Example: solve the serious packet loss problem of the Cisco Router

This article mainly uses an example to explain in detail how to solve the packet loss problem caused by CISCO router faults? The following article gives me a detailed answer.

Recently, some people in my department (Department A) reported that they could not access the network of the higher-level department (Department B. I use the ping command from the computer A1 (IP Address: 10.20.12.11/24) used by this person to send the ping command to the computer B1 (IP Address: 10.20.30.110/24) in department B's network) and B2 (IP Address: 10.20.30.111/24) Send test data packets, packet loss rate up to 100%, ping other computers in the Department shows that the connection is normal. Check the IP settings of the computer and find that the gateway is correctly set (the correct gateway is 10.20.12.1). Therefore, it is suspected that the router cannot work normally. When I came back to my office, I used my computer A2 and A3 to test and found the following strange phenomena:
(1) run the ping command on computer A2 (IP Address: 10.20.12.12/24) to ping the computers B1 and B2 in department B's network respectively. Then, it is found that B1 has a normal response, and the response rate of B2 is between 20% and 24 ~ In computer A3 (IP: 10.20.12.13/24), ping the computer B1 and B2 by using the ping command. The response shows that the connection is normal and the packet loss rate is 0.
(2) Change the IP address of computer A3 to 10.20.12.12/24 before testing. 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. Test the connection with B1 and B2. The result shows normal and the packet loss rate is 0.
(4) change the IP address of computer A1 to an unused IP Address: 10.20.12.23/24. Test the connection with B1 and B2. The result shows normal and the packet loss rate is 0.

Analysis of Cisco router troubleshooting

Considering that network connectivity is related to the IP address of the local machine, it is suspected that the fault may be caused by the firewall settings of department B's network. Because department B and department B are located in two different places in the city, it is not convenient to view the firewall settings, so I called to ask. However, the Department's network administrator told him that the firewall was set for the network IP segment, that is, the firewall allowed access to the entire network IP segment of Department.

The reason may be that some computers in this department have sent illegal access information to department B's network, and department B's firewall automatically intrude this IP address into the computer list, the IP packet sent by WAF is blocked, leading to abnormal network connection. Investigate all computers in the Department and change the IP addresses of computers that cannot normally access the external network (Department B network) to the new IP address.

However, a week later, the Network went down again, and packet loss occurred to several computers in department B that could have been normally accessed, however, some IP addresses that previously failed to access the external network can normally access the external network. It seems that the problem may not be on the firewall.

Run the Tracert command to track and test A2 (IP Address: 10.20.12.12/24) on the computer. The symptoms are as follows:

C: \> Tracert 10.20.30.110

Tracing route to 10.20.30.110 over a maximum of 30 hops

1 <1 MS <1 MS <1 MS 10.20.12.1

2 1 MS <1 MS 1 MS 192.168.10.2

3 1 MS 1 MS 1 MS 10.20.30.1

4 2 MS 1 MS 2 MS 10.20.30.110

Trace complete

Change the IP address of A2 on the computer to 10.20.12.22/24 and run the Tracert command to perform the test:

Tracing route to 10.20.30.110 over a maximum of 30 hops

1 <1 MS <1 MS <1 MS 10.20.12.1

2 1 MS ** 192.168.10.2

3*2 MS * 10.20.30.1

4 *** Request timed out

5 *** Request timed out

6 ** 2 MS 10.20.30.110

Trace complete

Trace complete

→

Sometimes, when Tracert is used to track and test the results of rows 4 and 5 and later, Request timed out is displayed.

Looking back at the recent changes in the network structure, the gateway in the Department used a three-tier switch (only using its routing function). At that time, the internal network could normally access the external network, if you change to a Cisco 3640 vro one week ago, You will reconfigure the 3640 vro according to the configuration in the layer-3 vswitch. Shortly after the change, the above network faults will occur. Check the port configuration and route configuration of Cisco 3640, and find that the port IP address is configured normally, but there are two routes in the route list:

Ip route 0.0.0.0 0.0.0.0 192.168.10.2

Ip route 0.0.0.0 0.0.0.0 10.255.12.254

Because the Department's network is the lowest subnet in the Unit's Wan, the whole unit's Wan is accessed according to business needs, and the Department's network is only connected to department B through A Cisco 3640 Router, then, access the unit Wide Area Network (see the preceding network topology) through department B's Router B ). To facilitate the configuration, we only need to add a default route "ip route 0.0.0.0 0.0.0 192.168.10.2" in Cisco 3640 to forward the Department's access to the external network to department B's Router B, to achieve network interconnection. Due to setup errors, an existing "ip route 0.0.0.0 0.0.0 10.255.12.254" node may be added to the route table ". As a result, when computers in the Intranet access the external network, the router sometimes cannot correctly send IP packets to 192.168.10.2. Because the router is powered on, the Intranet access route list to the Internet is saved in the memory, in this way, some computers can access the Internet normally, while others cannot. When the vro restarts (for example, the vro restarts on the morning of the next Monday), the access route list is re-established, allows internal computers to access the Internet to detect changes.
 
Troubleshooting of Cisco router faults

Delete A list of useless routes in vro privileged mode:

# No ip route 0.0.0.0 0.0.0.0 10.20.12.254

# Wr

After the router is restarted, all the computers in the department can access department B's network and the WAN. The fault is completely solved.

It refers to a table stored on a router or other Internet network device. The table contains paths to specific network terminals. In some cases, there are also some measurements related to these paths.

The route table is generally in the following format:

Destination Gateway Flag Refs Use Inerface

Specifically, Destination is the IP address of the target network or host; Gateway is used to reach the specified Destination; Flag is used to describe the characteristics of this route; Refcnt is used to create a connection, the number of times the route is used. Use indicates the number of group packets transmitted through the route. Interface indicates the network Interface name used by the route.

An example of a typical route table

Destination Gateway Flags Refs Use Interface

Default 26.112.191.98 UGS 0 0 net0

Default 26.112.191.98 UGS 0 18 net0

26.112.191 26.112.191.2 UC 1 0 net0

26.112.191.2 127.0.0.1 UGHS 3 92 lo0

127.0.0.1 127.0.0.1 UH 3 9786480 lo0

224 26.112.191.2 UCS 0 0 net0