how is share count arrived at?

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Mar 10th, 2008
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according to Cisco's document:

http://www.cisco.com/warp/public/103/eigrp-toc.html


share count is a function of the FD's for each available route: "How does the router divide the traffic between these paths? It divides the metric through each path into the largest metric, rounds down to the nearest integer, and uses this number as the traffic share count."


I have a switch on one side of an MPLS network that has two pathways to a switch on the other side. When I issue the show ip route for the network on the other end, I get:


Routing entry for 10.1.0.0/16

Known via "eigrp 64", distance 170, metric 563200

Tag 1803, type external

Redistributing via eigrp 64

Last update from 10.64.1.2 on Vlan1, 4w4d ago

Routing Descriptor Blocks:

* 10.64.1.3, from 10.64.1.3, 4w4d ago, via Vlan1

Route metric is 563200, traffic share count is 40

Total delay is 21000 microseconds, minimum bandwidth is 100000 Kbit

Reliability 255/255, minimum MTU 1500 bytes

Loading 1/255, Hops 1

Route tag 65000

10.64.1.2, from 10.64.1.2, 4w4d ago, via Vlan1

Route metric is 1070592, traffic share count is 21

Total delay is 25000 microseconds, minimum bandwidth is 5945 Kbit

Reliability 255/255, minimum MTU 1500 bytes

Loading 1/255, Hops 1

Route tag 1803


Notice my share count is 21 and 40. But no ammount of dividing the stated metrics into each other yeilds those two numbers.


So exactly what numbers are being used to arrive at the share count?

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Load Balancing

EIGRP puts up to four routes of equal cost in the routing table, which the router then load-balances. The type of load balancing (per packet or per destination) depends on the type of switching being done in the router. EIGRP, however, can also load-balance over unequal cost links.


Note: Using max-paths, you can configure EIGRP to use up to six routes of equal cost.


Let us say there are four paths to a given destination, and the metrics for these paths are:


path 1: 1100


path 2: 1100


path 3: 2000


path 4: 4000


The router, by default, places traffic on both path 1 and 2. Using EIGRP, you can use the variance command to instruct the router to also place traffic on paths 3 and 4. The variance is a multiplier: traffic will be placed on any link that has a metric less than the best path multiplied by the variance. To load balance over paths 1, 2, and 3, use variance 2, because 1100 x 2 = 2200, which is greater than the metric through path 3. Similarly, to also add path 4, issue variance 4 under the router eigrp command. Refer to How Does Unequal Cost Path Load Balancing (Variance) Work in IGRP and EIGRP? for more information.


How does the router divide the traffic between these paths?


It divides the metric through each path into the largest metric, rounds down to the nearest integer, and uses this number as the traffic share count.


router# show ip route 10.1.4.0

Routing entry for 10.1.4.0/24

Known via "igrp 100", distance 100, metric 12001

Redistributing via igrp 100, eigrp 100

Advertised by igrp 100 (self originated)

eigrp 100

Last update from 10.1.2.2 on Serial1, 00:00:42 ago

Routing Descriptor Blocks:

* 10.1.2.2, from 10.1.2.2, 00:00:42 ago, via Serial1

Route metric is 12001, traffic share count is 1

Total delay is 20010 microseconds, minimum bandwidth is 1000 Kbit

Reliability 1/255, minimum MTU 1 bytes

Loading 1/255, Hops 0For this example, the traffic share counts are:


for paths 1 and 2: 4000/1100 = 3


for path 3: 4000/2000 = 2


for path 4: 4000/4000 = 1


The router sends the first three packets over path 1, the next three packets over path 2, the next two packets over path 3, and the next packet over path 4. The router then restarts by sending the next three packets over path 1, and so on.



FROM:


http://www.cisco.com/warp/public/103/eigrp-toc.html




suelange Mon, 03/10/2008 - 13:58
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Yes, I read all that. That's the quote I pasted in my original question.


The problem is, in real life you aren't working with simple numbers like the example.


I cut and pasted the output from my switch in the original post. Where does it possibly get 40 and 21 for share counts given the metrics it is showing?

Thank you.

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