Cisco 15327 User Manual download pdf (Page 287)

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Ethernet Card Software Feature and Configuration Guide, R7.2

Chapter 17 Configuring Cisco Proprietary Resilient Packet Ring

Cisco Proprietary RPR Shortest Path

Figure 17-14 Shortest and Longest Path

By always using the shortest path, traffic between two nodes can achieve the lowest possible latency.

This is especially important for delay-sensitive traffic such as voice-over-IP (VoIP) or video broadcast

TV.

The ML-Series card implements shortest path through a hop-based topology discovery mechanism based

on the IEEE 802.17 standard. Each Cisco proprietary RPR station bidirectionally floods local topology

data around the Cisco proprietary RPR to it peers through a control frame. The topology state machine

process all the received control frames and builds a map based on the collective data. Layer 2 unicast

traffic direction relies on this map.

The ML-Series card can also base its Layer 2 unicast traffic load-balancing on the shortest discovered

path, although MAC address load-balancing is the default, and port-based load balancing is also an

option.

The topology discovery mechanism also reroutes data during a protection event, such as a fiber cut. The

station that detects the failure of a Cisco proprietary RPR link starts restoring the traffic with

Cisco proprietary RPR wrapping. This causes partial traffic reorder and delivery through the reverse,

suboptimal path along the Cisco proprietary RPR to the destination. After all the stations get updated

with the new topology, which includes the link failure, they steer the traffic away from the failed span.

These are the guidelines for configuring Cisco proprietary RPR shortest path and topology discovery:

•

You must configure

spr topology discovery-enable

on all the nodes on the Cisco proprietary RPR

for accurate topology discovery.

•

When topology discovery is configured, keep alives will automatically become enabled, if not

already enabled.

•

Disabling topology discovery does not disable keep alives, but a “keep alive enabled” warning

message is displayed.

•

DRPRI nodes do not support topology discovery.

•

Shortest path load balancing requires the spr or mpls working microcode image for the ML-Series

card. If you attempt to enable shortest path load balancing with another microcode image that does

not support it, a warning message appears. The denied configuration is saved.

•

If topology discovery is enabled, enable shortest path load balancing.

•

You can enable shortest path load balancing on a per-node basis. It is not dependant on the type of

load balancing running on other nodes.

RPR

151792

A B (shortest path)

RPR

A B (longest path)

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