OSPF uses a simple formula to calculate the OSPF cost for an interface with this formula:
cost = reference bandwidth / interface bandwidth
The reference bandwidth is a value in Mbps that we can set ourselves. By default this is 100Mbps on Cisco IOS routers. The interface bandwidth is something we can lookup.
Let’s take a look at an example of how this works. I’ll use this router:
The router above has two interfaces, a FastEthernet and a serial interface:
R1#show ip interface brief
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 192.168.1.1 YES manual up up
Serial0/0 192.168.2.1 YES manual up up
Let’s enable OSPF on these interfaces:
R1(config)#router ospf 1
R1(config-router)#network 192.168.1.0 0.0.0.255 area 0
R1(config-router)#network 192.168.2.0 0.0.0.255 area 0
After enabling OSPF we can check what the reference bandwidth is:
Router#show ip ospf | include Reference
Reference bandwidth unit is 100 mbps
By default this is 100 Mbps. Let’s see what cost values OSPF has calculated for our two interfaces:
Router#show interfaces FastEthernet 0/0 | include BW
MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec
Router#show ip ospf interface FastEthernet 0/0 | include Cost
Process ID 1, Router ID 192.168.1.1, Network Type BROADCAST, Cost: 1
The FastEthernet interface has a bandwidth of 100.000 kbps (100 Mbps) and the OSPF cost is 1. The formula to calculate the cost looks like this:
100.000 kbps reference bandwidth / 100.000 interface bandwidth = 1
What about the serial interface? Let’s find out:
“It now has a cost of 1 which means that a Gigabit interface would end up with a cost of 1.”
Did you mean 10?
check this command its not working
this one is working
Router#show ip protocols | include Reference
I have only seen
auto-cost reference-bandwidth
for OSPF (both in IOS 12 and 15). There is areference-bandwidth
command, but it is in the NX-OS and unrelated to OSPF.Hello Harshit
Fundamentally, reference bandwidth should always be the same on all routers taking part in OSPF. Now if you choose to use 1000 Mbps as the reference bandwidth or 10000Mbps, or even 100 Mbps, it doesn’t make a difference what the speeds on your interfaces are, as long as all routers are choosing costs based on the same reference bandwidth.
Now in the case where you have a GigabitEthernet interface connected to a FastEthernet interface, it shouldn’t make a difference. This is because if these two interfaces were connected, the GigabitEthernet link
... Continue reading in our forumThank you very much Laz. Wish you a good day!