HSR-PRP Design

The chapter shows the HSR and PRP designs.

The HSR- PRP implementation adds zero failover time hardware redundancy to the Raptor platform. To achieve that, the following is implemented:

A packet is sent to the network over the I-port and the redundant switch makes two copies and send them over the A-port and B-port.
The redundant switch will only forward the first copy of a packet that it receives on Port-A or Port-B to the I-port and discards the second copy of the packet.

Each redundant switch or node in the ring contains at least three ports: the redundant pair of ports A and B and the physical I -Port. The mapping of the I-Port and ports A and B of the redundant switch is static for each line card.

For HSR (IEC 62439-3 Clause 5), packets are transmitted with a HSR header and circulate in both direction through the ring topology. Unicast packets are forwarded until they reach the destination device, while multicast packets are forwarded until they reach the originator. HSR rings allow for lower deployment costs as they are only connected to each other. Non-HSR equipment cannot be connected directly to the HSR ring but can be connected through a HSR RedBox.

Figure 1. HSR Topology

For PRP, (IEC 62439-3 Clause 4) packets are transmitted with a PRP trailer (an identifier added to the frame) in parallel to LAN A & B, forming a double star topology. This is suitable for large scale networks. As the LANs use standard Ethernet equipment, even non PRP-capable devices (e.g singly attached nodes (SAN)) can be connected to one of the LANs as it ignores the PRP trailer.

Figure 2. PRP Topology

Figure 3. Physical and Logical Ports in HSR-PRP Topology

I-Ports are physical ports of the redundant switch connected to PONCAT (a.k.a. Gigabit / Extreme Ethernet) ports.
Port-A and Port-B are redundant switch ports. Each redundant switch or LRE (Link Redundancy Entity) must be represented by a logical interface (a.k.a redundant interface or RED 1 or RED2) in ISS. This logical interface will be used to configure the port A and B of the redundant switch.

Redundancy is achieved by duplicating the received I-Port Ethernet frames on both the Port-A and Port-B, while not forwarding the second duplicate Ethernet frames received on Port-A or Port-B ports to the I-Port.

In this way, if a link, a node, or even a switch in the network goes down, the other parts of the network will still work as expected.