OSPF cost and reference bandwidth

A real-life situation

The network from the first module has two paths from LAN A to LAN B. The top one, through R2, is gigabit all the way. The bottom one, through R3, includes an older 100 Mbps link between R1 and R3. With OSPF running on default settings, users complain that copies to the servers are sometimes fast and sometimes slow. show ip route on R1 shows two routes to LAN B with the same cost: OSPF is sending half the traffic over the slow link. It can't tell the two links apart, and this lesson shows why.

What OSPF cost is

Each interface gets a cost from one simple division:

cost = reference bandwidth ÷ interface bandwidth

The result is rounded down and is never less than 1. The reference bandwidth is 100 Mbps unless you change it. That made sense when 100 Mbps was fast, but today it means:

With the default, FastEthernet, gigabit and 10-gigabit all cost 1. That is exactly the problem in the situation above. Raising the reference to 1 000 Mbps separates 100 Mbps from 1 Gbps; a network with 10-gigabit links needs 10 000 Mbps or more.

How path cost adds up

A router adds the cost of each outgoing interface on the way to the destination, including the interface on the last router that connects to the destination LAN. Costs of incoming interfaces are not counted.

With the default reference, the 100 Mbps link would cost 1 too: both paths would total 3, and R1 would use both. That is equal-cost multi-path (ECMP). OSPF installs up to 4 equal paths by default (change it with maximum-paths under router ospf). ECMP is useful when the paths really are equal, and harmful when they only look equal.

How to configure it on Cisco IOS

There are three ways to influence cost. In order of priority, highest first:

1. Set the cost directly

2. Change the bandwidth the formula uses

3. Raise the reference bandwidth (recommended)

The reference must be the same on every router. Otherwise one router thinks a gigabit link costs 1 and another thinks it costs 10, and their path choices stop making sense together. Choose a value at least as high as your fastest link, so the fastest link costs 1 and everything slower costs more.

How to verify it

R1#show ip ospf interface brief
Interface    PID   Area            IP Address/Mask    Cost  State Nbrs F/C
Gi0/2        1     0               10.0.13.1/30       10    P2P   1/1
Gi0/1        1     0               10.0.12.1/30       1     P2P   1/1
Gi0/0        1     0               192.168.1.1/24     1     DR    0/0

R1#show ip route 192.168.4.0
Routing entry for 192.168.4.0/24
  Known via "ospf 1", distance 110, metric 3, type intra area
  Last update from 10.0.12.2 on GigabitEthernet0/1, 00:01:15 ago
  Routing Descriptor Blocks:
  * 10.0.12.2, from 4.4.4.4, 00:01:15 ago, via GigabitEthernet0/1
      Route metric is 3, traffic share count is 1

R1#show interfaces GigabitEthernet0/2 | include BW
  MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec,

What goes wrong and how to troubleshoot it

• Traffic is shared over a fast and a slow link. The default reference makes them equal. Raise auto-cost reference-bandwidth everywhere.
• Path choices change depending on which router you ask. The reference bandwidth differs between routers. Compare show ip ospf | include Reference.
• Traffic goes one way and returns another. ip ospf cost was set on one end of a link only. That isn't wrong, but it should be deliberate.
• A cost won't change. An ip ospf cost on the interface overrides both the reference bandwidth and the bandwidth command.

Common mistakes

• Counting incoming interface costs. Only outgoing interfaces count.
• Forgetting the last router's LAN interface when adding up a path.
• Mixing units: auto-cost reference-bandwidth is in Mbps, bandwidth is in kbps.
• Changing the reference bandwidth on one router only.
• Thinking bandwidth changes the real speed of the interface.

💡 Exam tip: be fast at 100 ÷ bandwidth in Mbps (and at other reference values), at adding path costs from a diagram, and at reading [110/3] as distance 110, cost 3. Know the three commands and that ip ospf cost wins over the other two.

Key takeaways

Check yourself