EIGRP Summarization
🌐 EIGRP Summarization – Complete Guide
📘 Introduction to EIGRP Summarization
EIGRP (Enhanced Interior Gateway Routing Protocol) is an advanced distance-vector routing protocol used on computer networks. Summarization in EIGRP is the process of consolidating multiple routes into a single advertisement, reducing the size of routing tables, speeding up convergence, and improving overall network performance.
This guide will walk you through every detail of EIGRP Summarization — from fundamentals to configuration and troubleshooting.
🧠 Why is Summarization Important in EIGRP?
EIGRP summarization helps by:
- 🔽 Reducing routing table size.
- ⚡ Improving convergence time.
- 🚦 Lowering CPU utilization on routers.
- 🔒 Increasing route stability by hiding flapping routes.
- 📉 Minimizing the number of routing updates.
✅ Types of EIGRP Summarization
There are two types of summarization in EIGRP:
1. Automatic Summarization (Legacy)
- Enabled by default in older versions of EIGRP (pre-IOS 15).
- Summarizes routes to their classful boundaries.
- Can cause routing issues like black holes if misused.
Example:
192.168.1.0/24 and 192.168.2.0/24 are summarized as 192.168.0.0/16.
To Disable auto-summarifzation:
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Router(config-router)# no auto-summary
2. Manual Summarization
- Done manually at the interface level in EIGRP.
- Offers precise control over what is advertised.
- Can be used on point-to-point and multipoint interfaces.
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Router(config-if)# ip summary-address eigrp <AS_number> <summary_IP> <subnet_mask>
🛠️ How Manual Summarization Works in EIGRP
- Summarization occurs outbound—on the interface sending updates.
- When a summary route is created:
- A null0 route is installed in the routing table to prevent routing loops.
- It acts as a discard route if no more specific route exists.
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interface Serial0/0
ip summary-address eigrp 100 192.168.0.0 255.255.252.0
### This summarizes:
192.168.0.0/24
192.168.1.0/24
192.168.2.0/24
192.168.3.0/24 …into 192.168.0.0/22.
🔄 EIGRP Summarization and Route Advertisement
- EIGRP does not automatically suppress more specific routes when a summary is configured. Both may be advertised unless explicitly filtered.
- Summarization is not recursive. A router must have all the specific routes in its table to advertise a summary.
⚠️ Null0 Route and Loop Prevention
When a manual summary is created, EIGRP automatically installs a summary route to Null0. This prevents traffic from being forwarded to nonexistent subnets.
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S 192.168.0.0/22 is directly connected, Null0
If no specific subnet matches a packet, it’s dropped safely.
🧪 Lab Example: EIGRP Summarization
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R1 -- R2 -- R3
R1 has:
192.168.1.0/24
192.168.2.0/24
192.168.3.0/24
Configuration on R1:
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interface Serial0/0
ip summary-address eigrp 100 192.168.0.0 255.255.252.0
Show Commands:
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show ip route
show ip eigrp topology
show ip protocols
🛑 Common Issues & Troubleshooting EIGRP Summarization
| 🧩 Issue | 🛠️ Cause | ✅ Solution |
|---|---|---|
| Routing Black Hole | Summary route points to Null0 | Ensure all subnets are reachable |
| No Summarization | Incorrect mask or syntax | Verify summary mask |
| Overlapping Routes | Improper subnetting | Plan subnetting and summaries properly |
| Duplicate Routes | Summary + specific routes both advertised | Use route filtering or adjust summarization |
💡 Best Practices for EIGRP Summarization
Always disable auto-summary in modern EIGRP deployments.
Use manual summarization at appropriate interfaces.
Verify the Null0 route exists for safety.
Plan IP addressing to allow clean summarization.
Use tools like show ip route and show ip eigrp topology to verify results.
Use route filtering (distribute-list / prefix-list) to control unwanted routes.
📚 Useful Commands Summary
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# Disable auto-summary
router eigrp 100
no auto-summary
# Apply manual summarization
interface s0/0
ip summary-address eigrp 100 192.168.0.0 255.255.252.0
# Verification
show ip route
show ip eigrp topology
show ip protocols
🏁 Conclusion
EIGRP summarization is a powerful tool for optimizing your routing environment. When used correctly, it leads to faster convergence, more stable networks, and simplified route tables. Always understand your network’s addressing scheme and use manual summarization strategically.
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