In the case that there are multiple equal-cost links and the default per-flow load balancing mode is adopted, why the Tracert test result outputs the

Created Apr 12, 2017 08:40:43Latest reply Apr 12, 2017 14:03:05 753 2 0 0
In the case that there are multiple equal-cost links and the default per-flow load balancing mode is adopted, why the Tracert test result outputs the interface IP address of each equal-cost link?
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user_2790689  Expert   Created Apr 12, 2017 13:43:53 Helpful(0) Helpful(0)

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  • convention:

Adamcolob     Created Apr 12, 2017 14:03:05 Helpful(0) Helpful(0)

ECMP supports only per-flow load balancing.

In per-flow load balancing mode, before forwarding a packet, the device calculates a cost value through the hash algorithm according to the quintuple information of the packet, including the protocol type, source IP address and mask, destination IP address and mask, source port number, and destination port number. Then, the device chooses a link according to the cost value to forward the packet.

UDP packets are used in the Tracert test. The destination port number is sum of the specified port number and the sequence number of the packet to be sent. By default, the destination port number is 33434 and the sequence number of the packet starts from 0 and is increased by 1. After reaching 65535, the port number starts from 33434 again.

The destination port number of the UDP packet in the tracer test changes. As a result, the hash result changes and the chosen route also changes. Therefore, the Tracert test result outputs the interface IP address of each equal-cost link.
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