What Is HSR?

Packet Forwarding Mode

As shown in the following figure, a terminal transmits all frames to RedBox1, which duplicates the frames before sending them to the destination through two different paths.

• If either of the paths is abnormal (for example, a link or node fault occurs), the frames can still be sent to the destination through the other path.
• If both paths can forward frames normally, RedBox1 marks all the frames to be sent in sequence. Upon receipt of the frames, the destination device sorts and deduplicates them. The complete frames that arrive first and are in the correct sequence are forwarded, and the duplicate frames that arrive subsequently are discarded. If some frames are missing on a path, the frames that arrive first from this path are cached and sorted. After the missing frames arrive from the other path, all these frames are assembled into complete frames in sequence and then forwarded. In this way, zero packet loss is achieved.

HSR ring

To connect two HSR rings, quadruple port boxes (QuadBoxes) are required. A QuadBox is bound to two RedBoxes with different IDs and has at least four Ethernet ports. Logically, two RedBoxes form a QuadBox. The two Eth-Trunk member interfaces of each RedBox are connected to the same HSR ring.

On the network shown in Figure 1-3, to improve link reliability, QuadBoxA and QuadBoxB connect two HSR rings (HSR Ring 1 and HSR Ring 2) and achieve zero loss of service data, ensuring the link connectivity of the two HSR rings. To ensure the connectivity between two HSR rings in the case of a QuadBox failure, two QuadBoxes need to be deployed for HSR ring interconnection.

Interconnection between HSR rings

Avoiding Cyclical Forwarding of Packets in the HSR Ring

Nodes in the HSR ring do not transmit packets that are already transmitted in the same direction. This avoids cyclical forwarding of packets in the HSR ring, prevents issues such as broadcast storms and MAC address flapping, and avoids communication quality deterioration and service interruptions. The following describes how a packet is processed when a RedBox injects the packet into the HSR ring.

1. Unicast packet with the destination inside the HSR ring: After the unicast packet reaches the destination node, the packet is not forwarded in the same direction.
2. Unicast packet with the destination outside the HSR ring: The unicast packet with the destination outside the HSR ring is forwarded by every node in the ring until it reaches the source node.
3. Multicast packet with destinations inside the HSR ring: The multicast packet with destinations inside the HSR ring is forwarded by every node in the ring until it reaches the source node.

Unicast or Multicast Packet Forwarding with Destination Nodes in Different HSR Rings

On the network shown in Figure 1-4, two QuadBoxes are deployed to implement HSR ring interconnection. The QuadBox must work in hsrredboxhsr mode. That is, HSR packets (carrying HSR tags) are forwarded across rings.

Example of forwarding packets whose destination nodes are in different HSR rings

Packets are forwarded as follows in HSR in this scenario (QuadBoxA and QuadBoxB process packets in the same way):

1. The RedBox (source node) sends frames bidirectionally. Specifically, it marks all the frames to be sent in sequence, makes two copies of each frame, and forwards the two copies separately in the clockwise and counterclockwise directions. For example, frame series a is sent clockwise and frame series b is sent counterclockwise. These two frame series are then forwarded by each node on HSR Ring 1 until they reach the source node.
2. The QuadBoxes sort and deduplicate the frame series a and frame series b received on HSR Ring 1 based on the receiving time to form a complete packet in the correct sequence, and directly discard the duplicate frames received later. The following assumes that the packet finally formed by QuadBoxA is packet A, and that by QuadBoxB is packet B.
3. QuadBoxes then forward the packets to another ring. QuadBoxA and QuadBoxB each modifies the Pathid field (specifically, the Net ID of the HSR ring) in the packet, makes two copies of the packet, and forwards the copies from two different ports on HSR Ring 2. For example, QuadBoxA modifies the Pathid field in packet A so that the field includes the Net ID of HSR Ring 2, copies packet A into frame series A1 and A2, and sends the frames from the two ports on HSR Ring 2. Similarly, QuadBoxB modifies the Pathid field, copies packet B into frame series B1 and B2, and sends the frames from the two ports on HSR Ring 2.
4. The QuadBoxes do not forward the received frames repeatedly on HSR Ring 2 because the frames have the same sequence ID. For example, after receiving frame series B2 from QuadBoxB, QuadBoxA finds that the sequence ID of frame series B2 is the same as that of frame series A1 and A2. In this case, QuadBoxA discards frame series B2. Likewise, QuadBoxB discards frame series A2 after receiving them from QuadBoxA.
5. Frames received by a QuadBox from another QuadBox in HSR Ring 2 are not forwarded to HSR Ring 1. On HSR Ring 2, after finding that the Pathid field includes the Net ID of HSR Ring 2, QuadBoxA or QuadBoxB does not inject the received frame series B2 or A2, respectively, into HSR Ring 1.
6. Finally, the destination node on HSR Ring 2 receives frame series A1 and B1, sorts and deduplicates the frames to form a complete packet, and forwards the packet to the destination terminals.