[c-nsp] EIGRP feasible successors

Howard, Christopher Christopher-Howard at utc.edu
Fri Oct 3 14:19:38 EDT 2014


Ok, that makes sense and thanks for your response.

I think partly what is confusing me is that I used the csr1000v to bring
up the same topology in a lab.  The distances are obviously different, but
the feasible distance of the successor is equal to the advertised distance
on the 3rd route, as it is in production.  However, no amount of variance
will cause that 3rd route to be installed, and I tried really high
variances in the lab where I'm only using a variance of 2 in production.

Everything looks identical between real and lab except for the fact that
the real ones are adding the third route and the lab isn't and I was
unsure why.

For now I've removed the variance so the 3rd route will go away.  Nothing
like trying to pump traffic down a 1G link when you have 2 10G links
sitting idle. :)

-Christopher



On 10/3/14, 2:07 PM, "Bruce Pinsky" <bep at whack.org> wrote:

>On 10/3/2014 7:03 AM, Howard, Christopher wrote:
>> I'm hoping for some clarification as to whether I'm incorrect or my
>>switch
>> is incorrect.
>> 
>> I have a switch (4500X) that has 3 different routes to another switch.
>> Two routes traverse 10G links and the other is a 1G link.  However,
>> traffic is getting transferred through the 1G link thanks to EIGRP.  I
>> think EIGRP is wrong.
>> 
>> 
>> First, the topology table says it has 3 successors, but only lists 2.  I
>> have filtered out to just one subnet, but there are others this way.
>> 
>> switch#sh ip eigrp vrf green topology
>> P 172.1.2.0/24, 3 successors, FD is 3072
>>         via 10.1.1.6 (3072/2816), Vlan910
>>         via 10.1.9.6 (3072/2816), Vlan2910
>> 
>> 
>> If I tell it to show me all links in the topology table, I can see the
>> third route.
>> 
>> switch#sh ip eigrp vrf green topology all-links
>> P 172.1.2.0/24, 3 successors, FD is 3072, serno 25436
>>         via 10.1.1.6 (3072/2816), Vlan910
>>         via 10.1.9.6 (3072/2816), Vlan2910
>>         via 10.1.5.3 (3328/3072), Vlan1910
>> 
>> 
>> Now, as I understand it, the first two routes are successors because
>>they
>> have the lowest feasible distance.  The third route should not be
>> considered a feasible successor because the advertised distance is equal
>> to the feasible distance of the successors (the feasibility condition
>> explicitly states less than).  However, it appears that the switch is
>> considering this third route as a successor.
>> 
>> 
>> And worse, due to the use of the variance command, the switch is using
>>the
>> third route as the active one.
>> 
>> switch#sh ip route vrf green 172.1.2.0
>>   Last update from 10.1.9.6 on Vlan2910, 3w5d ago
>>     10.1.9.6, from 10.1.9.6, 3w5d ago, via Vlan2910
>>       Route metric is 3072, traffic share count is 40
>>   * 10.1.5.3, from 10.1.5.3, 3w5d ago, via Vlan1910
>>       Route metric is 3328, traffic share count is 37
>>     10.1.1.6, from 10.1.1.6, 3w5d ago, via Vlan910
>>       Route metric is 3072, traffic share count is 40
>> 
>> 
>> I can remove the variance from EIGRP so that route will drop from the
>> route table, but am I incorrect in thinking that route should not be a
>> feasible successor in the first place?
>> 
>> 
>
>No, there are two conditions that must be met for variance to work.  The
>first is this one:
>
>"The route must be loop-free. This condition is satisfied when the
>reported
>distance is less than the total distance or when the route is a feasible
>successor."
>
>I think you are getting bit by the first part of the condition because the
>computed total distance is 3328 and the reported distance is 3072.
>
>Now variance works as multiples of the metric of the best route.  So, if
>you have any variance other than 1, that third path is going to be
>installed.
>
>-- 
>=========
>bep
>
>




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