Router Architecture
Router architecture refers to the components of the router that are used for the booting process. You can back up and manage these components using the Cisco IOS. Remove the cover on a router and you will see its working parts such as memory banks and flash memory cards.
Types Of Router Memory
- Boot ROM: This memory contains the EEPROM for storing the startup diagram and Rommon and is responsible for loading the IOS. If loading the IOS fails for any reason, it will load a backup emergency mode called Rommon, also called bootstrap mode. Rommon has limited functionality meant for recovering the IOS if possible and reloading another IOS. The prompt for Rommon looks like:
>
Rommon>
- NVRAM: This type of memory stores the router’s startup configuration file, which contains the saved configuration of the router. NVRAM also contains the configuration register, which contains instructions for bootup and can be used for password recovery or handling tasks on routers that have no configuration. The NVRAM is not erased when the router has to reboot or reload. You can modify the startup configuration file by making changes to the running configuration and then using the copy running-config startup-config command.
- Flash/PCMCIA: This is the EEPROM that contains the Cisco IOS in a compressed form and some of the configuration files. If the EEPROM is large enough, it may have more than one version of the IOS.
- DRAM: This is the primary, or temporary, memory of the router that works much like RAM on a laptop or desktop computer. It can store the full IOS, running configuration, and routing tables. It is erased if the router is rebooted, so if you make any changes on the running configuration file and don’t want to lose it, use the copy running-config startup-config command to save it. If you need to manually pull the startup configuration into DRAM, you use the copy startup-config running-config command.
- ROM Monitor: This contains a small code that checks for attached memory and interfaces and can perform limited system diagnostics and startup tasks.
- RxBoot: A kind of mini-IOS that is loaded before the full IOS is loaded. It is also known as the boot loader and can handle some router maintenance activities.
- You can view information about items like the current configuration register setting, internal hardware and attached interfaces by typing in commands like show version and show version Cisco Internetwork.
Managing The IOS
The router configuration should be saved when you have the running configuration set to your preferences by using copy running-config startup-config and should also be backed up whenever possible. This will help you recover more quickly when your network has a meltdown. A server or PC should be configured with TFTP server software that can be downloaded from sites like Solarwinds and can be used to back up your router configuration by using the command copy startup-config tftp: (don’t forget the colon). Before upgrading your router’s IOS, it is a good idea to back it up by using the command copy flash tftp: in case the new IOS causes issues with your router. You can use commands like copy tftp startup-config: to retrieve the startup configuration from the TFTP server or copy tftp flash: to retrieve the contents of the flash memory. It is also a good idea to have printed-out checklists and documentation of your router configuration settings that you can access if your TFTP server goes out with the rest of your network.
Boot Options
Under normal circumstances, the router will boot using the IOS it has on file. However, the IOS may be too big for the flash memory, in which case the router should boot from a TFTP server. To boot from TFTP, use these commands:
Router(config)# boot system tftp: <The boot file; should end with a .bin extension> ? <Hostname or IP address containing the file> <cr>
Router(config)# boot system tftp:
To boot from the router’s flash memory, use the command Router(config)# boot system flash ? WORD System image filename <cr>.
Simply typing in boot system ? should show you the current boot options that the router uses when it boots up.
Router Parts on eBay
It is possible to replace defective parts in a router, but be careful. Besides the possibility of voiding any existing warranties, installing a part the wrong way could do more damage to a router.
