Brocade Virtual ADX Switch and Router Guide (Supporting ADX v03.1.00) - Manual (Page 135)

Brocade Virtual ADX Switch and Router Guide 121
53-1003246-01
Configuring OSPF
6
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’s 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’s cost = 100/10 = 10
• 100 Mbps port’s cost = 100/100 = 1
• 1000 Mbps port’s cost = 100/1000 = 0.10, which is rounded up to 1
• 155 Mbps port’s cost = 100/155 = 0.65, which is rounded up to 1
• 622 Mbps port’s cost = 100/622 = 0.16, which is rounded up to 1
• 2488 Mbps port’s cost = 100/2488 = 0.04, which is rounded up to 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 – 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.
Interface types to which the reference bandwidth does not apply
Some interface types are not affected by the reference bandwidth and always have the same cost
regardless of the reference bandwidth in use:
• The cost of a loopback interface is always 0.
• The cost of a virtual link is calculated using the Shortest Path First (SPF) algorithm and is not
affected by the auto-cost feature.