EIGRP variance question

Posted on 2009-07-06
Last Modified: 2012-06-27
For the attached diagram, if variance has been configured on Router 4 to 2, can I say that only "4-5-6" and "4-7-8" will participate in load sharing ? The shortest FD is "4-5-6", with FD = 10 and AD = 5 (from Router5).  AD from Router 3 is 10 which couldn't be a feasible succesor for Router 4 as its FD (10) = AD (10) from router 3. Hence, it couldn't be added as feasilble successor and it don't have any benefit with variance. Am I correct ?

Question by:AXISHK
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Expert Comment

by:Don Johnston
ID: 24784015
Looks like 4-5-6 has an FD of 10.
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Expert Comment

by:Robert Sutton Jr
ID: 24786950
Agreed. And I also concur with what Djohnston said above.   =10

Author Comment

ID: 24790907

My conern is

On R4
  through  R3: FD (20) AD (10)
  through  R5: FD (10) AD(5)
  through  R7: FD (15) AD(5)

So on R4, through R5 will be the successor, and through R7 will be feasible successor. R3 wouldn't be the feasbile successor as its AD (10) is not smaller than the successor (FD=10). If I add varience 2 on R3, both R5 and R7 will have load balancing.

However, should I need to consider R2 ? R2 has FD(5) and AD(0) ? If I take it into the consideration, R2 will also fullfill the requirement as its AD(0) is smaller than the successor (FD(10).  But R2 is behind R3, so can I say only R3 will be considered ?

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Expert Comment

by:Don Johnston
ID: 24791006
> If I add varience 2 on R3, both R5 and R7 will have load balancing.


If you set the variance on R3 to 2, then R3 will use R2 and R4 as equal cost paths to the corp network. This could create some interesting behavior when R3 send a packet to R4 who then sends it right back to R3 (routing loop).

Author Comment

ID: 24791066
Sorry, typo mistake. Variance should set on Router4. So, by putting the variance on R4, only "4-5-6" and "4-7-8" will participate in load sharing, correct ?

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Expert Comment

by:Don Johnston
ID: 24791087
I think 4-3-2 will also be used. The idea is that since 10 is the FD, a variance of 2 makes the target FD 20. Any paths 20 or better are now considered equal.

That said, I've never tested it since usually the metrics are 6 digits or more and getting one that close is pretty unusual.

Author Comment

ID: 24791186
Before adding the variance would path "4-3-2" be a feasible successor for Router 4 ?

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Accepted Solution

Don Johnston earned 500 total points
ID: 24793229
Since the AD is not greater than the FD of the successor, yes.

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