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# EIGRP VARIANCE

Posted on 2011-10-26
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I would like to understand the purpose of the EIGRP Variance command.
I did some reading, and it says the purpose of it is to Loadbalance EIGRP Traffic.

Let's say we have the following Routers:

A--10->B--20->C  FD=30
A--20->D--30->C  FD=50
A--30->E--30-->C FD=60

how does Variance command come into play in this case?

thanks

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Question by:jskfan

LVL 17

Accepted Solution

Varience deals with unequal cost load balancing. when configuring the variance, EIGRP takes the varience that is configured and multiplies it by 20. In your example we would need to configure a varience of 30. 2 * 30 = 60.  This will set the minimum metric to reach router C to 60.  This means that all routes that are less than or equal to 60 will qualify to participate in unequal cost loadbalancing.

However, in your example A>B>C is the Successor and A>D>C is the Feasible Successor. there for only these two routes will be used when loadbalancing. Since A>E>C is not a Feasible Successor it will not participate in the loadbalancing.
0

LVL 17

Assisted Solution

Also, the router will load balance proportionally over unequal cost paths.
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Author Comment

when you use the variance:
30. 2 * 30 = 60

Now you are making the cost for A--10->B--20->C FD=60
and since
A--20->D--30->C  FD=50

would thisA--20->D--30->C FD=50   become the successor and the A--10->B--20->C FD=60 becomes the Feasible successors ? seeing that 50<60 ?

Also A--30->E--30-->C FD=60 , why this woulfd not be a Feasible Successors since it has the same FD as the old successor A--10->B--20->C FD=60

Thanks

0

LVL 17

Assisted Solution

first of all it is not 30.2 * 30.........it is 2 *30  The dot is a period.

Also notice I made a mistake in my previous post.  it was meant to be a configure a varience of 3, not configure a varience of 30.

----->would thisA--20->D--30->C FD=50   become the successor and the A--10->B--20->C FD=60 becomes the Feasible successors ? seeing that 50<60 ?

no, A>B>C is still the successor and A>D>C is still the feasible successor.  entering the variance 2 command only instructs the router to send traffic accross links that have a cost less than or equal to 60 but still meet the requirements of successor and feasible successor.

----->Also A--30->E--30-->C FD=60 , why this woulfd not be a Feasible Successors since it has the same FD as the old successor A--10->B--20->C FD=60

Again, the variance command doesn't change the existing cost, only tells the router which links to send traffic over.  Successor and feasible successor remain the same.
0

LVL 17

Assisted Solution

LOL ignor the following statement from my previous post.  Must not have gotten enough sleep last night :-D

----->Also notice I made a mistake in my previous post.  it was meant to be a configure a varience of 3, not configure a varience of 30.

0

LVL 17

Assisted Solution

Use the variance n command in order to instruct the router to include routes with a metric of less than n times the minimum metric route for that destination. The variable n can take a value between 1 and 128. The default is 1, which means equal cost load balancing.
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Author Comment

Let me Recapitulate:

A--10->B--20->C  FD=30
A--20->D--30->C  FD=50
A--30->E--30-->C FD=60

with variance of 2 it will become
A--20->D--30->C  FD=50
(--10->B--20->C)*2  FD=60

A--30->E--30-->C FD=60

You are saying the Successor and FS  don't change. which means:
(--10->B--20->C)*2  FD=60  = Successor
A--20->D--30->C  FD=50 = feasible Successor

This means that all routes that are less than or equal to 60 will qualify to participate in unequal cost loadbalancing.
SO: A--20->D--30->C  FD=50 AND A--30->E--30-->C FD=60 will qualify.

Correct ??

0

LVL 17

Assisted Solution

Before variance:
A--10->B--20->C metric=30
A--20->D--30->C metric=50
A--30->E--30-->Cmetric=60

A variance of 3 means that any route with a metric less than the best metric (minimum metric) to a destination multiplied by the variance, and meets the feasibility condition, will be added to the routing table.

A--10->B--20->C metric=30
A--20->D--30->C metric=50
A--30->E--30-->Cmetric=60

The command doesn't change the metrics or FD of the routes but instead tells the router to include paths to a destination that meet the conditions. route A>E>C is not included in the routing table because it doesn't meet the feasibility conditions.
0

LVL 17

Assisted Solution

Hi,

Variance is used by EIGRP to decide which routes are placed from the EIGRP topology table into the routing table. With a variance of 1 which is the default, only routes which have the lowest metric are placed into the routing table. If there are multiple paths with this same lowest metric then these are also placed into the routing table and load balanced equally.

If you increase the variance to 2, then EIGRP will multiply the lowest cost path by 2 and will place routes which are less than or equal to this into the routing table. EIGRP will only consider paths which it determines to be loop free i.e. those which meet the Feasibility Condition. EIGRP will load balance unequal cost paths proportionally.
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Author Comment

Before variance:
A--10->B--20->C metric=30
A--20->D--30->C metric=50
A--30->E--30-->Cmetric=60

After variance 3 is added I guess it will be:
A--10->B--20->C metric=90
A--20->D--30->C metric=50
A--30->E--30-->Cmetric=60

in this case either the Variance 3 is too high or thelinks below will also be added to the topolgy table and there will be an unequal cost balancing between all 3 links:

A--20->D--30->C metric=50
A--30->E--30-->Cmetric=60

0

LVL 17

Assisted Solution

Before variance:
A--10->B--20->C metric=30
A--20->D--30->C metric=50
A--30->E--30-->Cmetric=60

A--10->B--20->C metric=30
A--20->D--30->C metric=50
A--30->E--30-->Cmetric=60

The metrics DO NOT change. The variance command only tells the router which links to include in the unequal cost loadbalancing that also meet the feasibility condition.

Yes, in this scenario variance 3 is too high but the result would still be the same.  Since A > E > C does not meet the feasibility condition it would not be included anyway.
0

Author Closing Comment

Thank you Guys1
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