How to configure EIGRP Load balancing?
EIGRP supports two types of load balancing which are:
Equal cost load balancing.
Equal cost load balancing is done automatically. Six paths can be configured for load balancing, but there is a default of four paths.
Unequal cost load balancing.
If metrics are different load balancing will not happen automatically. Thus, a variance command must be used to make EIGRP load balance even on unequal metrics.
This can be configured as below.
On the topology above, router R1 needs to be configured so that traffic gets to 10.10.10.0/24
| Network | Neighbor | Feasible Distance | Advertised Distance |
| 10.10.10.0/24 | R2 | 30 | 10 |
| R3 | 20 | 10 | |
| R4 | 45 | 25 |
Router R1 by default will choose R1 => R3route to reach 10.10.10.0/24 because the metric has the lowest Feasible Distance of 20. If variance of 2 is configured on R1, then R1 => R2 is chosen to reach 10.10.10.0/24. As (20 + 10 = 30) < [2 x (FD) = 40]. Router R4 will not be chosen to send traffic to 10.10.10.0/24 and does not form part of load balancing as 25 is greater than 20.
To configure Unequal Load balancing the following commands must be run on router R1.
Router_R1(config)#router eigrp 10
Router_R1(config-router)#variance 2
Router R1(config-router)# maximum-paths 6
DUAL Assignment
What is/are the reported distance and feasible distance(s) of router E to the 10.1.1.0/24 network?
| EIGRP | Neighbor | Feasible Distance | Reported/Advertised Distance |
| 10.1.1.0/24 | E via D | 3 | 2 |
| E via C | 4 | 3 | |
| E via C via D | 5 | 4 | |
| E via D via C | 6 | 5 | |
- Which routers are the successor and feasible successors of router E to 10.1.1.0/24 network?
i. Successor is E via D = 3
ii. Feasible Successor is E via C
| EIGR | Feasible Distance | Reported/Advertised Distance | Topology |
| 10.1.1.0/24 | 3 | 2 | |
| E via D | 4 | 3 | Successor |
| E via C | 5 | 4 | Feasible Successor |
2. What happens when Routers B and D detect a link failure? (Give a step by step and detailed explanation of the recomputation that occurs in the routing information tables)
If a link failure is detected between B and D the following happens
- At router D to the network 10.1.1.0/24 does not have a Feasible Successor because the Advertised Distance (AD) via Router C is greater that the Feasible Distance via Router B, so DUAL performs the following steps.
i. DUAL sets the cost to the network 10.1.1.0/24 as unreachable usually a -1 represents unreachable.
ii. Advertised Distance is greater that Feasible Distance means there is no Feasible Successor saved in the topology table, there for the route changes from a passive state to active. In this activated state the router sends queries to all neighbors looking for a new successor.
iii.DUAL sends a query to Routers C and E to find an alternative path to the network.
iv. DUAL then marks Routers C and E as having query pending. - At Router E, marks the path to the network via Router D as unusable.
- At Router C, Dual marks the path C-D to network 10.1.1.0/24 as unusable.
Step 4
- Router D.
i. DUAL gets a reply from Router C that there is no change in the path to network 10.1.1.0/24.
ii. It removes the query flag from Router C
iii. DUAL remains active on the network waiting for a reply to its query from Router E. - There is no Feasible Successor at Router E because the Advertised Distance from Router C is not less that the Feasible Distance.
i. DUAL generates a query to Router C.
ii. DUAL marks Router C as query pending.
3. Dual mark path to network 10.1.1.0/24 via Router E as unusable at Router C.i. DUAL stays active on network 10.1.1.0/24 waiting for reply from Router E at Router D.
ii. Router E:
b. It removes the query flag from Router C.
a. DUAL gets a reply from Router C saying there is no change.
c. Calculates a new Feasible Distance and installs a new successor route in the topology table.
d.Then it changes the status of the network 10.1.1.0/24 from active to passive(Converged)
Then these steps happen as follows
- DUAL gets a reply from Router E.
- Then removes the query flag from Router E.
- Calculates the new Feasible Distance.
- Installs new successor routes in the topology table. Two routes C and E have same Feasible Distance and are all marked as successor.
- Changes route status to network 10.1.1.0/24 from active to passive.
Step 7
- Two successor routes are saved in the topology table for network 10.1.1.0/24 and they are both listed in the routing table and equal-cost load balancing is in effect.
- The network is stable and converged.
References
Balchunas, A. (2007). Enhanced Interior Gateway Routing Protocol. EIGRP v1.31, 1(1), 1. Retrieved from http://www.routeralley.com/guides/eigrp.pdfDomzał, J., Duliński, Z., Kantor, M., Rząsa, J., Stankiewicz, R., Wajda, K., & Wójcik, R. (2015). A survey on methods to provide multipath transmission in wired packet networks. Computer Networks, 77, 18–41. https://doi.org/10.1016/j.comnet.2014.12.001
Teare, D. (2010). Implementing Cisco IP Routing (ROUTE) Foundation Learning Guide. Cisco Press. https://doi.org/10.1017/CBO9781107415324.004
Wallace, K. (2014). CCNP Routing and Switching ROUTE 300-101 Offical Cert Guide.
