Consider a network that mixes link bandwidths from 128 kb/s to 40 Gb/s. Which value should be set
for the OSPF reference bandwidth?
A.
Set a value of 128.
B.
Set a value of 40000.
C.
Set a manual OSPF cost on each interface.
D.
Use the default value.
E.
Set a value of 40000000.
F.
Set a value of 65535.
B
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default ref bandwidth is 100m, when the link is mixed with huge and small bandwidth, Cisco advise to configure a manual metric value.
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Using a 50 Gbps reference BW allows to handle links with a lower speed limit of 762 kbps (50000 Mbps / 65535 gives this result)
so you need to take care of sub-T1 link speeds if there in your network use on them
https://supportforums.cisco.com/discussion/10577421/ospf-auto-cost-reference-bandwidth
so in our case if 40G –> 40 000 000 000 / 65535 = 610 kb/s (lower speed limit), but we must have range 40G — 128 kb/s
so, i think also answer – C
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