Figure 17-14 Shortest And Longest Path - Cisco ONS 15454 SDH Configuration Manual

Chapter 17 Configuring Cisco Proprietary Resilient Packet Ring
Figure 17-14 Shortest and Longest Path
A
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:
•
•
•
•
•
•
•
RPR
P0
P0
E0
E0
E1
B
A B (shortest path)
You must configure
spr topology discovery-enable
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.
Cisco ONS 15454 and Cisco ONS 15454 SDH Ethernet Card Software Feature and Configuration Guide, R8.0
P1
C
on all the nodes on the Cisco proprietary RPR
Cisco Proprietary RPR Shortest Path
RPR
P0
P0
E0
E0
E1
A
B
A B (longest path)
P1
C
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