EIGRP Variance Command Example

This tutorial will demonstrate how you can use the EIGRP variance command to make EIGRP load balance over feasible successors. If you have no idea what I’m talking about then it’s best to read my lesson about EIGRP unequal load balancing first before you continue.

To demonstrate EIGRP load balancing I will use the following topology:

EIGRP Variance Example Topology

The routers above are all running EIGRP. R1 is connected to R2, R3 and R4 using a FastEthernet, Ethernet and Serial link. On the right side you see R5 which has a loopback interface that is configured with network 5.5.5.5 /32. We will enable EIGRP on all interfaces and take a look what path R1 will choose when we want to reach 5.5.5.5 /32.

Let’s enable EIGRP on all routers using the ‘shotgun approach’:

R1,R2,R3,R4 & R5:
(config)#router eigrp 1
(config-router)#no auto-summary
(config-router)#network 0.0.0.0

If you are configuring this yourself, make sure you check that all routers have formed EIGRP neighbor adjacencies before you continue.

Let’s take a look to see what path R1 will choose to reach 5.5.5.5 /32:

R1#show ip route | begin 5.5.5.5
D       5.5.5.5 [90/158720] via 192.168.12.2, 00:00:04, FastEthernet0/0

It will take the path through R2 which makes sense since this is the FastEthernet interface. If you want to see more detailed information you can use the following command:

R1#show ip route 5.5.5.5
Routing entry for 5.5.5.5/32
  Known via "eigrp 1", distance 90, metric 158720, type internal
  Redistributing via eigrp 1
  Last update from 192.168.12.2 on FastEthernet0/0, 00:00:07 ago
  Routing Descriptor Blocks:
  * 192.168.12.2, from 192.168.12.2, 00:00:07 ago, via FastEthernet0/0
      Route metric is 158720, traffic share count is 1
      Total delay is 5200 microseconds, minimum bandwidth is 100000 Kbit
      Reliability 255/255, minimum MTU 1500 bytes
      Loading 1/255, Hops 2

To see exactly why R1 has selected R2 as the successor for this network we’ll have to take a look at the EIGRP topology table:

R1#show ip eigrp topology 5.5.5.5 255.255.255.255
IP-EIGRP (AS 1): Topology entry for 5.5.5.5/32
  State is Passive, Query origin flag is 1, 1 Successor(s), FD is 158720
  Routing Descriptor Blocks:
  192.168.12.2 (FastEthernet0/0), from 192.168.12.2, Send flag is 0x0
      Composite metric is (158720/156160), Route is Internal
      Vector metric:
        Minimum bandwidth is 100000 Kbit
        Total delay is 5200 microseconds
        Reliability is 255/255
        Load is 1/255
        Minimum MTU is 1500
        Hop count is 2
  192.168.14.4 (Serial2/0), from 192.168.14.4, Send flag is 0x0
      Composite metric is (2300416/156160), Route is Internal
      Vector metric:
        Minimum bandwidth is 1544 Kbit
        Total delay is 25100 microseconds
        Reliability is 255/255
        Load is 1/255
        Minimum MTU is 1500
        Hop count is 2
  192.168.13.3 (Ethernet1/0), from 192.168.13.3, Send flag is 0x0
      Composite metric is (412160/156160), Route is Internal
      Vector metric:
        Minimum bandwidth is 10000 Kbit
        Total delay is 6100 microseconds
        Reliability is 255/255
        Load is 1/255
        Minimum MTU is 1500
        Hop count is 2

Above you see the different values for the feasible distance and advertised distance. The lowest feasible distance is 158.720 and it’s the path through R2 which makes it the successor.

R3 and R4 have been selected as feasible successors because their advertised distance of 156.160 is lower than the feasible distance (158.729) of R2.

So far so good, we found the successor and we know that R3 and R4 are feasible successors. If we want to enable load balancing we have to use the following formula:

FD of feasible successor < FD of successor * multiplier

So the feasible distance of the feasible successor has to be lower than the feasible distance of the successor that is multiplied with some value. Let’s look at an example so this makes more sense. Let’s say we want to load balance over R3:

  • Feasible Distance of R2 (successor) = 158720
  • Feasible Distance of R3 (feasible successor) =412160
412160 / 158720 = 2.59

So if I set the multiplier at something higher than 2.59 then R3 will be used for load balancing. The multiplier is configured using the variance command:

R1(config)#router eigrp 1
R1(config-router)#variance 3

Let’s take a look at R1 to see if this has any effect:

R1#show ip route | begin 5.5.5.5
D 5.5.5.5 [90/412160] via 192.168.13.3, 00:00:42, Ethernet1/0
          [90/158720] via 192.168.12.2, 00:00:42, FastEthernet0/0

Above you can see that R1 has installed the path through R3 as well. EIGRP does “unequal” cost load balancing and to see how it shares traffic among the interfaces we have to use another command:

R1#show ip route 5.5.5.5        
Routing entry for 5.5.5.5/32
  Known via "eigrp 1", distance 90, metric 158720, type internal
  Redistributing via eigrp 1
  Last update from 192.168.13.3 on Ethernet1/0, 00:00:40 ago
  Routing Descriptor Blocks:
  * 192.168.13.3, from 192.168.13.3, 00:00:40 ago, via Ethernet1/0
      Route metric is 412160, traffic share count is 23
      Total delay is 6100 microseconds, minimum bandwidth is 10000 Kbit
      Reliability 255/255, minimum MTU 1500 bytes
      Loading 1/255, Hops 2
    192.168.12.2, from 192.168.12.2, 00:00:40 ago, via FastEthernet0/0
      Route metric is 158720, traffic share count is 60
      Total delay is 5200 microseconds, minimum bandwidth is 100000 Kbit
      Reliability 255/255, minimum MTU 1500 bytes
      Loading 1/255, Hops 2

As you can see EIGRP is sharing traffic in a 60:23 proportion, that means the FastEthernet link is used about 2.6 more often than the Ethernet link. What if we also want to use the serial link for load balancing? The feasible distance of R4 (2300416) is quite high. What kind of multiplier do we require to enable this link?

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Forum Replies

  1. Hi Rene,

    If we have three path to destination and we configure variance to use the three path just like in your example here :-

    My question is :-
    The traffic will be shared

    ... Continue reading in our forum

  2. i didn’t understand this
    could you explain more details about this . that will be hard when the numbers are high .
    and i show command its show me share count

    Router#show ip route 2.2.2.2
    Routing entry for 2.2.2.0/24
      Known via "eigrp 1", distance 90, metric 156416, type internal
      Redistributing via eigrp 1
      Last update from 192.168.0.1 on FastEthernet0/0, 00:01:11 ago
      Routing Descriptor Blocks:
        192.168.0.1, from 192.168.0.1, 00:01:11 ago, via FastEthernet0/0
          Route metric is 156416, traffic share count is 240
          Total delay is 5110 microseconds
    ... Continue reading in our forum

  3. Hello Florain

    The FS is a FS only if it is not in the routing table. If it is in the routing table it has the best metric and is thus considered a successor, even if there are more than one such as is the case with EIGRP load balancing.

    ... Continue reading in our forum

  4. Hello Marcel

    All routes learned via EIGRP are contained within the topology table, whether they are actively being used (in the routing table) or not. These routes are examined by EIGRP and only the route with the best metric is placed within the routing table. The rest of the routes still exist in the topology table however.

    So, the routing table will only have a single route to a particular destination by default (unless two routes have an equal metric of course). In order to determine if a route can be used for unequal load balancing, the variance value i

    ... Continue reading in our forum

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