Packet loss, as its name implies, refers to the loss of communication packets in communication. Below, I have compiled articles on how to deal with serious packet loss in routers. Let's see:

Some people in the department (hereinafter referred to as department A) reported that they could not access the network of the superior department (department B). I ping this person's computer A 1(IP address is10.20.12.1/24) to the computer B 1(IP address is/kloc-0) And B2(IP address is10.20.30.11/24), and the packet loss rate reaches 100%. When you ping other computers in the department, the connection is normal. Check the IP settings of his computer, and find that the gateway settings are correct (the correct gateway is10.20.12.1). It is suspected that the router cannot work normally. Using computers A2 and A3 in our office, we found the following strange phenomena:

(1) On the computer A2 (IP:10.20.12/24), use the ping command to ping the computers B 1 and B2 in the network B, and find that B 1 has a normal response, and the response rate of B2 is 20%. In computer A3(IP is10.20.12.13/24), ping computers B 1 and B2 respectively, the response shows that the connection is normal, and the packet loss rate is 0.

(2) Change the IP address of computer A3 to 1 0.20.12.12/24, and the ping result is the same as that of computer A2 in (1).

(3) Change the IP address of computer A2 to an unused IP address: 10.20. 12.22/24, and test the connection with B 1 and B2. The results show that it is normal and the packet loss rate is 0.

(4) Change the IP address of computer A 1 to an unused IP address: 10.20. 12.23/24, and test the connection with computer B 1 and B2. The results show that it is normal and the packet loss rate is 0.

Fault analysis

Considering that the network connectivity is related to the local IP address, referring to the network topology diagram of our unit, it is suspected that the fault may be caused by the firewall setting of network B. Because department B and this department are in two different places in the city, it is not convenient to check their firewall settings, so I called to ask. But the network administrator of the department told him that the firewall is set for the IP segment of the network, that is to say, the firewall is accessible to the entire IP segment of Department A.

Analyzing the above reasons, it may be that some computers in this department sent illegal access information to the network of department B, and the firewall of department B automatically intruded the IP column into the computer list, shielding the IP packets sent by it, which led to abnormal network connection. Check all the computers in this department, and replace the IP address of the computer that can't access the external network (Department B network) with a new IP address to solve the problem temporarily.

However, a week later, the network broke down again, and several computers that could normally access Department B lost packets, while several IP addresses that could not normally access the external network could normally access the external network. It seems that the problem may not be the firewall.

On computer A2(IP address is10.20.12.12/24), use Tracert command to track the test, and the phenomenon is as follows:

c:\ " Tracert 10 . 20 . 30 . 1 10

Trace the route to10.20.30.110 with a maximum of 30 hops.

1 1 millisecond1millisecond1millisecond10.20.12.1

2 1 ms 1 ms 1 ms192.168.10.2

3 1 ms 1 ms 1 ms 10.20.30. 1

42ms 1 ms 2ms10.20.30.110

Tracking completed

Change the IP address of A2 on the computer to 10.20. 12.22/24, and trace the test with the Tracert command. The phenomenon is as follows:

Trace the route to10.20.30.110 with a maximum of 30 hops.

1 1 millisecond1millisecond1millisecond10.20.12.1

2 1 ms * * 192. 168. 10.2

3 * 2ms * 10.20.30. 1

4 * * * The request timed out.

5 * * * Request timed out.

6 * * 2 ms 10 . 20 . 30 . 1 10

Tracking completed

Tracking completed

?

Sometimes, the fourth and fifth lines obtained by Tracert trace test and all subsequent show requests will time out.

Looking back at the recent changes in network structure, it turns out that the gateway of this department uses a three-layer switch (only using its routing function). At that time, the internal network could access the external network normally. I switched to Cisco 3640 router a week ago, and the 3640 router will be reconfigured according to the configuration in the Layer 3 switch. The above network failure occurred shortly after handover. Check the port configuration and routing configuration of Cisco 3640, and find that the port IP address configuration is normal, but there are two routes in the routing table:

The ip route is 0.0.0.0.0.0192.168.10.2.

The ip route is 0.0.0.0.0.010.20.12.254.

Because the departmental network is the lowest subnet in the company's WAN, we can access the whole WAN according to business needs, but the departmental network is only connected to the departmental B network through a Cisco 3640 router, and then connected to the company's WAN through the departmental B router (see the network topology diagram above). For easy setup, we just need to add a default route to Cisco 3640? Ip route 0.0.0.0.0.0192.168.10.2? You can forward your department's access to the external network to router B of department B, so as to realize the interconnection of the network. Perhaps due to the error of the setup personnel, a nonexistent next hop node was added to the routing table? Ip route 0.0.0.0.0.010.20.12.254? . When computers in the internal network access the external network, the router sometimes fails to send IP packets to192.168.10.2 correctly. After the router is powered on, the internal network access routing table is stored in memory, so that some computers can always access the external network normally, while others cannot. When the router restarts (for example, the router restarts the next Monday morning).

trouble clearing

Delete useless routing tables in router privileged mode:

# There is no ip route 0.0.0.0.010.20.12.254.

#wr

After restarting the router, all computers in this department can access the network of Department B and the unit WAN normally, and the fault is completely solved.

A table stored on a router or other Internet device, which contains paths to specific network terminals and, in some cases, some metrics related to these paths.

The general form of routing table is:

The destination gateway flag refers to the use interface.

Where the IP address of the destination network or host; Use the gateway to the gateway of the specified destination; Flag flag field, which describes some characteristics of the route; Refcnt indicates the number of times the route was used to establish a connection; Use indicates the number of packet messages transmitted through this route; Interface indicates the name of the network interface used by this route.

Examples of typical routing tables

The target gateway flag refers to the use interface.

The default is 26.112.191.98ugs00net0.

The default is 26.112.191.98ugs018net0.

26. 1 12. 19 1 26. 1 12. 19 1.2 UC 1 0 net 0

26. 1 12. 19 1.2

127 . 0 . 0 . 1 127 . 0 . 0 . 1 UH 3 9786480 lo0

224 26. 1 12. 19 1.2 UCS 0 0 net 0

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