HP 200 Series Services And Applications page 278

Routing with OSPF
Equal-Cost Multipath
Equal-Cost Multipath
Equal-cost-multipath routing is not yet available on HP routers. The
following discussion is intended only to explain the equal-cost-multipath
routing concept.
ROUTER
NET 2
NET 3
NET 1
NET 5
Metric B
Metric C
Metric A
Metric A
ROUTER
ROUTER
NET 4
Metric D
Metric B + Metric C = Metric D
Figure 8. Equal-Cost Routes from Net 1 to Net 5
Consider the network in figure 8. There are two equal-cost paths from net 1
to net 5. The sum of the metrics for net 2 plus net 3 is identical to the metric
for net 4. When packets destined for net 5 are received on net 1 by the
router, packets will be forwarded on both paths. Obviously, careful attention
must be paid to the assignment of link metrics to ensure that multiple paths
are used to forward data when that is desired.
Conclusion
OSPF provides support for large networks using hierarchical network
organization, improved metrics, and a link-state protocol. Additional
features that help conserve IP address space, make addressing more
flexible, and improve network security make OSPF attractive for use on
many small and medium-sized internets as well.
OSPF features available in future router releases, such as type-of-service
routing and equal-cost-multipath routing, will add even more value to an
already valuable internetworking tool.
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