PART 6: EIGRP Timers

Enhanced Interior Gateway Routing Protocol

EIGRP uses several key timers to maintain neighbor relationships, detect failures, and manage route processing activities. Understanding and properly configuring these timers is essential for optimal EIGRP operation.

Core EIGRP Timers

Hello Timer

This timer controls how frequently EIGRP routers send Hello packets to discover and maintain neighbor relationships:

• Default values:

  • 5 seconds for high-speed links (10 Mbps or faster)

  • 60 seconds for low-speed links (T1/E1 or slower)

Hold Timer

The Hold timer determines how long a router waits without receiving any EIGRP packets before declaring a neighbor dead:

• Default values:

  • 15 seconds for high-speed links (3× Hello interval)

  • 180 seconds for low-speed links (3× Hello interval)

Active Timer

This timer limits how long a router remains in Active state while waiting for replies to query packets:

• Default value: 3 minutes

• In IOS 15.x and later, an SIA-Query is sent after 1.5 minutes (halfway through the
  Active timer)
  

Important Timer Considerations

Several unique aspects of EIGRP timers differ from other routing protocols:

1. Independent neighbor timers: Unlike OSPF, EIGRP neighbors do not need to have matching Hello/Hold timers to form adjacencies
   

2. Hold timer negotiation: The Hold timer value is included in Hello packets, allowing each router to know how long its neighbor expects to receive updates
   

3. Multiple packet types reset Hold timer: Any EIGRP packet received from a neighbor (not just Hello packets) resets the Hold timer
   

4. Timer configuration: EIGRP allows Hold timers to be set lower than Hello timers, though this is not recommended as it may cause unstable neighbor relationships
   

5. Supplemental failure detection: For faster convergence, EIGRP can be used with Bidirectional Forwarding Detection (BFD) to detect failures in milliseconds
   

Timer Configuration

Cisco IOS/IOS XE (Numbered Mode)

R1(config-if)# ip hello-interval eigrp <AS> <seconds>
R1(config-if)# ip hold-time eigrp <AS> <seconds>
R1(config-router)# timers active-time <minutes>

Cisco IOS XE (Named Mode)

R1(config-router-af-interface)# hello-interval <seconds>
R1(config-router-af-interface)# hold-time <seconds>
R1(config-router-af-topology)# timers active-time <minutes>

Cisco IOS XR

Router(config)# router eigrp <process-name>
Router(config-eigrp)# address-family ipv4
Router(config-eigrp-af)# interface GigabitEthernet0/0/0/0
Router(config-eigrp-af-if)# hello-interval <seconds>
Router(config-eigrp-af-if)# hold-time <seconds>
Router(config-eigrp-af)# timers active-time <minutes>

Timer Optimization Strategies

For network optimization, consider these timer adjustment strategies:

1. Default timers for stable networks: Use default timers in stable networks with reliable links

2. Faster convergence: Reduce Hello/Hold timers on critical links to detect failures more quickly (e.g., 1-second Hello, 3-second Hold)

3. WAN link considerations: In bandwidth-constrained environments, longer Hello intervals reduce protocol overhead

4. BFD integration: For sub-second failure detection, implement BFD alongside EIGRP rather than extremely short Hello timers

5. Active timer adjustment: For large networks with complex topology, consider increasing the Active timer to prevent SIA (Stuck-In-Active) conditions

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