IEWB RSv5 Advanced Technology Labs - EIGRP Unequal Cost Load Balancing

Hi All,

As I'm doing this lab, I noticed a strange behavior with the 'variance' command. Its very possible that I'm missing something, which is why I wanted to ask here so if someone can point out the cause of this behavior or if otherwise the documentation of this command is not accurate.

The issue is around the fact that for a route to be affected by the variance command it needs to pass the feasibility condition, i.e it is a feasible successor in the EIGRP topology table.

Moving on to the lab, Im following everything as is and did the math to know what delay I should configure on R6's VLAN 67 and VLAN 146 interfaces. Keep in mind that I'm using EVE-NG so the delay and interfaces different than whats in the Lab.

Focusing on R6 and before changing anything:

R6(config-subif)#do sh ip eigrp top all | sec 155.1.9.0/24
P 155.1.9.0/24, 1 successors, FD is 76800, serno 662
via 155.1.67.7 (76800/51200), Ethernet0/1.67


R6(config-subif)#do sh ip eigrp top 155.1.9.0 255.255.255.0
EIGRP-IPv4 Topology Entry for AS(100)/ID(150.1.6.6) for 155.1.9.0/24
State is Passive, Query origin flag is 1, 1 Successor(s), FD is 76800
Descriptor Blocks:
155.1.67.7 (Ethernet0/1.67), from 155.1.67.7, Send flag is 0x0
Composite metric is (76800/51200), route is Internal
Vector metric:
Minimum bandwidth is 10000 Kbit
Total delay is 3000 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 2
Originating router is 150.1.9.9

Now, I'm going to change the delay on VLAN67 to 300 and VLAN146 to 2100.

Here is where things get interesting:

R6(config-subif)#do sh ip eigrp top all | sec 155.1.9.0/24
P 155.1.9.0/24, 2 successors, FD is 76800, serno 701
via 155.1.67.7 (128000/51200), Ethernet0/1.67
via 155.1.146.1 (640000/102400), Ethernet0/1.146


R6(config-subif)#do sh ip eigrp top | sec 155.1.9.0/24
P 155.1.9.0/24, 2 successors, FD is 76800
via 155.1.67.7 (128000/51200), Ethernet0/1.67


R6(config-subif)#do sh ip eigrp top 155.1.9.0 255.255.255.0
EIGRP-IPv4 Topology Entry for AS(100)/ID(150.1.6.6) for 155.1.9.0/24
State is Passive, Query origin flag is 1, 2 Successor(s), FD is 76800
Descriptor Blocks:
155.1.67.7 (Ethernet0/1.67), from 155.1.67.7, Send flag is 0x0
Composite metric is (128000/51200), route is Internal
Vector metric:
Minimum bandwidth is 10000 Kbit
Total delay is 5000 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 2
Originating router is 150.1.9.9
155.1.146.1 (Ethernet0/1.146), from 155.1.146.1, Send flag is 0x0
Composite metric is (640000/102400), route is Internal
Vector metric:
Minimum bandwidth is 10000 Kbit
Total delay is 25000 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 4
Originating router is 150.1.9.9


R6(config-router)#do sh ip rou 155.1.9.0
Routing entry for 155.1.9.0/24
Known via "eigrp 100", distance 90, metric 128000, type internal
Redistributing via eigrp 100
Last update from 155.1.67.7 on Ethernet0/1.67, 00:00:08 ago
Routing Descriptor Blocks:
* 155.1.67.7, from 155.1.67.7, 00:00:08 ago, via Ethernet0/1.67
Route metric is 128000, traffic share count is 1
Total delay is 5000 microseconds, minimum bandwidth is 10000 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 2

Now according to the above, the route that is being learned from 155.1.146.1 has an RD of 102400 which is currently higher than the FD 76800 which means it did not satisfy the FC. All the show output above also confirms that as it does not show in the topology table without using all-links keyword or with the exact prefix.

So, no matter how much the value of the variance command is, this route should NEVER be considered by EIGRP as it cannot guarantee that its loop free - correct? well, not in this case...

I'm going now to configure variance of 5.


R6(config-router)#do sh ip rou 155.1.9.0
Routing entry for 155.1.9.0/24
Known via "eigrp 100", distance 90, metric 128000, type internal
Redistributing via eigrp 100
Last update from 155.1.146.1 on Ethernet0/1.146, 00:00:07 ago
Routing Descriptor Blocks:
155.1.146.1, from 155.1.146.1, 00:00:07 ago, via Ethernet0/1.146
Route metric is 640000, traffic share count is 1
Total delay is 25000 microseconds, minimum bandwidth is 10000 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 4
155.1.67.7, from 155.1.67.7, 00:00:07 ago, via Ethernet0/1.67
Route metric is 128000, traffic share count is 5
Total delay is 5000 microseconds, minimum bandwidth is 10000 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 2


R6(config-router)#do sh ip rou | sec 155.1.9.0
D 155.1.9.0/24 [90/640000] via 155.1.146.1, 00:00:17, Ethernet0/1.146
[90/128000] via 155.1.67.7, 00:00:17, Ethernet0/1.67


R6(config-router)#do sh ip eigrp top 155.1.9.0 255.255.255.0
EIGRP-IPv4 Topology Entry for AS(100)/ID(150.1.6.6) for 155.1.9.0/24
State is Passive, Query origin flag is 1, 2 Successor(s), FD is 76800
Descriptor Blocks:
155.1.67.7 (Ethernet0/1.67), from 155.1.67.7, Send flag is 0x0
Composite metric is (128000/51200), route is Internal
Vector metric:
Minimum bandwidth is 10000 Kbit
Total delay is 5000 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 2
Originating router is 150.1.9.9
155.1.146.1 (Ethernet0/1.146), from 155.1.146.1, Send flag is 0x0
Composite metric is (640000/102400), route is Internal
Vector metric:
Minimum bandwidth is 10000 Kbit
Total delay is 25000 microseconds
Reliability is 255/255
Load is 1/255
Minimum MTU is 1500
Hop count is 4

So as you can see, even though that route does not satisfy the FC, EIGRP still decided to use it and install it in the routing table, Even unequally load balancing (ratio is 5 to 1)

Anyone can shed some light on why this is happening, that would be really great.

Thanks in advance.

Sign In or Register to comment.