Use commands to change the route start sequence

In a single machine, the network administrator can set the startup sequence of the PC, such as starting from the hard disk, or from the disc and network drive. In fact, you can also change the startup sequence in the vro. Sometimes, it is very important to change the router startup sequence when troubleshooting router startup errors.

Here we will introduce a command to quickly change the Boot sequence of a vro. With this command, the network administrator can change the router startup sequence as needed.

Before talking about this topic, I will introduce the self-check process of the router. Knowing this process can help you understand the basic ideas and importance of changing the router startup sequence.

Vro startup self-check process

When a vro is powered on and started, like a normal PC, the system performs a series of steps to test whether the vro-related hardware works properly and load the software required for running the vro. Vro officially referred to this series of steps as the startup sequence. Generally, there are four steps to start a vro.

Step 1: perform self-check on the vro

When a Cisco router is powered on and started, the router first executes the power-on self-check. The self-check will check the relevant hardware of the router to verify whether all the components of the device are currently operational and whether there are some hardware faults. If the self-check function is enabled, different interfaces of the vro are checked to check the working status of the vro. The boot self-check program is stored in read-only memory and runs from read-only memory.

Step 2: search for and load the operating software

If the self-check succeeds, the BootStrap program will find the available IOS (Cisco network operating system) software and load it. This program is located in read-only memory and is mainly used to execute some preparation jobs before the vro operating system runs. The BootStrap program is responsible for finding the location of each Cisco network operating system in the network, and then loading it. In general, this application looks for Cisco Network Operation pain points from flash memory, TFTP server, ROM and other locations in turn. That is to say, by default, Cisco routers load the Cisco Router network operating system from flash memory. However, if there are some problems with flash memory, the network administrator will have to change this order to enable the router from the TFTP server or ROM, so that our network administrator can find the problem of the router.

This is like a normal PC. By default, it is started from the hard disk. However, when there is a problem with the hard disk boot program, we sometimes need to start from the CD. Find and modify the problem, or back up some important files to prevent loss. This is similar to the router operation principle.

Step 3: Find the vro configuration file

After the vro network operating system is loaded smoothly, it will find a valid configuration file in NVRAM. NVRAM is short for non-volatile RAM. It is mainly used to save configurations related to vrouters and vswitches. After a vro or vswitch is reloaded, the content in non-volatile RAM is not deleted. For example, the configuration register is stored here. To enable the vro operating system properly, you must find a configuration file for Startup-config. This file is generated only when the Administrator copies the Running-Config file to non-volatile RAM. However, this is also an exception. In some new ISR routers, a small Startup-config file is loaded in advance by default.

Step 4: Apply related configurations

If the startup configuration file exists in non-volatile RAM, the vro network operating system copies the file to RAM and calls the Runing-config file. This file stores vro configuration parameters. The router operating system operates according to these configuration parameters. After the vro completes this operation, the vro starts successfully and you can perform network operations.

However, if the startup configuration file is not available in non-volatile RAM, the vro network operating system will send a broadcast to all the interfaces for carrier detection. The purpose is to find available TFTP hosts for relevant configuration. If the vro still cannot find the available Startup-config file after broadcasting, the vro will start the Setting Mode and ask the network administrator to re-configure it.

Therefore, in daily work, it is very important to back up these configurations. In this case, when a problem occurs in the configuration file, you can immediately start the router through the configuration file on the TFTP host.

In the above four startup steps, when necessary, our network administrator mainly intervened in the second step. In other words, you need to adjust the vro startup sequence so that it does not start network operating system software from flash memory, but starts from the TFTP host. When flash memory problems occur, this will be the only way to bring the router back to life.

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