OSPF: auto-cost reference-bandwidth

Reference: Routing TCP/IP volume 1 page 345

Cost calculation for OSPF:

10^8/bandwidth

This works well with 100mpbs link whereby the cost is 1.

however if it exceeds the 100mbps boundary the cost calculation will still be 1, because decimal/fraction of 1 is unacceptable in ospf cost metric.

hence any fraction of 1 will be rounded back to 1.

this will cause the faster link (1gbps) to be the same cost with other 100mbps link making the routing decision to be hop-based instead of cost base.

to resolve this issue, cisco routers have this feature: auto-cost reference-bandwidth

Let’s look at this request:

A company will be upgrading its network links to gigabit, assuming the highest link is a 1gbps, you must configure the routers to accurately calculate the cost metric so that the gigabit is taken into account.

The command must be consistent throughout the ospf speaking routers.

WARNING: setting a reference bandwidth too high will cause some lower bandwidth links to lost ospf adjacency, ospf does have a cap to the cost calculation if the cost calculation exceeded the highest cap ospf will deem that route dead.

R5(config-router)#auto-cost reference-bandwidth ?
<1-4294967>  The reference bandwidth in terms of Mbits per second

(DO NOT SET THIS TOO HIGH)

R5(config-router)#auto-cost reference-bandwidth 1000
% OSPF: Reference bandwidth is changed.
Please ensure reference bandwidth is consistent across all routers.
R5(config-router)#

R2(config)#router ospf 1
R2(config-router)#auto-cost reference-bandwidth 1000

R3(config)#router ospf 1
R3(config-router)#auto-cost reference-bandwidth 1000

R4(config)#router ospf 1
R4(config-router)#auto-cost reference-bandwidth 1000

R1(config)#router ospf 1
R1(config-router)#auto-cost reference-bandwidth 1000

Testing the effect:

R1#sh ip route ospf
172.16.0.0/16 is variably subnetted, 5 subnets, 2 masks
O       172.16.0.0/22 is a summary, 00:21:40, Null0
172.30.0.0/24 is subnetted, 3 subnets
O IA    172.30.20.0 [110/2010] via 172.30.0.3, 00:00:20, FastEthernet0/0
O IA    172.30.10.0 [110/2010] via 172.30.0.2, 00:00:20, FastEthernet0/0
10.0.0.0/22 is subnetted, 2 subnets
O IA    10.10.0.0 [110/2011] via 172.30.0.2, 00:00:20, FastEthernet0/0
O IA    10.20.0.0 [110/2011] via 172.30.0.3, 00:00:20, FastEthernet0/0

The cost metric actually increased 10 folds. The reference is 1gbps instead of 100mbps. Hence caution is needed to never set the reference bandwidth values into insane values.

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