How does OSPF handle same cost paths?

[Krishnendu AR]

Lets assume there are 2 routes to a remote destination with equal cost. But one is 1Gbps line and other one is 100Mbps.

How does OSPF handle this situation?

Krishna

[Thotsaphon Lueangwattanaphong]

Hello,

    You will see routes installed in the routing table. And then CEF(if enabled) will take care of the rest. Actually Per destination load balancing is used.

HTH,

Toshi

[Krishnendu AR]

What if I want to make the 1Gbps line my preffered route? How should I tweak the cost formula?

Krishna

[Thotsaphon Lueangwattanaphong]

Hello,

    Sure you can. You can modify cost on 100M to be worst(65535) as well.

HTH,

Toshi

[cadet alain]

Hi,

You can

1) modify the cost per interface with the interface command:  ip ospf cost

2) modify the reference-bandwidth used to automatically calculate the cost with this  OSPF process command:

ospf auto-cost reference -bandwidth 

But this command will affect the cost of all interfaces and first command overrides the cost per interface

Regards.

Alain

[NickNac79]

Hi All,

Just wanted to add quickly to what Alain said about the reference bandwidth.

The default reference bandwidth of 100 calculates all links of speed 100Meg AND ABOVE with a cost of 1.  This became a problem for us when we moved to 1Gig, and then 10Gig and 44 Gig interlinks.  We were having to manually configure the link costs for every link and sometimes people would forget and routing wouldn't work as expected.

What we now do is on every OSPF device, we set the reference bandwitdh as stated above by Alain.  the Value we chose to set on all devices is 100,000 as this would alllow us to in future have 100Gig links with a cost of 1.

I'm not suggesting you adopt this, as the decision is based upon the speed of your links and the size of your network.  You may also decide to manually set speeds, which has less of an impact when implementing on a live network.

Below is a chart that I made up at the time in order to make the decision.  Note that if any route's cost grows to 65535 it gets classed as unreachable and will not get installed in the routing table, so if you have lots of low bandwitdh links, you need to take this into account.

	Reference Bandwidth	44Gig	10Gig	1Gig	100Meg	10Meg	8Meg	2meg	1Meg	512k	256k
Default	100	1	1	1	1	10	13	50	100	195	391
	1,000	1	1	1	10	100	125	500	1,000	1,953	3,906
	10,000	1	1	10	100	1,000	1,250	5,000	10,000	19,531	39,063
	44,000	1	4	44	440	4,400	5,500	22,000	44,000	65,535	65,535
	100,000	2	10	100	1,000	10,000	12,500	50,000	65,535	65,535	65,535
	200,000	5	20	200	2,000	20,000	25,000	65,535	65,535	65,535	65,535
	300,000	7	30	300	3,000	30,000	37,500	65,535	65,535	65,535	65,535

Be careful doing this, as Alain said - applying the command will change the automatically calculated costs of every interface on the device which hasn't had its cost set manually, so this might cause routing issues if you are not careful.

Hope that Helps,

Nick