Open Shortest Path First (OSPF) is a widely used link-state routing protocol in enterprise networks, known for its fast convergence and scalability. One of its powerful features is the ability to perform load balancing over multiple paths to the same destination, provided those paths have equal costs (metrics). This is commonly referred to as Equal-Cost Multipath (ECMP) routing. In this blog post, we’ll dive into how OSPF handles ECMP by default, why you might want to adjust it, and how to configure the maximum number of next hops using Cisco IOS commands. Whether you’re optimizing traffic distribution or troubleshooting uneven load balancing, this guide will help you get it right.
What is ECMP in OSPF?
Before we get into the details, let’s clarify what ECMP means in the context of OSPF. When OSPF calculates the shortest path to a destination using Dijkstra’s algorithm, it might discover multiple paths with the exact same cost. Instead of picking just one, OSPF can install all of them into the routing table, allowing the router to distribute traffic across these paths. This improves bandwidth utilization, provides redundancy, and enhances overall network performance.
However, OSPF doesn’t install an unlimited number of these paths. There’s a configurable limit to prevent overwhelming the router’s forwarding engine or causing instability in very large networks.
The Default Behavior: Maximum of 4 Equal-Cost Paths
By default, Cisco IOS limits OSPF to installing up to 4 equal-cost paths per destination in the routing table.community.cisco.com+5 more This means if there are more than 4 paths with identical metrics, OSPF will only select and use 4 of them for load balancing. The remaining paths are ignored, even if they’re equally viable.
Why 4? It’s a balance between performance and resource consumption. Installing too many paths can increase the complexity of the forwarding information base (FIB) and potentially lead to higher CPU and memory usage on the router. For most networks, 4 paths provide sufficient load balancing without overcomplicating things.
To verify the current setting on your router, you can use the show ip protocols command. Look for the “Maximum paths” value under the OSPF section—it should say 4 if unchanged.
When and Why to Increase the Maximum Paths
In larger or more complex topologies—such as data centers, service provider networks, or environments with redundant links—you might have more than 4 equal-cost paths to a destination. Limiting to 4 could underutilize available bandwidth or create bottlenecks. Increasing the maximum allows OSPF to leverage all available paths, distributing traffic more evenly and improving throughput.
Common scenarios include:
- High-redundancy designs: Multiple parallel links between routers for fault tolerance.
- Traffic engineering: Ensuring balanced load across diverse paths.
- Scalability testing: In lab environments or when upgrading hardware that supports more paths.
Keep in mind potential downsides:
- Hardware limitations: Not all Cisco platforms support the maximum value equally. Older routers might experience performance hits.
- Uneven load balancing: OSPF uses per-packet or per-destination hashing, which might not distribute traffic perfectly if flows are uneven.
- Interaction with other protocols: This setting is OSPF-specific and doesn’t affect protocols like EIGRP or BGP unless configured separately.
Configuring the Maximum Paths in OSPF
To adjust this limit, use the maximum-paths command under the OSPF router configuration mode. The syntax is straightforward:
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router ospf <process-id>
maximum-paths <number>
- <process-id>: The OSPF process number (e.g., 1).
- <number>: The maximum number of paths, ranging from 1 to a platform-dependent maximum (typically up to 16, but up to 32 on newer IOS XE releases).freeccnaworkbook.com+4 more
Example Configuration
Suppose you have a router with OSPF process 1, and you want to allow up to 8 equal-cost paths:
- Enter global configuration mode: textCollapseWrapCopy
configure terminal - Enter OSPF configuration mode: textCollapseWrapCopy
router ospf 1 - Set the maximum paths: textCollapseWrapCopy
maximum-paths 8 - Exit and save: textCollapseWrapCopy
endwrite memory
After applying this, OSPF will recalculate routes and install up to 8 paths if available. You can verify with show ip route <destination> to see multiple next hops listed, or show ip ospf for process details.
Reducing Paths for Testing or Control
You can also decrease the value below 4 (down to 1) if you want to disable load balancing entirely or force traffic over fewer paths. For instance:
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maximum-paths 1
This effectively turns off ECMP for that OSPF process.
Platform-Specific Notes
- Cisco IOS and IOS XE: Defaults to 4, max often 16–32 depending on the version and hardware.cisco.com
- IOS XR: Similar command, but check the specific documentation as syntax might vary slightly.cisco.comcisco.com
- Other vendors: This post focuses on Cisco, but protocols like OSPF are standardized—commands may differ (e.g., on Aruba/HPE, it’s also maximum-paths with a default of 4).arubanetworking.hpe.com
Always test changes in a lab first, as altering routing behavior can impact live traffic. Monitor with tools like show ip cef <destination> internal to see how many paths are actually being used.serverfault.com
Conclusion
Adjusting the maximum equal-cost paths in OSPF is a simple yet effective way to fine-tune load balancing in your network. Starting from the default of 4, you can scale up to 16 or even 32 paths on supported platforms using the maximum-paths command. This flexibility makes OSPF a robust choice for dynamic environments. If you’re dealing with OSPF in your setup, give this a try and share your experiences in the comments!