Equalizing Path Costs For Ospf Networks; Figure 4-6 Ospf Cost Metric - ND SatCom SKYWAN IDU 7000 Manual

Data Networking
®
SkyWAN Internet Protocol Features
4.2.6

Equalizing Path Costs for OSPF Networks

In OSPF routing networks each router interface has an assigned cost metric value. The short-
est path first algorithm determines the optimal path to any reachable destination by minimizing
the summary cost metric.
In the sample network displayed in figure 4-6 the destination network is reachable from
IDU 1-3 via either IDU 4 or IDU 5.
Figure 4-6 OSPF Cost Metric
In the first setup the cost metric for every interface is set to "1" which is the IDU default value
for an OSPF interface. OSPF will select the path R 1 via IDU 4 as this has a summary cost
metric of "2", compared to a cost metric summary of "3" for the path R 2 via IDU 5. As long as
IDU 4 and its Ethernet interface is up, all IP unicast traffic will be routed via IDU 4 using the
frequency channel 2 which is the home channel of IDU 4. Only in case of failure of this station,
traffic will be rerouted via IDU 5, using then frequency channel 3. This situation may lead to an
uneven usage of channel 2 and 3, where channel 2 may be completely congested whereas
channel 3 has still lot of capacity left.
Load balancing would help to equalize the usage of both channels. For this purpose the cost
metrics of both paths have to be identical as OSPF only supports ECMP balancing mode. In
this example this can be achieved by setting the cost metric of the IDU 4 Ethernet interface to
"2", which would mean that now both paths have the same metric summary cost of" 3".
116 Network Design and Engineering Guide 2010-10-26