| CPC H04L 45/566 (2013.01) [H04L 45/28 (2013.01); H04L 45/42 (2013.01)] | 20 Claims |

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1. A packet transmission system, comprising:
a destination end device;
a source end device configured to send a first packet, wherein the first packet carries first source routing information, a source routing validity identifier, and a destination identifier, wherein the first source routing information comprises port identifiers of each network device of a plurality of network devices on a first forwarding path, and wherein the destination identifier indicates the destination end device; and
a first network device of the plurality of network devices and on the first forwarding path, wherein the first network device is configured to:
receive the first packet;
either forward the first packet based on the first source routing information when the first packet satisfies a source routing forwarding condition or forward the first packet based on the destination identifier when the first packet does not satisfy the source routing forwarding condition and satisfies a forwarding table forwarding condition, wherein the forwarding table forwarding condition is that the source routing validity identifier indicates that source routing encapsulation in the first packet is valid and an egress port in the first source routing information is in a non-working state, and wherein the egress port is indicated by a port identifier of the first network device; and
forward the first packet.
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