Hi Stingo,

This is basically because the Eth-Trunk interface is a layer 2 interface and will not necessarily be related to a single VLAN. It could be configured as a trunk port carrying multiple VLANs. If it was a L3 Eth-Trunk, then the software would automatically adjust the cost. You will find if you check that VLAN interfaces have a fixed cost based around 1 Gbps.

Remember that on a switch, a VLAN could and probably will be asociated with multiple physical interfaces, so the bandwidth assigned to it cannot logically be taken from any one single physical interface.

If you have a specific case where the VLAN is just used across one physical link then you can manually change the OSPF cost to represent the actual bandwidth of that link. The switch cannot make any assumptions about this and it could make the network very unstable if the switch were to try and automatically calculate the costs based on the L2 interfaces that a particular VLAN is associated with.

Regards Nigel

Thanks!

I changed cost manually,but I thought, may be it could work at L2.. 

it isn't really comfortable in my case.