17-35
Ethernet Card Software Feature and Configuration Guide, R7.2
January 2009
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 topo logy discovery mechanism based
on the IEEE 802.17 standard. Each Cisco proprietary RPR station bid irectionally floods local topology
data around the Cisco proprietary RPR to it peers through a contr ol 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 p ort-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 destinatio n. 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
P0
E0
P0
E0
P1
E1
A
151792
B
C
A B (shortest path)
RPR
P0
E0
P0
E0
P1
E1
A
B
C
A B (longest path)