What's wrong with the router pinging?

Source: Internet
Author: User
Tags echo message

The router does not ping. Many reasons ... Now let's talk briefly about the following points

Ask someone to ping your IP to see.

Use ping to determine TCP/IP failure--from Yesky

1, Ping 127.0.0.1:

127.0.0.1 is a local circular address and if this address is not ping, the local machine TCP/IP protocol does not function.

2, ping This computer IP address:

Use Ipconfig to view the native IP, and then ping the IP, the general rule indicates that the network adapter (NIC or modem) is working properly, not the general rule is that the network adapter fails.

3, Ping the same network segment of the computer IP:

Ping the IP of a computer on the same network segment, the general rule indicates that the network line has a fault, if the network also contains routers, you should ping the routers in the network segment of the IP, not general rule This section of the line has problems; The general rule again ping the router in the target computer in the network segment of the port IP, is not general routing failure; General rules again ping the destination machine IP address.

4, Ping website:

To detect a network with a DNS service, you can ping the network name of the computer, such as Ping, after the IP address of the destination machine has been ping in the previous step and you are still unable to connect to the machine. sina.com.cn, the IP that the URL points to will normally appear, which indicates that the DNS settings of this computer are correct and the DNS server is working properly, and vice versa may be one of the faults; You can also check for WINS resolution failures by pinging the computer name (WINS is a service that resolves the computer name to an IP address).

The use of PING commands there's so much to note--from Tsinghua it

Network engineers will use Ping, which is an effective way to check routing problems. But also often hear engineers complain: Impossible, how can not pass?

Such confusion usually occurs when you think the route is set correctly. Some of the problems encountered by the author, welcomed the reader to add (can be sent to me, or posted to the "Network World" Forum for Reference).

The simplest three cases: 1. Too impatient. That is, the network cable just plugged into the switch to ping the gateway, ignoring the generation of tree convergence time. Of course, newer switches support fast spanning trees, or some administrators simply turn off the spanning tree protocol of the user's port (access port), and the problem is solved. 2. Access control. No matter how many jumps in the middle span, as long as there are nodes (including the end node) to the ICMP filter, ping is not normal. The most common is the behavior of firewalls. 3. Some router ports do not allow users to ping.

It has also been the case, more covert.

1. The network due to the delay between the equipment is too large, resulting in ICMP echo message can not be received in the default time (2 seconds). There are several reasons for the delay, such as the line (satellite network latency up to 540 milliseconds), the router processing time delay, or routing design unreasonable cause circuitous path. Using extended ping to increase the timed out time, ping-pass is too much of a routing delay.

2. The introduction of NAT will cause a one-way ping. NAT can play the role of hidden internal address, when from the ping outside, can ping because the NAT table mapping relationship exists, when the outside of the host ping Intranet, it is impossible to find the border router Nat table entries.

3. Multi-routing load balancing situation. For example, ping remote destination host, the successful reply and timed out staggered appear, the result found that there are two routes to the destination network in the gateway router, the two routes have equal weights, but there is a problem after a route is checked.

4.IP address assignment is not contiguous. Address planning problems like the presence of landmines in the network, or the failure of address overlap or mask partitioning, may be problematic during ping. For example, an extreme case, a, b two hosts, after multiple hops connected, a can ping the Gateway B, and B's gateway set correctly, but a, B is ping impassability. After checking, there is also a second address on B's network card, and this address overlaps with the network segment where A is located.

5. Extend ping for the specified source address. Log on to the router, ping the remote host, and when ICMP echo request is sent from the serial WAN interface, the router specifies an IP address as the source IP, which may not be the IP of this interface or the interface has no IP address at all. A downstream router may not be able to route to this IP segment, resulting in a failure to ping. You can use an extended ping to specify a good source IP address.

It is also common for a ping problem to occur when the configuration of the host gateway and intermediate route is considered correct. At this point you should forget about the word "impossible" and combine the extended parameters of the ping with feedback, traceroute, router debug, and port mirroring and sniffer tools.

For example, when a, b two host through the Multi-hop router connection, both the gateway is set correctly, on a can ping B, but on B can not ping a. Can be mirrored in the switch, and sniffer grab packet to find out where the ICMP message terminated, what the message content, you can find the source IP address in the ICMP message is not as expected, at this time it is easy to imagine that the router can be the NAT function, In this way, we can gradually find some neglected problems. And Ping impassability when the feedback information is "destination_net_unreachable" or "Timed out" is also a difference.

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