Wide-Area Networks - CCIE Routing and Switching Exam Certification Guide - page 197

neighbors is also congested, it propagates the minimum of the advertised usage and its
own usage. This procedure ensures that no one node hogs all the bandwidth, creating star-
vation or excessive delay.

•

Local optimization —A ring node can transmit more than its fair share of bandwidth to
another node on a local segment, as long as nodes on other sections of the ring are not
adversely impacted.

•

Scalability —The fairness algorithm has highly efficient and scalable bandwidth control
and can handle up to 128 nodes running at high speeds (OC-48c/STM-16c and OC-192c/
STM-64c) over widely distributed geographic areas.

Packet Priority
SRP provides packet priority marking and expedited priority packet handling. SRP provides
special handling in the transmit queue and transit buffer. Transmit packets are originated onto
the DPT ring by the transmitting node. Transit packets originate in another ring node. A 3-bit
priority field is in the SRP MAC header. There is a mapping between these bits and the IP pre-
cedence bit values in the type of service (ToS) field in the IP header. The node sourcing the
packet onto the ring sets the SRP priority bits. However, only two queues (high priority and low
priority) are in SRP for the transmit queue and the transit queue. The SRP node utilizes config-
urable threshold values to determine into which queue the packets are placed. The packet
scheduler in the node utilizes specific packet handling rules to determine which queue is
serviced next. The rules are as follows:

High-priority transit packets first 1

High-priority transmit packets 2

Low-priority transmit packets

3

Low-priority transit packets

4

This hierarchy is modified by placing a threshold on the low-priority transit queues to ensure
that the transit queue does not overflow while the node is servicing low-priority locally sourced
traffic, and the transit queue does not have to wait too long behind locally sourced low-priority
traffic.

Ring Resiliency
DPT uses Intelligent Protection Switching (IPS), which uses SRP control packets to proactively
monitor ring performance to perform fault isolation and to provide self-healing rings by wrapping
around outages within 50 ms. If multiple events occur concurrently, IPS uses event hierarchy
rules to determine which events to handle and which events to handle first.

IPS is multilayer aware. It monitors and handles events at Layers 1, 2, and 3 instead of just
Layer 1. An event that occurs at Layer 3 is important, but it might not justify a ring wrap. In this