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17-2

Ethernet Card Software Feature and Configuration Guide, R7.2

January 2009

Chapter 17 Configuring Cisco Proprietary Resilient Packet Ring

Understanding Cisco Proprietary RPR

Cisco proprietary RPR is a MAC protocol operating at the Layer 2 level. It is well suited for transporting

Ethernet over a SONET/SDH ring topology and it enables multiple ML-Series cards to become one

functional network segment or shared packet ring (SPR). Cisco proprietary RPR overcomes the

limitations of earlier schemes, such as IEEE 802.1D Spanning Tree Protocol (STP), IEEE

802.1W Rapid

Spanning Tree Protocol (RSTP), and SONET/SDH when used in this role.

In Software Release 7.2 and later, the ML-Series card supports IEEE 802.17b based RPR (RPR-IEEE)

in addition to Cisco proprietary RPR. Throughout this book, Cisco proprietary RPR is referred to as

Cisco

proprietary RPR, and IEEE 802.17 based RPR is referred to as RPR-IEEE. This chapter covers

Cisco

proprietary RPR. Chapter 26, “Configuring IEEE 802.17b Resilient Packet Ring” covers IEEE

802.17b based RPR.

Role of SONET/SDH Circuits

The ML-Series cards in an SPR must connect directly or indirectly through point-to-point STS/STM

circuits. The point-to-point STS/STM circuits are configured on the ONS node and are transported over

the ONS node’s SONET/SDH topology with either protected or unprotected circuits.

On circuits unprotected by the SONET/SDH mechanism, Cisco proprietary RPR provides resiliency

without using the capacity of the redundant protection path that a SONET/SDH protected circuit would

require. This frees this capacity for additional traffic. Cisco proprietary RPR also utilizes the bandwidth

of the entire ring and does not block segments like STP or RSTP.

Packet Handling Operations

When an ML-Series card is configured with Cisco proprietary RPR and is made part of an SPR, the

ML-Series card assumes a ring topology. If a packet is not destined for network devices bridged through

the Ethernet ports of a specific ML-Series card, the ML-Series card simply continues to forward this

transit traffic along the SONET/SDH circuit, relying on the circular path of the ring architecture to

guarantee that the packet will eventually arrive at the destination. This eliminates the need to queue and

process the packet flowing through the nondestination ML-Series card. From a Layer 2 or Layer

perspective, the entire Cisco proprietary RPR looks like one shared network segment.

An ML-Series card configured with Cisco proprietary RPR has three basic packet-handling operations:

bridge, pass-through, and strip.

Figure 17-1 illustrates these operations. Bridging connects and passes

packets between the Ethernet ports on the ML-Series and the packet-over-SONET/SDH (POS) ports

used for the SONET/SDH circuit circling the ring. Pass-through lets the packets continue through the

ML-Series card and along the ring, and stripping takes the packet off the ring and discards it.

The Cisco proprietary RPR protocol, using the transmitted packet's header information, allows the

interfaces to quickly determine the operation that needs to be applied to the packet. It also uses both the

source and destination addresses of a packet to choose a ring direction. Flow-based load sharing helps

ensure that all packets populated with equal source- and destination-address pairs will be sent in the

same direction, and arrive at their destination in the correct order. Ring direction also enables the use of

spatial reuse to increase overall ring aggregate bandwidth. Unicast packets are destination stripped.

Destination stripping provides the ability to have simultaneous flows of traffic between different parts

of a ring. Traffic can be concurrently transmitted bidirectionally between adjacent nodes. It can also can

span multiple nodes, effectively reusing the same ring bandwidth. Multicast packets are source stripped.

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