1 00:00:01,130 --> 00:00:07,790 So the first step, the router performs a power-on self-test or POST, then it loads and runs the bootstrap 2 00:00:07,790 --> 00:00:08,270 code, 3 00:00:08,660 --> 00:00:11,030 thirdly, it finds the Cisco iOS software. 4 00:00:11,510 --> 00:00:16,880 So the bootstrap code determines where the Cisco iOS software is that should be run. 5 00:00:17,510 --> 00:00:23,110 Normally this is located in Flash memory but it could also be stored on a TFTP or FTP server. 6 00:00:23,690 --> 00:00:30,350 The configuration register and the configuration file determine where Cisco iOS files are located and 7 00:00:30,350 --> 00:00:35,030 which image file to use. If a Cisco iOS image cannot be located 8 00:00:35,490 --> 00:00:39,410 a scaled-down version of the Cisco iOS is copied from ROM into RAM. 9 00:00:39,920 --> 00:00:43,400 That only applies, however, to older versions of Cisco routers. 10 00:00:44,000 --> 00:00:50,060 In the past, a mini operating system would be used, but today the router will boot into ROM monitor mode 11 00:00:50,060 --> 00:00:56,300 if no Cisco iOS is available. After the bootstrap code is found, the correct image, it loads the image 12 00:00:56,300 --> 00:00:59,330 into RAM and starts the Cisco iOS software. 13 00:00:59,930 --> 00:01:05,480 Older routers used to run the operating system from flash and memory 14 00:01:05,930 --> 00:01:09,760 but today, once again, the entire operating system is loaded into RAM. 15 00:01:10,370 --> 00:01:17,030 Once the operating system is loaded, the router will find the configuration, which is typically stored 16 00:01:17,030 --> 00:01:18,110 in NVRAM. 17 00:01:18,350 --> 00:01:24,830 So to load the startup-config from NVRAM and then apply it to startup-config is loaded in RAM 18 00:01:24,830 --> 00:01:28,190 and becomes the current or running-config on the router. 19 00:01:28,730 --> 00:01:32,150 The running configuration of a router contains the IP addresses. 20 00:01:32,600 --> 00:01:38,150 It determines which routing protocols need to be started, which passwords need to be configured and 21 00:01:38,150 --> 00:01:40,190 other characteristics of the router. 22 00:01:40,670 --> 00:01:47,540 If no configuration file exists, the router enters the setup utility or attempts an auto-install of a 23 00:01:47,540 --> 00:01:53,540 configuration file from a TFTP server. The router then runs the configured iOS software. 24 00:01:54,110 --> 00:01:58,010 Now it's important to remember that Cisco creates the iOS software. 25 00:01:58,160 --> 00:02:04,610 You can't change that, but you can change how it's applied by changing the configuration on a router. 26 00:02:05,030 --> 00:02:07,070 In the same way as with windows, 27 00:02:07,070 --> 00:02:11,120 you can't change the Windows operating system, but you could change your desktop, 28 00:02:11,120 --> 00:02:13,640 you could change the fonts and other options 29 00:02:13,640 --> 00:02:21,290 in Windows. The Cisco iOS software can be manipulated through the configuration, so you may decide 30 00:02:21,290 --> 00:02:27,800 to run the OSPF routing protocol on one router, but use EIGRP on another router. 31 00:02:28,340 --> 00:02:33,920 Your configuration determines the behavior of the Cisco iOS on a router. 32 00:02:35,770 --> 00:02:43,330 On this router as an example, the router does have a startup-config, so show startup-config shows us 33 00:02:43,480 --> 00:02:45,670 the configuration of this router. 34 00:02:46,790 --> 00:02:51,110 On this router we could erase the startup config. 35 00:02:53,700 --> 00:03:04,680 So now the show startup command or show startup-config command shows us that there's no router configuration. 36 00:03:05,310 --> 00:03:06,990 If we reload this router 37 00:03:08,770 --> 00:03:11,290 it will boot with no configuration 38 00:03:13,290 --> 00:03:16,110 and we should be prompted to enter the startup wizard. 39 00:03:18,710 --> 00:03:21,080 On this router, which does have a startup-config, 40 00:03:21,230 --> 00:03:28,700 so once again, shows start when we reload the router, it should boot with the startup configuration 41 00:03:28,700 --> 00:03:31,490 applied as the running-config. 42 00:03:33,140 --> 00:03:35,450 So this router doesn't have a startup config. 43 00:03:36,800 --> 00:03:40,820 Where's this router on the left does have a startup-config. 44 00:03:41,860 --> 00:03:44,650 So we should see the difference when they boot up. 45 00:03:51,860 --> 00:03:58,400 We can see the routers trying to get a configuration from a TFTP server, so it's trying to boot and 46 00:03:58,400 --> 00:03:59,840 retrieve a configuration. 47 00:04:01,390 --> 00:04:04,240 So it's sending a broadcast looking for a network config. 48 00:04:06,600 --> 00:04:08,640 This router is continuing to boot up 49 00:04:11,800 --> 00:04:18,310 and notice, a configuration has been applied, the router has a name, and if we scroll up 50 00:04:21,630 --> 00:04:32,220 we can see that various interfaces came up, but in this case, show run shows us that a configuration 51 00:04:32,220 --> 00:04:33,200 has been applied. 52 00:04:33,480 --> 00:04:40,290 So the configuration was copied from startup to running, whereas in this router the system configuration 53 00:04:40,290 --> 00:04:41,550 dialogue starts up 54 00:04:42,030 --> 00:04:46,440 and I pressed enter a few times and hence it was shown a few times 55 00:04:46,440 --> 00:04:50,100 and then I broke out of the initial configuration dialogue 56 00:04:50,580 --> 00:04:59,370 and notice in this case shows start shows us that there's no startup-config and the router has a default 57 00:04:59,820 --> 00:05:02,080 configuration applied. 58 00:05:02,790 --> 00:05:10,290 Now it did configure an IP address automatically on the serial interface using what's called slop. 59 00:05:12,220 --> 00:05:21,340 That's why we were seeing this error where it was trying to connect to a TFTP server and download a network 60 00:05:21,340 --> 00:05:24,820 config, as well as a Cisco net config file. 61 00:05:27,280 --> 00:05:34,540 On some routers, when there's a serial interface, the router will automatically apply an IP address 62 00:05:34,540 --> 00:05:41,410 based on the neighboring router to this interface and try and pull the config down from that neighboring 63 00:05:41,410 --> 00:05:41,830 router. 64 00:05:42,900 --> 00:05:49,500 This router, however, has a configuration so we can see OSPF has been enabled, there's a loopback 65 00:05:49,500 --> 00:05:54,780 IP address, there's an IP address on the serial interface and any other configuration would have been 66 00:05:54,780 --> 00:06:00,930 applied with this router that doesn't have a startup-config boots up. 67 00:06:01,230 --> 00:06:05,310 No startup-config is applied and it uses a default config 68 00:06:06,300 --> 00:06:13,740 and may do some automatic configuration or use automated methods to try and find a configuration, the 69 00:06:13,740 --> 00:06:15,390 moral of the story is as follows. 70 00:06:15,810 --> 00:06:21,540 If there's no sort of config, the router will typically boot up with some default values. 71 00:06:22,440 --> 00:06:24,840 Such as having most interfaces shut down. 72 00:06:25,930 --> 00:06:31,630 Whereas if a startup-config is available in the router that startup-config will be applied.