Using a switch to illustrate a method and technique for locating a faulty network node
1. Use the fault log of the switch as first-hand material
The premise of judging network failure is to have a clear understanding of the topology of the network, but when troubleshooting a relatively unfamiliar network, familiarity with the network is an asymptotic process. If problems are quickly discovered, the failure is judged Click to view the log information on the switch is an effective method. The unit's networking mainly uses Huawei 3500 and H3C3100 switches, both of which support log viewing, which provides us with a convenient location to quickly locate the cause of the failure. Log in to the Huawei 3500 switch and see a lot of information as shown below:
% Nov 11 07:50:10 2010 rcq_3526 SYSM / 5 / IP MOVE: Rcv src IP packet from port 3 but it`` ''s nexthop arp 219. *. *. 72 with 000f-e23f-3180 resided in port 26
Through the analysis of the above information, we can roughly conclude that there is a network device with an IP address of 219. *. *. 72 that carries out ARP address spoofing attacks on port 3 of the Huawei 3500 switch. The next thing we do is to judge In which VLAN this IP address is located, and further located on which port of this switch this IP address is located, through the dis cu command to find, the following useful information appears:
vlan 101
descripTIon link to *****
interface Vlan-interface101
descripTIon link to *****
ip address 219. *. *. 65 255.255.255.240
Through the description information displayed on the switch, we can determine that this IP address belongs to VLAN101, and the description information can also roughly determine that this VLAN is the connected unit information, but as a network administrator, we also know that many of the description information on the switch It was written during the initial configuration. In many cases, the network connection was changed but the description information was not modified in time. Therefore, what we need to do below is to verify whether the IP address and the gateway belong to the same network segment. The network manager of this unit has told us that he is used to using the smallest address in the network as the gateway address. According to this rule, the subnet mask of 219. *. *. 72 and 219. *. *. 65 is 255.255. In the case of 255.240, it should belong to a network segment, but in order to make this problem clear, we still introduce the method of judgment in detail.
2. Fully understand and flexibly apply the relationship between IP address, subnet mask and gateway
Taking the above example as an example, the subnet mask is 255.255.255.240, indicating that only the fourth bit of the IP address in this network segment is different, then we first convert the 248 in the subnet mask to a binary number, and then Then convert 65 and 72 into binary numbers, and then perform an AND operation with the subnet mask, as shown below:
Decimal number binary number
6501000001
7201001000
24011110000
The result of the AND operation of 65 and the subnet mask 01000000
72 and the subnet mask AND operation result 01000000
It can be seen from the displayed results that the results of the AND operation of 65 and 72 and the subnet mask are the same, which means that 219. *. *. 65 and 219. *. *. 72 belong to the same network segment . Knowledge of IP addresses, subnet masks, and gateways is the basic knowledge we learn from computer network knowledge, but skilled and flexible applications are essential for us to judge network failures. If the IP address in the network cannot be correctly judged by the subnet mask, the IP address that does not belong to this network segment will be set to the corresponding port, causing the problem of network disconnection. Fortunately, this does not appear in this example. phenomenon.
3. Ingenious use of IP address conflicts to find the node where the faulty network is located
Then through dis cu to view the configuration information, we found that port 3 is set to TRUNK mode, the port allows multiple VLANs including VLAN101 to pass. As follows:
interface Ethernet0 / 3
port link-type trunk
port trunk permit vlan 101 109
An H3C3100 switch is connected under port 3, and there are multiple ports belonging to VLAN101, but at the same time because the description information is imperfect, we cannot know for a moment which network device address under which port is 219. *. *. 72. It is a solution to go to the site where the H3C switch is placed for inspection, but it saves time and effort, we take another method, that is, on the superior Huawei 3500 switch, we set an empty port to belong to VLAN101, and put our own laptop The IP address is set to 219. *. *. 72, then you will immediately see the IP address conflict prompt in the "Event Viewer".
At this time, we log in to the H3C3100 switch again, shut down all three ports belonging to VLAN101, and then perform the undo shutdown operation in turn. Each time the undo shutdown operation is performed, plug and unplug the network cable on our laptop. There is also a prompt of IP address conflict, which means that there is an IP address of 219. *. *. 72 in the network connected to the port. In fact, we are very lucky to find the faulty network node when trying the first port , Reset the network port to the shutdown state, set the other two ports to the undo shutdown state, and then log in to the Huawei 3500 switch, and find that no new alarm information has been generated. Ask the units connected under the switch to There is no packet loss.
Although there were some twists and turns in the troubleshooting of this network fault, it was finally solved smoothly. Especially when looking for the network node that caused the fault, the method of setting the IP address conflict was used to solve the network fault by generating the network fault. This is also a living learning and application of network knowledge.
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