Ali Abbas

Every cisco routers has a configuration register which is saved in NVRAM and is a 16 bit value.

This post will not tackle all the 16 bits of the configuration register, but only the 13th bit which is used to either load IOS or ROMMON. Another post will be made to detail all the 16 bit configuration register.

Before continuing, it is important to understand the basic “boot process” of a router. When you power a router on, it first performs a POST, then loads the bootstrap program from ROM to RAM, which in return loads the appropriate IOS (bootstrap can load an IOS from tftp)/ROMMON or RXBOOT. Once loaded, the bootstrap program gives the hand to the IOS to handle the commands from there on.

The most important bit is the low order bit which is (2 for IOS, 1 for Rxboot and 0 for ROMMON)

Please note that by default, the low order boot bit is 2 thus 0x 2102

Example:

home-booth(config)#conf t
home-booth(config)#config-register 0×2100
home-booth(config)#do wr
Building configuration…
[OK]
home-booth(config)#do reload

Proceed with reload? [confirm]

(… a bit later)

%SYS-5-RELOAD: Reload requested by console. Reload Reason: Reload Command.
System Bootstrap, Version 12.3(8r)T8, RELEASE SOFTWARE (fc1)
Cisco 1841 (revision 5.0) with 114688K/16384K bytes of memory.

rommon 1 > System Bootstrap, Version 12.3(8r)T8, RELEASE SOFTWARE (fc1)
Cisco 1841 (revision 5.0) with 114688K/16384K bytes of memory.

rommon 1 >
rommon 1 > confreg 0×2102
rommon 2 >

What could ROMMON be useful for? Simple… restoring a router with a corrupted/broken IOS image!

Configuring Cisco IOS shortcuts command are quite easy and neat… just use the exec command “alias”.

command structure: alias <mode> <shortcut> <command>

Example in exec mode: sh ip int br (show ip interface brief)

so following, we would get “alias exec s sh ip int br”

Example in configure mode: router ospf

and we would get “alias configure ro router ospf”

More mixed examples [ alias / original command / command to enter ]

shr / show run | inc ip  / alias exec shr sh run | inc ip

et0 / interface fa0/0 / alias configure et0 interface fa0/0

reload /  sh run / alias exec reload sh run <— rewrite IOS command

I hope that was informative!

Hello there,

Just a quick overview over a couple of cisco hidden command useful in certain circumstances

1. ttcp

ttcp is only available on routers/layer 3 switches (of course )

For those familiar with iperf, ttcp is a kinda”ish” iperf

Start ttcp on one router in point 2 point scenario and ttcp on the other endpoint. One router will be in receiving mode, while the other will be in sending mode… running the command is quite straight forward, thus I will not demonstrate it here.

2. test crash

Ok this used to be hidden, but isn’t in newer IOS version. Test crash allows to simulate a crash.. so you came to the office one morning and found your switch dead… not log, not knowing what happened. test crash can help you dig into the problem by simulating the crash

3. test transmit

This is a very cool feature (ah I love cisco ). You can simulate traffic to interfaces, using specific encapsulations and protocols…

4. ip route profile and show ip route profile

Used in conjonction, this “debug” command enables you to see route fluctuation.

5. csim start “number”

This emulates a voice call to the specified number… practical for VoIP testing environment.

6. show controller switch

Need to get a quick overview on your switch traffic stats. Suspect a congestion network? show controller switch will give you the quick heads up on where you stand

7. debug ip osfp monitor

This command enables you to monitor the OSPF SPF process

8. ip-con 2 con

Available on cisco 7500… it enables you to enter the VIP console to execute VIP commands on VIP boards

9.  ip osfp interface retry <X>

Back to OSPF (the coolest routing protocol), by default OSPF pools retry on the interface 10 times… having too much fluctuation of states on the interface of your router? increase the pooling period.

10. show region

Displays how the memory is partitioned on the device

If you dream of Cisco Routers and Switches like me

If you can’t help it but swear by Cisco (beside their load balancers, let’s be honest it sucks)

If you tried to convince your wife to have each room of the house on a different subnet

then you need to download this … http://alouche.net/cisco/data-center-poster.pdf

**** the following poster is provided and copyrighted freely by Cisco Inc…