From: Mike Williams (ccie2be@swbell.net)
Date: Thu Jun 12 2003 - 17:35:51 GMT-3
When you use the variance command, EIGRP will take the minimum metric
(best) metric for a given destination, then it will consider other
routers with a higher metric, as long as the higher metric is < (best
metric) * variance.
Let's say you have three possible routes to the 10.1.1.0/24 network and
they have EIGRP metrics of 20, 35, and 50. Normally (without the
variance) the route with the metric of 20 would be installed and that's
it. If you go and configure a variance of 2, now EIGRP will install the
routes with metric of 20 and 35 (since 35 is less than or equal to 20 x
2 = 40). It will then proceed to send 2 packets over the metric 20 link
and 1 over the metric 35 link and back and forth.
Here is the output from a site we have. This site uses variance to load
balance across a 384kbps ADSL circuit and a 768Kbps Frame relay circuit.
Routing entry for 10.1.0.0/16
Known via "eigrp25", distance 90, metric 3459328, type internal
Redistributing via eigrp 25
Last update from 10.63.0.81 on ATM0/0.35, 00:00:05 ago
Routing Descriptor Blocks:
10.63.0.197, from 10.63.0.197, 00:00:05 ago, via Serial0/0.1
Route metric is 3459328, traffic share count is 2
Total delay is 4930 microseconds, minimum bandwidth is 768 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 10/255, Hops 1
* 10.63.0.81, from 10.63.0.81, 00:00:05 ago, via ATM0/0.35
Route metric is 6792704, traffic share count is 1
Total delay is 4930 microseconds, minimum bandwidth is 384 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 31/255, Hops 1
The thing to note here is that it shows the "traffic share count". As
you can see the 768Kbps link has a share count of 2 and the 384 has a
share count of one. This means it will send 2 packets down the 768Kbps
pipe then one down the 384Kbps pipe, just as planned. Just be careful.
Many times, depending on the interface, just because the bandwidth of
one link is 2x or 3x the bandwidth of the smaller link, that doesn't
mean a variance of 2 or 3 will load balance properly. This is because
the EIGRP metric depends on the interface types. So if you have a
768Kbps ATM PVC and a 768Kbps Serial connection, the delay on the ATM is
80 and the delay on the serial is 20000(!!) This make a noticable
difference in the EIGRP metric. So what you need to do is to do a 'sho
ip eigrp topology <network> <subnet mask>' and analyze the metrics over
the difference links. Then you must adjust the delay so that the
metrics are the multiplier you want. So, if you have 2 links and one is
2 times the bandwidth of the other but the delays are way off, you may
want to change the delay on one interface so they are the same on both.
This will usually make the metrics within 2x of each other, and a
variance of 2 will work great then.
Good Luck!
Mike W.
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
John Matijevic
Sent: Thursday, June 12, 2003 12:18 PM
To: ccielab@groupstudy.com
Subject: eigrp and variance command
Hello,
I want to better understand the variance command, I know that it is used
to provide unequal load balancing among mutliple paths, and you specify
by variance 1-128 metric varinace multiplier. I have read the doc cd
regarding this and could not get more specific info that I need.
Basically I would like to know how to verify that the command took
place, do I need to look at the metrics on the routing tables? Also how
is the varinace applied to the metric what is the formula? I am
basically just entering the command and assuming thats its load
balancing but im not sure how to verify. Any help regarding the matter
is greatly appreciated. Sincerely, Matijevic
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