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.

Advertisements
This entry was posted in OSPF and tagged , . Bookmark the permalink.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s