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Home > Archive > CCNP > June 2001 > Can someone clarify this paragraph for me?
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Can someone clarify this paragraph for me?
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| In CiscoPress "BSCN book" ( By Tera), on Page 269-270, it gives the following example followed by an explaination, here is what it says:
Path 1: 1100
Path 2: 1100
Path 3: 2000
Path 4: 4000
“ By default, the router will route to the destination using both paths 1 and 2. To load-balance over paths 1,2,and 3, you would use the variance 2 command because 1100 * 2 = 2200, which is greater than the metric through path 3. Similarly, to also include path 4, you would issue the variance 4 command under the routing protocol configuration mode.”
My question is, why when we include paths 1,2 and 3, we didn’t use variance 3 as we did when we include path 4 by issuing variance 4 ?
I though that the number that follows the variance command should be equal to number of equal load-balancing we have.
Thanks in advance. | |
| strikeattack 2001-06-09, 6:27 pm |
| This is my description of the VARIANCE command:
VARIANCE [MULTIPLIER BETWEEN 1 and 128]; this command allows the router to load balance across different routes that have different metrics. The default value is 1. The multiplier defines the range of metric values that will be accepted for load balancing. For example, if the feasible distance to a destination network is 50, and the variance integer is set to 2, a router will load balance routes up to MAXIMUM-PATHS and route traffic through any link that has a feasible distance between 50 and 100.
The number that you specify is a multiplier of the administrative distance that you specify. For example, in your situation, if you wanted to load balance across all paths, you would issue a VARIANCE 4 command. If you wanted only the first three paths, you would issue a VARIANCE 2 command.
I hope this helps! | |
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| It helps alot.
Thanks strikeattack. | |
| dmaftei 2001-06-09, 7:54 pm |
| quote:
Originally posted by strikeattack
The number that you specify is a multiplier of the administrative distance that you specify.
The variance has nothing to do with the administrative distance. As you said in the first paragraph, it is a multiplier applied to a metric.
fit, in your example you have four routes, with metrics 1100, 1100, 2000 and 4000. Note that these are routes learned by one routing protocol; the variance doesn't make sense across protocols. When you issue the variance 2 command, you tell your router: for a certain route, take the lowest cost (1100 in your case), multiply it by 2 (you get 2200), and consider for load balancing all the routes with metrics that fall in the range you just got: 1100 through 2200. Therefore, with the variance 2 command, Path 1, Path 2 and Path 3 fall in the range you obtained.
With a variance 3 command you get the range 1100 through 3300, so the paths considered for load balancing will be 1, 2 and 3.
With a variance 4 command you get the range 1100 through 4400, so the paths considered for load balancing will be 1, 2, 3 and 4.
Does this clarify the issue for you?
Cheers! | |
| strikeattack 2001-06-09, 8:45 pm |
| Thanks for the correction, I meant to type feasible distance, not administrative distance. Oh, and one more thing that I forgot to mention that very well may pop up on test. Some protocols, like EIGRP for example, will AUTOMATICALLY load balance across 4 equal cost paths, and a maximum of 6. | |
| dmaftei 2001-06-09, 8:48 pm |
| You're welcome, Mike! | |
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| dmaftei, thanks very much.
strikeattack, thanks alot. | |
| RTRGOD 2001-06-16, 8:28 pm |
| Also, don't forget that VARIANCE only works with IGRP and EIGRP. |
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