PART 10: EIGRP Load Balancing
Enhanced Interior Gateway Routing Protocol
Load balancing allows routers to distribute traffic across multiple paths to the same destination, improving bandwidth utilization and network resilience. EIGRP offers several approaches to load balancing, including unique capabilities not found in other routing protocols.
Understanding Load Balancing Fundamentals
When multiple paths to the same destination exist, routers use a decision process to select which path(s) to use:
First, compare Administrative Distance (AD) values, selecting the path with the lowest AD
If AD values are equal (same routing protocol), compare metric values
If metric values are equal, install multiple paths in the routing table (automatic load balancing)
EIGRP Load Balancing Methods
EIGRP supports four primary methods for implementing load balancing:
1. Automatic Equal-Cost Load Balancing
This occurs automatically when two or more paths to the same destination have identical Feasible Distance (FD) values. The router installs all equal-cost paths in the routing table and distributes traffic between them.
The maximum number of equal-cost paths can be adjusted using:
# Numbered mode
Router(config-router)# maximum-paths <1-32>
# Named mode
Router(config-router-af-topology)# maximum-paths <1-32>2. Unequal-Cost Load Balancing with Variance
EIGRP's variance feature enables unequal-cost load balancing, a capability unique to EIGRP among IGPs. This allows traffic distribution across paths with different metrics.
Prerequisites for variance:
Must have Feasible Successors in the topology table (backup paths that satisfy the Feasibility Condition)
Backup path's Reported Distance (RD) must be less than the current route's Feasible Distance (FD)
Configuration:
# Numbered mode
Router(config-router)# variance <multiplier>
# Named mode
Router(config-router-af-topology)# variance <multiplier>The variance multiplier works by:
Multiplying the best path's FD by the variance value
Including any paths with metrics less than this multiplied value in the routing table
Traffic distribution is proportional to the metric values (better paths receive more traffic)
For example, with a variance of 2, paths with metrics up to twice the best path's metric will be included in the routing table.
3. Direct Metric Manipulation with Offset-List
Third methods can modify EIGRP metrics to influence load balancing.
Offset lists artificially adjust metrics by adding a specified value to routes matching an access list:
# Numbered mode
Router(config)# access-list 1 permit 192.168.10.0 0.0.0.255
Router(config-router)# offset-list 1 10 in GigabitEthernet0/0
# Named mode
Router(config)# access-list 1 permit 192.168.10.0 0.0.0.255
Router(config-router-af-topology)# offset-list 1 10 in GigabitEthernet0/0Offset lists can be applied:
Inbound or outbound
To specific interfaces or all interfaces
To all routes (using "0" instead of an ACL number)
4. Interface Bandwidth and Delay Adjustment
Directly modifying interface bandwidth and delay values affects metric calculations:
Router(config-if)# bandwidth <value-in-kbps>
Router(config-if)# delay <value-in-tens-of-microseconds>This approach is less recommended because:
It affects all routing protocols using these values
It can create confusion in documentation
Calculating the precise values needed can be challenging
To revert to default values:
Router(config-if)# default bandwidth
Router(config-if)# default delay
Practical Considerations
When implementing EIGRP load balancing:
Traffic distribution methodology: By default, Cisco Express Forwarding (CEF) distributes traffic proportionally based on metrics
Path selection verification: Use
show ip route <prefix>to verify which paths are installedTopology requirements: Feasible Successors must exist for variance to work
Configuration verification: Use
show ip protocolsto verify variance settingsMetric manipulation impact: Changes to bandwidth and delay values affect all routing protocols