EIGRP Variance Not Working

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Jul 18th, 2010
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Hello,


I have a home lab setup and working towards the CCNP.  I have six 1841 routers connected via serial links in a big circle.  Very basic configs on each one and I have EIGRP configured on each devices as well.


I can route just fine.  I can pass traffic to all neighbors and all links are up/up.  Problem is I was hoping to load balance in both directions (no reason just playing with the configs).  I set the variance command to 7 at first, then 10, then 128.  Nothing seemed to work.  As you can see some routes load balance, but not all of them will.


Here are the topology and route tables:



Router2#show ip eigrp top
IP-EIGRP Topology Table for AS 1


Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,
       r - Reply status


P 10.0.1.4/30, 1 successors, FD is 2169856
         via Connected, Serial0/1/0
P 10.0.3.0/24, 1 successors, FD is 28160
         via Connected, FastEthernet0/0.1
P 10.0.1.0/30, 1 successors, FD is 2169856
         via Connected, Serial0/1/1
P 10.0.1.8/30, 1 successors, FD is 2681856
         via 10.0.1.6 (2681856/2169856), Serial0/1/0
P 10.0.1.12/30, 1 successors, FD is 3193856
         via 10.0.1.6 (3193856/2681856), Serial0/1/0
P 10.0.1.16/30, 1 successors, FD is 3193856
         via 10.0.1.1 (3193856/2681856), Serial0/1/1
P 10.0.1.24/30, 2 successors, FD is 3196416
         via 10.0.1.1 (3196416/2684416), Serial0/1/1
         via 10.0.1.6 (3196416/2684416), Serial0/1/0
P 10.0.4.0/24, 1 successors, FD is 2172416
         via 10.0.1.6 (2172416/28160), Serial0/1/0
P 10.0.5.0/24, 1 successors, FD is 2684416
         via 10.0.1.6 (2684416/2172416), Serial0/1/0
P 10.0.6.0/24, 2 successors, FD is 3196416
         via 10.0.1.1 (3196416/2684416), Serial0/1/1
         via 10.0.1.6 (3196416/2684416), Serial0/1/0
P 10.0.1.20/30, 1 successors, FD is 2681856
         via 10.0.1.1 (2681856/2169856), Serial0/1/1
P 10.0.2.0/24, 1 successors, FD is 2172416
         via 10.0.1.1 (2172416/28160), Serial0/1/1





Router2#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
       i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
       * - candidate default, U - per-user static route, o - ODR
       P - periodic downloaded static route

Gateway of last resort is not set

     10.0.0.0/8 is variably subnetted, 12 subnets, 2 masks
C       10.0.1.0/30 is directly connected, Serial0/1/1
C       10.0.1.4/30 is directly connected, Serial0/1/0
D       10.0.1.8/30 [90/2681856] via 10.0.1.6, 00:22:13, Serial0/1/0
D       10.0.1.12/30 [90/3193856] via 10.0.1.6, 00:22:11, Serial0/1/0
D       10.0.1.16/30 [90/3193856] via 10.0.1.1, 00:24:39, Serial0/1/1
D       10.0.1.20/30 [90/2681856] via 10.0.1.1, 00:25:26, Serial0/1/1
D       10.0.1.24/30 [90/3196416] via 10.0.1.1, 00:23:46, Serial0/1/1
                     [90/3196416] via 10.0.1.6, 00:22:11, Serial0/1/0
D       10.0.2.0/24 [90/2172416] via 10.0.1.1, 00:25:26, Serial0/1/1
C       10.0.3.0/24 is directly connected, FastEthernet0/0.1
D       10.0.4.0/24 [90/2172416] via 10.0.1.6, 00:22:12, Serial0/1/0
D       10.0.5.0/24 [90/2684416] via 10.0.1.6, 00:22:11, Serial0/1/0
D       10.0.6.0/24 [90/3196416] via 10.0.1.1, 00:23:46, Serial0/1/1
                    [90/3196416] via 10.0.1.6, 00:22:11, Serial0/1/0

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Richard Burts Sun, 07/18/2010 - 14:36
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There is not enough information in your post to give a complete answer and explanation. show ip eigrp topology will show the entries in the table that are being used but will not show entries that it received from neighbors but is not using. And we would need to see at least some of these entries to fully answer your question. You might try show ip eigrp topology all, which will show all entries (but that would be a lot of detail). A better thing would be to post the output of show ip eigrp topology x.x.x.x m.m.m.m (where x is the subnet/network and m is the mask). This will show all the entries for a particular subnet/network and would give us what we need.


Based on what is in your post I offer these observations:

- the 2 routes that do have multiple entries are doing equal cost load sharing, so variance is not at work with these.

- if you try variance and it is not including the other prefix the most common cause is that the other path does not qualify as a feasible successor. Variance can only put a prefix into the table if the prefix does qualify as a feasible successor.


HTH


Rick

west33637 Mon, 07/19/2010 - 12:44
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Hello. The variance command helps install unequal metric routes into the routing table. The actual load-balacing is done by the routers forwarding mechanism.


For the routes that have only one successor in the EIGRP topology. Do a more specific show command to find out why. for example


show ip eigrp topology 10.0.1.8 255.255.255.252


This output will show you the feasible distance advertised by the successor.  the feasible distance is 2681856. If there are no other neighbors advertising a metric equal to or lower than 2681856, they will not be installed as successors and therefore will not be installed in the routing table.


If you now configure the variance as 128 - any neighbor router that is advertising a metric lower than or equal to 128 x 2681856 (variance x feasible distance) will be installed as successors and also in the routing table. the reason you have only one successor in the above instance is because you do not have any other router that is advertising a metric equal to or lower than the variance multiplied by the feasible distance (128 x 2681856).



In the case of the routes that did do load balancing - do the same command


show ip eigrp topology 10.0.1.24 255.255.255.252


In this case the feasible distance is 3196416. In this case you would see 2 successors whether you configure the variance command or not because you have 2 routers advertising a metric that is lower or equal to the feasible distance. With your variance configured at 128, any route that is advertising a metric lower than or equal to (128 x 3196416) will be installed as a successor and therefore installed in the routing table. This is how variance helps to install unequal cost metrics into the routing table. so while to feasible distance is 3196416, the variance 128 command allows a router that is advertising a much less desirable metric (in this case 128 x 3196416) to be installed into the routing table.


Once the unequal cost metric paths are installed into the routing table, CEF does the actual load balancing between the paths based on the ratio of the metric they advertise.


Please remember to rate this post if it helps.

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