Network enchanting phenomena encountered at work and Their Applications

In actual work, we occasionally encounter some very strange phenomena: inexplicable, strange, and interesting. Here are several examples of wheat:
Basic enchanting Phenomenon
A vro uses the command erase startup-config or write erase to delete the startup configuration. Run the command show startup-config to check whether the configuration has been deleted. However, after the device is powered down and restarted, the system still has a working configuration: The Host Name, IP address, and route configuration are all in.
Cause: the system Flash has an older configuration backup named router-confg. If the startup configuration does not exist when the vro starts, it will execute the AUTOINSTALL process and try to obtain its configuration through the network. The result is that the configuration file in Flash is loaded. For detailed procedures, see the Autoinstall document.
Application: this function can be used in the Lab for quick Lab Setup, such as configuring hostname, password, alias, placing the port in the UP state, and configuring the ip domain-lookup environment. If you are doing a BGP experiment, you can even use this configuration to establish full-network connectivity, and then create a tunnel port to implement the experiment topology. During remote configuration, you can save multiple configuration files and switch between them. The configuration file named router-confg can be used as a final backup-if you accidentally delete the configuration, there will still be an available configuration after the system restarts.
Conclusion: The initial enchanting phenomenon is caused by insufficient understanding of the functions of the device, which can be eliminated by reading documents and practices.
Intermediate enchanting Phenomenon
The distance between the two 3550 vswitches is estimated to be around 9KM. 5486 (lx gbic) is used on both sides. If it is suspected that the actual distance is over 10 KM, it cannot be changed to a pair of 5487 (zx gbic). One end uses 5486, use 5487 on the other end!
Cause: the actual distance between the two ends may exceed the working distance of GBIC. The attenuation on the optical fiber may be insufficient if the two ends use 5486, manual increase is required (10-DB should be added below 25KM, and 5-DB should be added for 25-50KM ). However, this cannot be explained at the end, because 5486 and 5487 use different laser wavelengths. According to the document, 5486 uses a laser with a wavelength of 1310nm and 5487 uses a wavelength of 1550nm. Will Cisco have changed the product specifications but not updated the documentation?
Application: It is recommended that you do not connect to the device using this method before Cisco makes an explanation.
Conclusion: It is difficult to analyze the phenomenon of intermediate monsters. manufacturers need to cooperate or inspect and analyze the use of professional equipment, such as testing the actual wavelength of light waves of 5486 and 5487, and detecting the line distance and the situation of continuous equipment.
Here we will send you a Tips. If you forget to configure the password of the 3500 switch and have installed it on the remote end, how can you set the device? Telnet reports the following error: Password required, but none set. In this case, you can use the Cluster management method to control the device. First, find an adjacent device that can be managed, such as 3550/3500/2900, and perform the Cluster Command Switch. The global configuration Command (take 3550 as an example): cluster enable lab. After the configuration is complete, run the show cluster candidates command to find the MAC address of the vswitch without a password, and then run the configuration command: cluster member 2 mac-address 0050.50df.5040 to add the vswitch to the Cluster as member 2, then you can use rcommand 2 to configure the target switch. This method is effective for vswitches such as 3550/3500, 2900XL, and 1900.
This Tips tells us that no one can remotely control the switch without setting the password of the virtual terminal. (There is another possibility: remote management can be performed through http as long as the vswitch is configured with an IP address and no password is configured)
Advanced enchanting Phenomenon

A 8540 vswitch runs the OSPF routing protocol with other trunk switches and learns the OSPF route: 172.16.0.0/12. It also uses the Static Routing Protocol to direct to the edge router, for example, ip route 172.28.128.0 255.255.255.0 192.168.1.1. The two route records are displayed in the routing table at the same time. The switch itself can be pinged to a host in the CIDR Block 172.28.128.0, but not to the PC connected to the switch. Trace indicates that the packets sent by the PC are forwarded using records 172.16.0.0/12 on 8540!
Cause: it may be a software BUG.
Application: it is difficult to apply; application is BT.
Conclusion: if this problem cannot be circumvented, contact Cisco to open a CASE. This enchanting phenomenon is not met by everyone, and not by everyone.

Concerning the 8540 non-static route packet forwarding problem, we have requested support from cisco tac and finally identified it as a software BUG. The internal BUG_ID is CSC03111802. We recommend that you avoid using the CPU port ATM0 (ATM2/0/0) of the vswitch to connect to the PVC. Instead, you can use the ARM module to do this.