EIGRP Stub Routing

📘 EIGRP Stub Routing – Notes

🔹 What is EIGRP and Why Do We Need Stub Routers?

• EIGRP (Enhanced Interior Gateway Routing Protocol) is like a group of routers constantly chatting and sharing directions (routes).
• Routers use DUAL (Diffusing Update Algorithm) to find the best paths. If a path fails, they send queries asking neighbors if they know another way.
• Problem: Sending queries to routers that are dead-ends (like small offices or remote sites) wastes bandwidth and CPU, and may even cause Stuck-in-Active (SIA) events.

👉 Stub Routers solve this problem by limiting EIGRP queries and advertisements.

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🔹 What is an EIGRP Stub Router?

• A Stub Router is a router that:
  • Acts as a non-transit router (traffic doesn’t pass through it to reach other routers).
  • Doesn’t receive queries from neighbors.
• Think of it as the quiet router in the group — it only speaks when needed, and others know not to ask it complicated questions.

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🔹 Why Use EIGRP Stub Routing? (Key Benefits)

1. Improved Performance
   • Stops sending Hellos on interfaces with no EIGRP routers.
   • Reduces unnecessary query traffic.
2. Enhanced Security
   • Using passive interfaces, prevents rogue routers from forming neighborships.
3. Increased Stability
   • Prevents SIA (Stuck-in-Active) events.
4. Faster Convergence
   • Core routers don’t wait for responses from stub routers, so the network adapts quicker.

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🔹 How to Configure a Stub Router

1. Optional – Passive Interface

Stops Hellos on interfaces with no EIGRP neighbors.

router eigrp 1
passive-interface g0/1

The network is still advertised, but no neighborship is formed.

2. Enable Stub

router eigrp 1
eigrp stub

By default, this advertises directly connected routes and summary routes.

3. Verify

show ip eigrp neighbors detail

Displays if the neighbor is a stub and what routes it advertises.

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🔹 Controlling Stub Advertisements

• eigrp stub connected → Only advertises directly connected networks.
• eigrp stub static → Advertises static routes.
• eigrp stub redistributed → Advertises redistributed routes.
• eigrp stub summary → Advertises summary routes.
• eigrp stub receive-only → Doesn’t advertise anything; only receives routes.
• eigrp stub leak-map <map-name> → Selectively advertises routes using ACLs + route maps.

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🔹 How Neighbors Recognize a Stub

• The stub router adds a TLV code in its Hello messages.
• Neighbors recognize it and stop sending queries to the stub.

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🔹 Analogy – Easy Way to Remember

• Imagine your network is an office building:
  • EIGRP query = someone shouting: “Where’s the stapler?”
  • Normal router = open office, where people may ask each other and forward the question.
  • Stub router = a tiny filing room at the end of the hall. Nobody asks them because they only deal with their files and won’t forward the question.

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✅ Key Takeaway:
Stub routers are best for remote sites or edge routers. They reduce unnecessary queries, improve security, prevent SIA events, and speed up convergence.

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