(OSPF & EIGRP ) Question

[Mohamed Sobair]

Hi All,

I am aiming to have load balance & redundancy of a traffic by using OSPF & Eigrp.

My question is:

I know OSPF could load balance over equal cost, what is the command to enable it? and if one of my interfaces goes down do I still have redundancy?

In other case: If I am using EIGRP, which could load balance over unequal cost by the Variance command, now my issue will be if the traffic is balanced & then one of my interfaces goes down , do I still have redundancy? I mean my traffic wouldnt pass through the failed interface?

Appreciate any feedback,

Thanks,

[pkhatri]

Hi,

OSPF equal-cost load-balancing is on by default; you don't have to do anything to enable it. If one of the interfaces used by an equal-cost path goes down, the others will still be able to be used, giving you redundancy. By default, you can have 4 equal-cost paths, but you can increase that to 6 by using the 'maximum-paths 6' command.

The same applies to EIGRP, except that you can have unequal-cost load-sharing by specifying a variance greater than 1. The redundancy offered by the multiple paths is still available.

Hope that helps - pls do rate the post if it does.

Paresh

[Mohamed Sobair]

Hi Paresh,

The above post was helpfull, but I still have one question:

I know the cost on OSPF = (Interface)bandwith/10.8

If I am going to use Eigrp, the cost in this case is varies for multiple options, could you please show me how is it calculate it ? based on actual environment,

Thanks for your prompt action,

[pkhatri]

Hi,

The cost of an OSPF interface is actually 10^8/(interface bandwidth)...

For EIGRP, the metric is calculated depending on the minimum bandwidth as well as the cumulative delay along the path. There are other components of the EIGRP metric, but only bandwidth and delay are used by default. The actual formula is outlined in this document:

http://www.cisco.com/en/US/customer/tech/tk365/technologies_white_paper09186a0080094cb7.shtml#eigrpmetrics

As an example, say you have a path between two points, A and Z:

A -- B -- C -- Z

To determine the metric, EIGRP will use the minimum of the following bandwidths: bandwidth(A-B), bandwidth(B-C), bandwidth(C-Z). In addition, it will add up the following delays: delay(A-B), delay(B-C), delay(C-Z). Then, it will plug those values into the above formula.

Hope that helps - pls do rate the post if it does.

Paresh

[pkhatri]

Hi again,

I've attached a simple Microsoft Excel-based EIGRP metric calculator that I've written.

Simply plug in the minimum Bandwidth and cumulative delay into cells B8 and C8 respectively, and you will get the metric in cell D8.

Hope that helps - pls do rate the posts if it does.

Paresh

[Mohamed Sobair]

Hi Paresh,

Thanks so much for the helpful post......

[pkhatri]

Looks like we posted responses at the same time.... You might want to try out that metric calculator I attached to my last post. It is quite accurate and also performs correct rounding-down etc to give you the exact metric as you would see on a router.

Paresh