EIGRP Timers
🕒 EIGRP TIMERS: THE CLOCKWORK BEHIND RAPID CONVERGENCE
Overview:
Enhanced Interior Gateway Routing Protocol (EIGRP) is known for its fast convergence powered by the Diffusing Update Algorithm (DUAL) and intelligent timer mechanisms.
Unlike older distance vector protocols that depend on long periodic updates, EIGRP uses timers to proactively monitor neighbor status and respond instantly to topology changes.
Understanding the three major timers — Hello, Hold, and Active (SIA) — is vital for CCNP ENARSI learners, SOC Analysts, and Network Engineers focused on network stability and convergence optimization.
🧩 PART 1: THE NEIGHBOR TIMERS (HELLO AND HOLD TIME)
EIGRP routers maintain dynamic neighbor relationships using Hello and Hold timers.
🔹 1. The Hello Timer (Hello Interval)
Purpose:
Hello packets act as heartbeats to detect and maintain EIGRP neighbor adjacencies.
Key Points:
- Sent periodically to inform neighbors of the router’s presence.
- Used for neighbor discovery and recovery.
- Not acknowledged (sequence number = 0).
- Contain EIGRP K-values.
Default Values: | Interface Type | Default Hello Interval | |———————–|————————| | High-Speed (≥ T1, e.g. Gigabit Ethernet) | 5 seconds | | Low-Speed (≤ T1, e.g. Serial) | 60 seconds |
Multicast Address:
- IPv4 →
224.0.0.10 - IPv6 →
FF02::A
🔹 2. The Hold Timer (Hold Time)
Purpose:
Specifies how long a router waits without receiving Hello packets before declaring a neighbor down.
Default Behavior:
- Hold Time = 3 × Hello Interval
| Interface Type | Hello (sec) | Hold (sec) |
|---|---|---|
| High-Speed | 5 | 15 |
| Low-Speed | 60 | 180 |
Mechanism:
Each received packet (Hello or others) resets the Hold Timer countdown.
⚠️ Important Note: Mismatched Timers
- Adjacency can still form if Hello/Hold timers differ.
- But instability occurs if the neighbor’s Hello interval > local Hold time.
- Result: Adjacency flaps (forms and drops repeatedly).
⚙️ PART 2: THE DUAL TIMER (ACTIVE TIME)
When no Feasible Successor (backup route) exists, EIGRP must search for a new path using DUAL.
This process is governed by the Active Timer.
🔹 1. Stuck In Active (SIA)
When a route loses its primary path:
- The router enters Active (A) state (from Passive).
- Sends Query packets to neighbors for alternate routes.
- Starts the Active Timer countdown.
🔹 2. The Active Timer (SIA Timer)
Purpose:
Controls how long DUAL allows a route to remain in the Active state waiting for replies.
Default Value:
3 minutes (180 seconds)
SIA Mechanism:
- After 90 seconds (½ of Active timer), if no reply → send SIA Query.
- If still no response by 180 seconds, router declares Stuck In Active (SIA).
Resolution:
- Router removes all routes learned from that neighbor.
- Resets adjacency.
- Maintains loop-free operation and network stability.
🟢 Tip:
Using EIGRP Stub Routing reduces SIA occurrences significantly.
🔧 PART 3: CONFIGURATION AND VERIFICATION
Timers can be modified per interface to optimize performance.
🖥️ Hello and Hold Timers Configuration
| Mode | Command | Description |
|---|---|---|
| Classic Mode (Interface Submode) | ip hello-interval eigrp <as-number> <seconds> | Set Hello interval |
ip hold-time eigrp <as-number> <seconds> | Set Hold time | |
| Named Mode (AF-Interface Submode) | hello-interval <seconds> | Set Hello interval |
hold-time <seconds> | Set Hold time |
⏱️ Active Timer Configuration
| Mode | Command | Description |
|---|---|---|
| Classic Mode (Router Submode) | timers active-time {disabled | 1-65535} | Configure Active Timer |
| Named Mode (Topology Base Submode) | timers active-time {disabled | 1-65535} | Configure Active Timer |
🔍 Verification Commands
| Purpose | Command |
|---|---|
| View Hello and Hold timers | show ip eigrp interfaces detail [interface-id] |
show ipv6 eigrp interfaces detail | |
| View Active timer configuration | show ip protocols or show eigrp protocols |
🧠 QUICK RECAP TABLE
| Timer Type | Function | Default Value | Scope | Impact |
|---|---|---|---|---|
| Hello Timer | Neighbor discovery/keepalive | 5s (Fast) / 60s (Slow) | Per-interface | Maintains neighbor adjacency |
| Hold Timer | Neighbor lifetime before timeout | 15s / 180s | Per-interface | Detects neighbor failure |
| Active Timer (SIA) | Duration for DUAL to wait for replies | 3 minutes | Per-process | Prevents routing blackholes |
✅ KEY TAKEAWAYS
- Hello & Hold Timers ensure neighbor stability and quick detection of failures.
- Active Timer ensures DUAL convergence without loops.
- Misconfigured timers can cause route flapping and network instability.
- Stub Routing and proper timer tuning improve convergence speed and reliability.
💡 Pro Tip for Interviews:
- Q: Can EIGRP neighbors form with mismatched Hello/Hold timers?
A: Yes, but the adjacency may flap if the Hello interval is greater than the Hold time. - Q: What is the purpose of the Active Timer?
A: It limits the time a route can stay in the Active state to prevent indefinite waiting and routing loops.
📘 In Summary:
Mastering EIGRP timers—Hello, Hold, and Active—means mastering EIGRP’s stability and speed.
They form the invisible clockwork ensuring that EIGRP reacts quickly, communicates reliably, and converges faster than almost any other interior routing protocol.
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