Managing a Cisco Internetwork

- Cisco Router Components -- Bootstrap, POST, ROM Monitor (all in ROM); Mini-IOS (RXBOOT, or bootloader), RAM, ROM, Flash, NVRAM, Configuration register.
- Cisco Router Boot Sequence -- POST; Bootstrap loads the IOS; the IOS loads a configuration ("startup-config", if present in NVRAM).
- Configuration register -- 16 bits, read 15-0 from left to right. Default is 0x2102 = 0010 0001 0000 0010 -- load IOS from flash and look for "startup-config".
- Configuration register common values in bits (and hex):
- 0-3 -- 0x0000-0x000F -- Boot field, as follows:
- 00 -- ROM monitor mode -- register = 2100. You must manually boot the router with the "b" command.
- 01 -- Boot image from ROM -- register = 2101.
- 02-F -- Specifies a default boot filename.
- 6 -- 0x0040 -- ignore NVRAM contents.
- 7 -- 0x0080 -- OEM bit enabled.
- 8 -- 0x0100 -- Break disabled.
- 10 -- 0x0400 -- IP broadcast with all zeros.
- 11-12 -- 0x0800-0x1000 -- console line speed.
- 13 -- 0x2000 -- Boot default ROM software if network boot fails.
- 14 -- 0x4000 -- IP broadcasts do not have net numbers.
- 15 -- 0x8000 -- Enable diagnostic messages and ignore NVM contents.
- "sh version" shows the current value of the configuration register.
- Change the config register (run this from "config t" mode) -- "config-register 0x0101" -- Boot into ROM mode and then show the current config register value.
- Recovering Passwords:
- Perform a "break" while the router boots -- you should see "rommon 1 >".
- "confreg 0x2142" -- turns on bit 6 -- ignore NVRAM contents. Note: On a Cisco 2500 router, type "o" after the break, then "o/r 0x2142".
! To boot from a different file, use "boot system <ios_name>", or "boot system tftp <ios_name> <ip_address>".
- Reboot the router -- "reset" ("I" on a 2500 -- meaning "initialize").
- Fix password and reboot -- "copy start run", "config t", "enable secret <new_pass>", "config-register 0x2102", Ctrl-Z, "copy run start", "reload".
- Back up and Restore the Cisco IOS:
- Check the flash and the TFTP server -- "sh flash", "ping 192.168.1.4".
- Backup the IOS image -- "copy flash tftp".
- Restore of Upgrade the Cisco Router IOS -- "copy tftp flash" (you may be asked to erase existing flash data). You can also erase it manually with "erase flash:".
- Set up a Cisco router to be a TFTP server -- "config t", "tftp-server flash:."
- Back up or Restore the Cisco Configuration -- "copy run tftp", "copy tftp run". Erase the startup config -- "erase start".
- Cisco Discovery Protocol (CDP) -- collect hardware and protocol information about neighbor devices. "sh cdp" shows CDP values:
- CDP timer -- how often CDP packets are transmitted to all active interfaces (default 60 seconds) -- "config t", "cdp timer 90".
- CDP holdtime -- how long to hold packets received from neighbor devices (default 180 seconds) -- "config t", "cdp holdtime 240".
- "sh cdp nei" (or "show cdp neighbor") shows CDP information ONLY about directly connected devices. "sh cdp neighbor detail" = "sh cdp entry *" BOTH display extended information ("sh cdp entry *" does NOT work on a 1900 switch).
- "sh cdp traffic" shows CDP traffic summary. "sh cdp interface" shows interface CDP information.
- CDP is enabled by default. Disable globally -- "no cdp run". To disable per interface -- "config t", "int s0", "no cdp enable".
- "telnet 192.168.2.100" = "192.168.2.100" -- "telnet" can be skipped, it is assumed.
- Connect via telnet to several routers simultaneously -- "telnet 192.168.1.100", "Ctrl-Shift-6 then x" (suspend), "telnet 192.168.2.100".
- "sh sessions" shows telnet connections from your router to another device. "sh users" shows telnet sessions in your router.
- "disconnect <ID>" disconnects a session. "clear line <ID>" disconnects a telnet session in your router.
- Use a host table to resolve addresses -- "config t", "ip host myrouter 192.168.1.100". Then you can telnet via "telnet myrouter", or just "myrouter". Remove with "no ip host myrouter".
- Using a DNS server for name resolution -- "config t", "ip domain-lookup", "ip name-server 192.168.1.4", "ip domain-name mydomain.com".
- "sh hosts" displays the host table, or DNS information, if set up.
- "ping" and "traceroute" can be used from user mode and enable mode, but not from config mode.
- If you try to run traceroute with IPX or AppleTalk, you will get an error. Those two are not supported yet