the local port and remote port, in the form of an ASCII character string such as ethernet0
■ Capabilities list: Supported features; for example, the device acting as a source-route bridge and also as a router
■ Platform: The hardware platform of the device; for example, Cisco 7200 series router
show cdp neighbors
For each CDP neighbor, the following information is displayed:
■ Device ID
■ Local interface
■ Holdtime value, in seconds
■ Device capability code
■ Hardware platform
■ Remote port ID
show cdp entry
■ Neighbor device ID
■ Layer 3 protocol information (for example, IP addresses)
■ Device platform
■ Device capabilities
■ Local interface type and outgoing remote port ID
■ Holdtime value, in seconds
■ Cisco IOS Software type and release
show cdp traffic
■ Syntax error
■ Checksum error
■ Failed encapsulations
■ Out of memory
■ Invalid packets
■ Fragmented packets
■ Number of CDP version 1 packets sent
■ Number of CDP version 2 packets sent
show cdp interface
■ Line and data-link status of the interface
■ Encapsulation type for the interface
■ Frequency at which CDP packets are sent (default is 60 seconds)
■ Holdtime value, in seconds (default is 180 seconds)
Creating a Network Map of the Environment
■ Connectivity
■ Addressing
■ Media types
■ Devices
■ Rack layouts
■ Card assignments
■ Cable routing
■ Cable identification
■ Termination points
■ Power information
■ Circuit identification information
Summary of Discovering Neighbors on the Network
The following list summarizes the key points that were discussed in the previous sections:
■ The CDP is an information-gathering tool used by network administrators to get information about directly connected devices.
■ CDP exchanges hardware and software device information with its directly connected CDP neighbors.
■ CDP on a router can be enabled or disabled as a whole or on a port-by-port basis.
■ The show cdp neighbors command displays information about the CDP neighbors of a router.
■ The show cdp entry, show cdp traffic, and show cdp interface commands display detailed CDP information on a Cisco device.
■ Using the information obtained from the show cdp commands output, a network topology map can be created to aid troubleshooting.
Stages of the Router Power-On Boot Sequence
Step 1 Perform power-on self test (POST). This is a series of hardware tests that verifies whether all components of the Cisco router are functional. During this test, the router also determines what hardware is present. POST executes from microcode resident in the system ROM.
Step 2 Load and run bootstrap code. Bootstrap code is used to perform subsequent events, such as locating the Cisco IOS Software, loading it, and then running it. When the Cisco IOS Software is loaded and running, the bootstrap code is not used until the next time the router is reloaded or power cycled.
Step 3 The bootstrap code determines where the Cisco IOS Software to be run is located. Normally, the Cisco IOS Software image is located in the flash memory. The configuration register and configuration file determine where the Cisco IOS Software images are located and which image file to use.
Step 4 When the bootstrap code has found the proper image, it then loads that image into RAM and starts the Cisco IOS Software. Some routers do not load the Cisco IOS Software image into RAM, but execute it directly from flash memory.
Step 5 The default is to look in NVRAM for a valid saved configuration file, called startup-config.
Step 6 The desired configuration for the router is loaded and executed. If no configuration exists, the router will enter the setup utility or attempt an AutoInstall to look for a configuration file from a TFTP server.
Internal Router Components
■ CPU: This is the processor that runs the Cisco IOS and features like route processing.
■ RAM: This read/write memory contains the software and data structures that enable the router to function. The principal software running in RAM is the Cisco IOS Software image and the running configuration. The RAM also contains the routing tables and packet buffers. RAM is volatile; its memory contents will be lost when power is turned off.
■ ROM: This type of memory contains microcode for basic functions to start and maintain the router, including bootstrap and POST. The ROM contains the ROM Monitor (ROMMON) used for router disaster recovery functions, such as password recovery. The ROM also contains a subset of Cisco IOS, which is used for Cisco IOS image file recovery, such as when the Cisco IOS image file in flash memory is erased.
ROM is nonvolatile; it maintains the memory contents even when the power is off.
■ Flash memory: Flash read/write memory is primarily used to store the Cisco IOS Software image. Some routers run the Cisco IOS Software image directly from flash memory and do not need to transfer it to RAM. Some routers maintain a subset of the Cisco IOS Software in flash memory rather than in ROM. Flash memory is nonvolatile; it maintains the memory contents even when the power is off.
■ NVRAM: This read/write memory is mainly used to store the saved configuration file, called the startup-config file. NVRAM uses a built-in battery to maintain the data when power is removed from the router.
■ Configuration register: The configuration register is used to control how the router boots. The configuration register is part of the NVRAM.
■ Interfaces: Interfaces are the physical connections to the external world for the router and include the following types, among others:
— Ethernet, Fast Ethernet, and Gigabit Ethernet
— Asynchronous and synchronous serial
— Token Ring
— FDDI
— ATM
— Console and auxiliary ports
ROM Functions
■ Bootstrap code: The bootstrap code is used to bring the router up during initialization. It reads the configuration register to determine how to boot, and then, if instructed to do so, loads the Cisco IOS Software.
■ POST: POST is the microcode used to test the basic functionality of the router hardware and determine which components are present.
■ ROMMON: This is a low-level operating system normally used for manufacturing, testing, troubleshooting, and password recovery. In ROMMON mode, the router has no routing or IP capabilities.
NOTE Depending on the specific Cisco router platform, the components listed can be stored in flash memory or in bootstrap memory to allow a field upgrade to later versions.
How a Device Locates and Loads Cisco IOS Image and Configuration Files
The bootstrap code is responsible for locating the Cisco IOS Software. It searches for the image according to the following sequence:
1. The bootstrap code checks the boot field of the configuration register. The boot field is the lower 4 bits of the configuration register and is used to specify how the router boots. These bits can point to flash memory for the Cisco IOS image, the startup-config file (if one exists) for commands that tell the router how to boot, or a remote TFTP server. Or, these bits can specify that no Cisco IOS image is to be loaded and to just start the Cisco IOS subset image in ROM.
For example, a configuration register value of 0x2102 (the 0x indicates that the digits that follow are in hexadecimal notation) has a boot field value of 0x2 (the rightmost digit in the register value is 2 and represents the lower 4 bits of the register). If the boot field value of the configuration register is from 0x2 to 0xF, the bootstrap code parses the startup-config file in NVRAM for the boot system commands that specify the name and location of the Cisco IOS Software image to load. Several boot system commands can be entered in sequence to provide a fault-tolerant boot plan. The boot system command is a global configuration command that allows you to specify the source for the Cisco IOS Software image to load. Some of the syntax options available include the following:
— boot system flash [filename]
— boot system tftp [filename][server-address]
— boot system rom
2. If there are no boot system commands in the configuration, the router defaults to loading the first valid Cisco IOS image in flash memory and running it.
3. If no valid Cisco IOS image is found in flash memory, the router attempts to boot from a network TFTP server using the boot field value as part of the Cisco IOS image file name.
NOTE Booting from a network TFTP server is a seldom-used method of loading a Cisco IOS Software image. Not every router has a boot helper image, so Steps 4 and 5 do not always follow.
4. By default, if booting from a network TFTP server fails after five tries, the router will boot the boot helper image (the Cisco IOS subset) from ROM.
5. If there is no boot helper image or if it is corrupted, the router will boot the ROMMON from ROM.
Configuration Register
The configuration register includes information specifying where to locate the Cisco IOS Software image.
You can change the default configuration register setting with the config-register global configuration command,
The configuration register is a 16-bit register. The lowest 4 bits of the configuration register (bits 3, 2, 1, and 0) form the boot field. A hexadecimal number is used as the argument to set the value of the configuration register. The default value of the configuration register is 0x2102. The boot field would be the last digit in the hexadecimal register. The setting of this field determines how the device boots.
The guidelines for changing the boot field are as follows:
■ The boot field is set to 0 to enter ROMMON mode automatically. This value sets the boot field bits to 0-0-0-0. In the ROMMON mode, the router displays the “>” or “rommon>” prompt, depending on the router processor type. From the ROMMON mode, you can use the boot command to manually boot the router.
■ The boot field is set to 1 to configure the system to boot the Cisco IOS subset automatically from ROM. This value sets the boot field bits to 0-0-0-1. The router displays the Router(boot)> prompt in this mode.
■ The boot field is set to any value from 0x2 to 0xF to configure the system to use the boot system commands in the startup-config file in NVRAM. The default is 0x2. These values set the boot field bits to 0-0-1-0 through 1-1-1-1.
NOTE When using the config-register command, all 16 bits of the configuration register are set. Be careful to modify only the bits that you are trying to change, for example, the boot field, and leave the other bits as they are. Remember that the other configuration register bits perform functions that include selecting the console baud rate and determining whether to use the saved configuration in NVRAM.
Summary of Managing Cisco Router Startup and Configuration
The following list summarizes the key points that were discussed in the previous sections:
■ When a router boots, it performs tests, finds and loads software, finds and loads configurations, and finally runs the software.
■ The major internal components of a router include the CPU, RAM, ROM, flash memory, NVRAM, and the configuration register.
■ When a router boots, it searches for the Cisco IOS Software image in a specific sequence: location specified in the configuration register, flash memory, a TFTP server, and ROM.
■ The configuration register includes boot information specifying where to locate the Cisco IOS Software image. The register can be examined with a show version command, and you can change the register value with the config-register global configuration command.
Cisco IOS File System and Devices
The Cisco IFS feature provides a single interface to all the file systems that a Cisco router uses, including the following:
■ Flash memory file systems
■ Network file systems: TFTP, Remote Copy Protocol (RCP), and FTP (This lesson only discusses commands used to transfer Cisco IOS images and configuration files to and from a TFTP server.)
■ Any other endpoint for reading or writing data (such as NVRAM, the running configuration in RAM, and so on)
Before copying the Cisco IOS image software from flash memory in the router to the network TFTP server, you should follow these steps:
Step 1 Make sure that you have access to the network TFTP server. You can ping the TFTP server to test connectivity.
Step 2 Verify that the TFTP server has sufficient disk space to accommodate the Cisco IOS Software image. Use the show flash command on the router to determine the size of the Cisco IOS image file.
Step 3 Check the filename requirements on the TFTP server. This can differ, depending on whether the server is running Microsoft Windows, UNIX, or another operating system.
Step 4 Create the destination file to receive the upload, if required. This step depends on the network server operating system.
The show flash command, shown previously, is an important tool to gather information about the router memory and image file. The show flash command can determine the following:
■ Total amount of flash memory on the router
■ Amount of flash memory available
■ Name of all the files stored in the flash memory
The name for the Cisco IOS image file contains multiple parts, each with a specific meaning. For example, the filename c2800nm-ipbase-mz.124-5a.bin contains the following information:
■ The first part of the image name identifies the platform on which the image runs. In this example, the platform is c2800.
■ The second part of the name specifies where the image runs and whether the file is compressed. In this example, mz indicates that the file runs from RAM and is compressed.
■ The third part of the name indicates the version number. In this example, the version number is 124-5a.
■ The final part of the name is the file extension. The .bin extension indicates that this is a binary executable file.
Managing Device Configuration Files
Configuration files are stored in the following locations:
■ The running configuration is stored in RAM.
■ The startup configuration is stored in NVRAM.
Configuration files are stored in the following locations:
■ The running configuration is stored in RAM.
■ The startup configuration is stored in NVRAM.
NOTE When a configuration is copied into RAM from any source, the configuration merges with, or overlays, any existing configuration in RAM, rather than overwriting it. New configuration parameters are added, and changes to existing parameters overwrite the old parameters. Configuration commands that exist in RAM for which there is no corresponding command in NVRAM remain unaffected. Copying the running configuration from RAM into the startup-config file in NVRAM will overwrite the startup-config file in NVRAM.
Using show and debug Commands on Cisco Devices
The show and debug commands have the following functions:
■ show: To snapshot problems with interfaces, media, or network performance
■ debug: To check the flow of protocol traffic for problems, protocol bugs, or misconfigurations
Use debug commands to isolate problems, not to monitor normal network operation. Because the high overhead of debug commands can disrupt router operation, debug commands should be used only when looking for specific types of traffic or problems and when those problems have been narrowed to a likely subset of causes.
The following are some considerations when using debug commands:
■ Be aware that the debug commands can generate too much data that is of little use for a specific problem. Normally, knowledge of the protocol or protocols being debugged is required to properly interpret the debug outputs.
■ Because the high CPU overhead of debug commands can disrupt network device operation, debug commands should be used only when looking for specific types of traffic or problems and when those problems have been narrowed to a likely subset of causes.
■ When using the debug troubleshooting tools, be aware that output formats vary with each protocol. Some generate a single line of output per packet, whereas others generate multiple lines of output per packet.
■ Some debug commands generate large amounts of output; others generate only occasional output. Some generate lines of text, and others generate information in field format.
■ Use of debug commands is suggested for obtaining information about network traffic and router status. Use these commands with great care.
■ If you are not sure about the impact of a debug command, check http://www.cisco.com for details or consult with a technical support representative.
Summary of Managing Cisco Devices
The following list summarizes the key points that were discussed in the previous sections:
■ The Cisco IOS File System feature provides a single interface to all the file systems (NVRAM, RAM, TFTP, flash) that a router uses.
■ As a network grows, storage of the Cisco IOS Software and configuration files on a central server enables control of the number and revision level of software images and configuration files that must be maintained.
■ Having proper backup of the current device configuration stored in a TFTP server can help reduce device downtime.
■ The Cisco IOS Software copy commands can be used to move configurations from one component or device to another, such as RAM, NVRAM, or a file server.
■ Network equipment is often moved from one location to another or removed from the network, or the configuration can change. The process is referred to as adds, moves, and changes.
■ The show and debug commands are built-in tools for troubleshooting. The show command is used to display static information, while the debug command is used to display dynamic data.
Chapter Summary
The following list summarizes the key points that were discussed in this chapter:
■ The CDP is an information-gathering tool used by network administrators to get information about directly connected Cisco devices, including the following for each device: device identifier, address list, port identifier, capabilities list, and platform. You can view this information by using the show cdp command
■ When a router boots up, it performs a series of steps, including performing tests, finding and loading the Cisco IOS Software, finding and loading configurations, and running the Cisco IOS Software.
■ The Cisco IFS feature provides a single interface to all the file systems that a router uses. As any network grows, storage of Cisco IOS images and configuration files on a central TFTP server enables control of the number and revision level of Cisco IOS images and configuration files that must be maintained
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