Dell FCX624-S Laptop User Manual


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946 PowerConnect B-Series FCX Configuration Guide
53-1002266-01
Configuring OSPF
29
Changing the reference bandwidth for the cost on OSPF interfaces
Each interface on which OSPF is enabled has a cost associated with it. The Layer 3 Switch
advertises its interfaces and their costs to OSPF neighbors. For example, if an interface has an
OSPF cost of ten, the Layer 3 Switch advertises the interface with a cost of ten to other OSPF
routers.
By default, an interface OSPF cost is based on the port speed of the interface. The cost is
calculated by dividing the reference bandwidth by the port speed. The default reference bandwidth
is 100 Mbps, which results in the following default costs:
10 Mbps port – 10
All other port speeds – 1
You can change the reference bandwidth, to change the costs calculated by the software.
The software uses the following formula to calculate the cost.
Cost = reference-bandwidth/interface-speed
If the resulting cost is less than 1, the software rounds the cost up to 1. The default reference
bandwidth results in the following costs:
10 Mbps port cost = 100/10 = 10
100 Mbps port cost = 100/100 = 1
1000 Mbps port cost = 100/1000 = 0.10, which is rounded up to 1
155 Mbps port cost = 100/155 = 0.65, which is rounded up to 1
622 Mbps port cost = 100/622 = 0.16, which is rounded up to 1
2488 Mbps port cost = 100/2488 = 0.04, which is rounded up to 1
For 10 Gbps OSPF interfaces, in order to differentiate the costs between 100 Mbps, 1000 Mbps,
and 10,000 Mbps interfaces, you can set the auto-cost reference bandwidth to 10000, whereby
each slower link is given a higher cost, as follows:
10 Mbps port cost = 10000/10 = 1000
100 Mbps port cost = 10000/100 = 100
1000 Mbps port cost = 10000/1000 = 10
10000 Mbps port cost = 10000/10000 = 1
The bandwidth for interfaces that consist of more than one physical port is calculated as follows:
Trunk group – The combined bandwidth of all the ports.
Virtual interface – The combined bandwidth of all the ports in the port-based VLAN that
contains the virtual interface.
The default reference bandwidth is 100 Mbps. You can change the reference bandwidth to a value
from 1 through 4294967.
If a change to the reference bandwidth results in a cost change to an interface, the Layer 3 Switch
sends a link-state update to update the costs of interfaces advertised by the Layer 3 Switch.
NOTE
If you specify the cost for an individual interface, the cost you specify overrides the cost calculated
by the software.