FastEthernet cost for OSPF
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Author:  dcolmena2000 [ Tue Dec 11, 2012 10:26 pm ]
Post subject:  FastEthernet cost for OSPF


FastEthernet ports should have an associated cost of 1 in OSPF.
However, I did a lab and I had 10, as if it were an Ethernet port.

I know we can go into the interface and enter commands like
"ip ospf cost 1" or "bandwidth 100000".

However, I am just curious. Why GNS3 shows a cost of 10 for
a FastEthernet port instead of 1 ????

Thank you for your answer.

Author:  dcolmena2000 [ Sat Jan 26, 2013 6:31 pm ]
Post subject:  Re: FastEthernet cost for OSPF

Hi all,

No answer to my question. Sniff. It is very simple to reproduce the issue.

R1 ---------------------- R2
   F0/0              F0/0
   .1                  .2

Here is the configuration of R1, and R2 has a similar configuration:
R1(config)# interface FastEthernet 0/0
R1(config-if)# ip address
R1(config-if)# no shutdown
R1(config-if)# exit
R1(config)# router ospf 1
R1(config-router)# network area 0
R1(config-router)# end
R1#show ip ospf interface FastEthernet 0/0
FastEthernet0/0 is up, line protocol is up
Internet Address, Area 0
Process ID 1, Router ID, Network Type BROADCAST, Cost: 10

As you can see, the last line says that the cost is 10. It should be 1.
Any idea????


Author:  claydon_dan [ Sat Jan 26, 2013 9:51 pm ]
Post subject:  Re: FastEthernet cost for OSPF

I have just tested this and can confirm it is caused by the network links within GNS3/Dynamips. By default the links only operate at 10Mbps/Half-Duplex, hence a cost of 10. Setting the following on your interfaces will fix that:
R1(config-if)# speed 100
R1(config-if)# duplex full

These were the results from my tests.
Test #1 - As per your post
Result - Cost:10

Test #2 - With Speed and Duplex set to 100/Full
Result - Cost: 1

Test #3 - Tested on two physical 2610XM Routers, using the original config (without speed and duplex settings)
Result - Cost: 1

Hope that helps

Author:  dcolmena2000 [ Tue Jan 29, 2013 2:00 am ]
Post subject:  Re: FastEthernet cost for OSPF

Hi Daniel,

Thank you so much for your contribution.
I always had this problem with the OSPF cost when I was teaching.
Now I know how to fix it.


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