OSPF interface path cost vrs bandwidth statements

[jkrawczyk]

Hi all;

I am setting up a live secondary ite-site VPN to encapsulate GRE traffic to my remote locations. One thing I'm pondering on is how to assure that my promary GRE will be utilized 100% of the time while my primary DS3 is up but when this DS3 ISP connection would fail, I need my inter business traffic to flow through my secondary ISP links (4XT1).

I was thinking on setting the bandwidth statement in my primary (ds3) GRE interfaces to bandwidth 46080 and my secondary GRE interfaces to bandwidth to 6144.

Does anyone believe that this will meet my needs or do I need to look at the ospf cost or maybe static routes.

Regards

jeff

[sundar.palaniappan]

Jeff,

You may be aware, OSPF uses the forumla 100 mbps/interface bandwidth to calculate the metric (COST). You can influence the OSPF metric calculation by setting the interface bandwidth to a higher value for the primary link and lower value for the backup link. Lower cost is preffered. If routing is working as intended then you can leave it this way.

Alternatively, you can use the 'ip ospf cost (cost)' command on an interface to influence the route selection. If you configure 'ip ospf cost' then it would override the forumlated cost calculation.

HTH

Sundar

[jkrawczyk]

Thanks Sundar, please correct me here if I'm in error.

So a DS3 = 100,000,000 / 45000000 = cost of 2

So a 4XT1 = 100,000,000 / 6144000 = cost of 16

So If configure my backup tunnel5701 as

ip ospf cost 16, my DS3 will be the always path unless by promary DS3 goes down?

Regards

Jeff

[sundar.palaniappan · Accepted solution]

Your calculation is correct. The calculation is based on default interface bandwidth or 'bandwidth' configured on the interface.

If you decide to use 'ip ospf cost' you might as well configure the cost on both interfaces, 2 & 16, to rule out any surprises. Ofcourse, you need to configure the cost on both sides of the link to eliminate any assymetric routing problems.

HTH

Sundar