Wide-Area Networks - CCIE Routing and Switching Exam Certification Guide - page 196

Both ATM and PPP are discussed in this chapter.

PoS specifies STS-3c/STM-1 (155 Mbps) as the basic data rate, with a usable data bandwidth
of 149.760 Mbps.

Dynamic Packet Transport (DPT)/Spatial Reuse Protocol (SRP)
DPT is a Cisco developed, resilient optical packet ring technology that is optimized for data
transmission. DPT uses dual, counter-rotating rings that are referred to as inner and outer,
which can be used for data and control packet transmission concurrently. DPT operates by
sending the data packets in one direction on one fiber ring and the corresponding control
packets in the opposite direction on the other fiber ring.

The full capacity of the fiber rings can be utilized for data and control traffic. It is not required
to reserve half of the capacity for redundancy.

DPT uses the Spatial Reuse Protocol (SRP) MAC layer protocol, which, according to Cisco,
was designed to be scalable and provide optimized IP packet aggregation and transport in local-
area networks (LANs), metropolitan-area networks (MANs), and wide-area networks (WANs).
It can support up to 128 nodes running at high speeds (OC-48c/STM-16c and OC-192c/STM-64c).
The fairness algorithm and packet priority ensures IP packets with bounded end-to-end delay
requirements are delivered successfully.

Bandwidth Efficiency
SRP uses destination stripping to increase bandwidth capacity. Other ring technologies, such as
Token Ring or Fiber Distributed Data Interface (FDDI), use source stripping—packets traverse
the entire ring before being stripped by the source. With SRP, the packet travels on the ring from
the source to the destination, and the destination strips the packet, which frees up bandwidth on
other segments of the ring for use by other stations.

Fairness

A fairness algorithm is implemented to ensure that all stations on DPT rings can use a fair share
of the ring. A distributed copy of the SRP fairness algorithm is run on each DPT station. As
defined by Cisco, the fairness algorithm ensures the following:
•

Global fairness —Each ring node gets its fair share of the ring by controlling the rate at
which packets are transmitted onto the ring. When a node is congested, it sends a control
message to its upstream neighbor, indicating its own transmit usage. The upstream neigh-
bor adjusts its transmission rate to not exceed the advertised value. This upstream neighbor
then propagates the advertised usage to its upstream neighbor. If one of the upstream