Analysis on vro IP Address Settings

Analysis of vro IP Address Settings

WhenVroWhen a packet that does not know the IP address of the router, that is, the router does not know the target route, it will try to send an ARP broadcast for resolution. If a target host responds to this ARP broadcast, the router forwards the data packet to the target host.

If the router does not receive a response, it will send an ARP request for the next four packets. If the 6th packets arrive, the MAC address of the target host has not been resolved, by default, the vro will discard 6th and subsequent data packets in the next 20 seconds, and return the ICMP message "the host is not reachable" to the source host.

It can also be proved from the first sentence in the record of computer B that the router sends an ARP query to the CIDR Block and finds the computer whose IP address is 222.222.222.191, the router considers that there is no target host in the CIDR block. Therefore, an ICMP message is returned to the source computer indicating that the target host cannot be reached to notify the source host that there is a problem and the original data packet is discarded. Now the problem is clear. The ICMP logs recorded by the router are the "Destination Unreachable" information sent by the router to the source address. So why are these vro IP addresses looking for computers on campus? From the collected data analysis, it is not difficult to find that these external hosts are mainly looking for three internal fixed computers. After checking the history logs, we can find the same records of the three computers:

 
 

  
  
07:52:19 Jul 09 07:50:02 %ACL_LOG-I-PERMIT, ACL [out]   
  
  
on "uplink" TCP 222.222.222.136:3159 -> 61.173.209.101:6881   
  
  
08:00:15 Jul 09 07:53:50 %ACL_LOG-I-DENY, ACL [out]   
  
  
on "uplink" TCP 222.222.222.12:3194 -> 219.121.133.197:6883   
  
  
08:00:15 Jul 09 07:57:59 %ACL_LOG-I-DENY, ACL [out]   
  
  
on "uplink" TCP 222.222.222.220:3196 -> 210.238.6.177:6881  
 
 

The ports connecting the three hosts to the target host are fixed between 6881 and 6889, and these ports are common ports for BT download. It is no wonder that such logs have not appeared before until BT became popular recently. The main reason is that when these hosts use BT for download, a record is left on the BT server so that other hosts can download resources to these hosts. When these hosts are shut down, the router tells them they cannot find these hosts.

Because the Log service records information above Layer 3, and the packets received by the router are discarded on Layer 2, the exception packets of these inputs are not recorded in the log. To reduce the log volume of the vro, use "ip disable icmp-messages destination-unreachables" in configuration mode to disable the forwarding of such information. Both of these faults are caused by ICMP. In a sense, they are not caused by system configuration problems, but by external factors. This type of fault can be identified only after some analysis, and then configured accordingly.

Router IP Address Settings troubleshooting

First, use the dedicated CONSOLE cable of the router to connect the COM port of the notebook with the CONSOLE port of the router, and enable the Super Terminal to establish a connection with the router on the notebook. After the power of the router is connected, you can see the following loading information on the Super Terminal:

 
 

  
  
Processor: R5000 rev 2.1 [0x2321], 198 MHz, (bus: 66 MHz), 64 MB DRAM   
  
  
I-Cache 32 KB, linesize 32. D-Cache 32 KB, linesize 32  
  
  
L2-Cache 512 KB, linesize 32, cache enable  
  
  
Mounting 8MB  Linear external flash card …Done  
  
  
initializing system …….Failure  
  
  
Autoboot in 2 seconds – press ESC to abort and enter prom. 
 
 

The last statement indicates that the system initialization fails. After you see "Autoboot in 2 seconds-press ESC to abort and enter prom.", press the "ESC" key to enter the start mode. By default, the IOS system image file in the flash card is booted when the router is started. If the IOS file is lost, the system initialization fails. To successfully boot the system, you must download the system image file again. We can use the following steps to restore the lost IOS:

Set the vro IP address of the laptop and the IP address of the vro Ethernet Management port to the same subnet. For example, set the IP address of your laptop to 10.10.10.2 and mask to 255.255.255.0. And connect the NIC of the computer with the en0 of SSR with the crossover line. The Analysis of router IP address problems is over. You should know more about it.