The
tracert command is used to discover the gateways that packets
actually pass through from the source to the destination. The
tracert command is used to check the network connectivity and
locate network faults.
The process
of a tracert command is as follows:
The sender
sends a packet with TTL 1. When the TTL expires, the first hot returns an ICMP
error message indicating that the message cannot be forwarded anymore.
The sender
sends a packet with TTL 2. When the TTL expires, the second hot returns an ICMP
error message indicating that the message cannot be forwarded anymore.
The sender
sends a packet with TTL 3. When the TTL expires, the third hot returns an ICMP
error message indicating that the message cannot be forwarded anymore.
The sender
repeats the preceding process by increasing the TTL value until the packet
reaches the destination.
- When
performing the tracert operation, a network device sends UDP packets. The UDP
port number of the three UDP packets starts from 33434 and is incremented by 1
every time the packets pass a hop. When one node on the path has equal-cost
routes, the node performs a hash operation based on flows. Therefore, the UDP
packets are distributed to different routes, and a maximum of three IP addresses
on the equal-cost routes are shown each time.
The following figure shows information about tracert packets sent by a network device. The first hop has only one route, so only one next-hop 192.168.2.1 is displayed. The second hop has two next hops (192.168.11.2 and 192.168.21.2), so the three packets are distributed to two links.
- When
performing a tracert operation, a PC sends ICMP packets, which are irrelevant to
port numbers. If a network device on the path has equal-cost routes, the ICMP
packets are distributed to only one link, and only one next-hop IP address is
displayed. However, if the network device performs load balancing based on
packets, the ICMP packets are distributed to different links.
The following figure shows information about the tracert packets sent by a PC. Three packets arrive at each hop together. For example, three packets have TTL 5.
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