1.9 Troubleshoot EIGRP (classic and named mode; VRF and global)
📘CCNP Enterprise – ENARSI (300-410)
EIGRP (Enhanced Interior Gateway Routing Protocol) is a Cisco routing protocol used inside a network (an IGP – Interior Gateway Protocol). Sometimes, you have small branch networks or remote routers that don’t need to know every single route in the network. For efficiency and security, EIGRP allows you to configure stub routers.
1. What is an EIGRP Stub?
- A stub router is a router that only advertises certain types of routes to its neighbors instead of sharing the full routing table.
- Purpose: Reduce unnecessary EIGRP traffic and prevent remote routers from sending queries to the stub router.
Think of it as a router saying:
“I’m at the end of the network. I only know about my local network, and I won’t forward traffic to other routers unless it’s destined for me.”
2. Why Use EIGRP Stubs?
- Reduce network traffic – Stub routers don’t forward EIGRP queries to other routers unnecessarily.
- Prevent loops – Since stub routers don’t act as transit routers, this reduces the chance of routing loops.
- Optimize convergence – EIGRP convergence is faster because fewer routers need to be queried.
Exam Tip: You might be asked “Why configure a stub router?” – Always mention less EIGRP query traffic and no transit routing.
3. Types of Routes a Stub Router Can Advertise
When configuring a stub, you can specify which types of routes it will advertise to its neighbors. The main types are:
| Option | Description |
|---|---|
| connected | Advertises only routes that are directly connected to the stub router. |
| summary | Advertises summarized routes (aggregated routes) only. |
| redistributed | Advertises routes that were redistributed from another routing protocol. |
| leak-map | Advanced: Allows selectively leaking certain routes while still being a stub. |
| receive-only | Router only receives routes from neighbors and does not advertise anything back. |
Example Exam Question:
“Which stub type advertises only directly connected networks?” → Answer: connected
4. How Stub Routers Affect EIGRP Queries
- Normally, if a router loses a route, it sends an EIGRP query to its neighbors asking, “Do you know a path to this network?”
- If a neighbor is a stub, it won’t send queries back. It only replies with what it knows.
- This prevents unnecessary queries from propagating to remote branch routers.
Key Exam Point:
Stub routers cannot be used as transit routers. They only terminate traffic for networks they know.
5. How to Configure EIGRP Stubs (Basic Commands)
There are two main ways to configure stubs: classic mode and named mode.
A. Classic EIGRP Mode
Router(config)# router eigrp 100
Router(config-router)# eigrp stub connected summary
100→ EIGRP AS (Autonomous System) numberconnected summary→ Only advertise connected and summarized routes
Other options: connected, connected redistributed, receive-only, etc.
B. Named EIGRP Mode
Named mode is more flexible and recommended in newer Cisco IOS versions.
Router(config)# router eigrp NAME
Router(config-router)# address-family ipv4 unicast autonomous-system 100
Router(config-router-af)# eigrp stub connected summary
- Named mode separates address-family configurations (IPv4 or IPv6).
- Same stub options are available here as in classic mode.
6. Verification Commands (Exam-Relevant)
After configuring, you should know how to verify stub behavior.
- Check if stub is enabled:
show ip eigrp neighbors
- Neighbors will show “S” next to the stub router in the output.
- Check routing table:
show ip route eigrp
- Only routes allowed by stub configuration are advertised.
- Check EIGRP topology table:
show ip eigrp topology
- You can see which routes are considered successors and feasible successors.
7. Exam Tips for EIGRP Stubs
- Remember that stub routers cannot forward traffic to unknown networks – they are end points.
- Always know the difference between classic and named mode configuration.
- Stub types are often tested in scenario questions.
- Verify neighbors and check for “S” in neighbor table output.
- Related topics: Successor & Feasible Successor – stub routers still follow the same logic internally for route selection.
✅ Summary for Easy Memorization
- Stub router = leaf router in network (does not forward transit traffic).
- Reduces EIGRP queries → faster convergence and lower CPU use.
- Advertises only selected routes → connected, summary, redistributed, or receive-only.
- Cannot act as a transit router.
- Two configuration modes: classic (
router eigrp) and named (router eigrp NAME). - Verify:
show ip eigrp neighbors(look for S),show ip route eigrp,show ip eigrp topology.
