EIGRP fundamentals

Enhanced Interior Gateway Routing Protocol (EIGRP) is an advanced distance-vector routing protocol that uses characteristics of both distance-vector and link-state protocols, making it a hybrid routing protocol. It operates at the Network Layer (Layer 3) of the OSI model and uses protocol number 88 for communication.

Note: EIGRP helps routers or Layer 3 switches dynamically discover and maintain the best paths for forwarding packets within an Autonomous System (AS).

Administrative Distance (AD) in EIGRP

Administrative Distance defines the trustworthiness of routing information. Lower AD values indicate higher trust. EIGRP assigns the following AD values:

Key EIGRP Messages

EIGRP uses multiple message types for neighbor communication and routing updates. These messages can be multicast (224.0.0.10) or unicast, depending on the purpose:

1. Hello Message: Used for neighbor discovery and keep-alives. Sent every 5 seconds (Hello timer). If no hello is received within 15 seconds (Dead timer), the neighbor is declared dead.
2. Full Update: Sent after neighbor adjacency is formed. Contains all known best routes.
3. Partial Update: Triggered by topology changes. Contains only the new or changed routes.
4. Query Message: Sent when a destination is no longer reachable. Multicast to neighbors requesting alternative routes.
5. Reply Message: Sent in response to a query. Provides alternate route information.
6. Acknowledgement Message: A hello packet with no data, used to acknowledge updates, queries or replies.
7. NULL Update: Used for calculating timers like SRTT (Smooth Round Trip Time) and RTO (Retransmission Timeout).

Note: Hello and Acknowledgement packets do not require acknowledgment. However, Query, Reply and Update messages are reliable and require acknowledgment.

Composite Metric in EIGRP

EIGRP calculates its routing decision using a composite metric, which can include five variables:

• K1 -> Bandwidth
• K2 -> Load
• K3 -> Delay
• K4 -> Reliability
• K5 -> MTU

Note: By default, only K1 (bandwidth) and K3 (delay) are used. The default K values are (K1=1, K2=0, K3=1, K4=0, K5=0). This allows EIGRP to select the path with the lowest composite metric, ensuring efficient routing.

Neighbor Adjacency Requirements

For two routers to form an EIGRP neighbor relationship, the following conditions must be satisfied:

• K values must match.
• Autonomous System (AS) number must be the same.
• Subnet mask must be identical.
• Authentication (if enabled) must match -> EIGRP supports MD5 authentication only.

EIGRP Timers

EIGRP relies on timers for maintaining neighbor relationships:

• Hello Timer: 5 seconds (interval between hello messages).
• Dead Timer (Hold Time): 15 seconds (neighbor declared dead if no hello received).

Note: These timers ensure stable communication and quick detection of failures.

Pros of EIGRP

• Fast convergence using the Diffusing Update Algorithm (DUAL).
• Supports VLSM and CIDR for efficient IP address usage.
• Efficient bandwidth usage with partial updates instead of full table exchanges.
• Scalability across large enterprise networks.
• Unequal-cost load balancing supported.

Suggested Quiz

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5 Questions

What is the protocol number used by EIGRP?

• A

  89

• B

  88

• C

  87

• D

  90

Explanation:

EIGRP uses IP protocol number 88.

By default, EIGRP Hello packets are sent every how many seconds on most interfaces?

• A

  60 sec

• B

  5 sec

• C

  10 sec

• D

  30 sec

Explanation:

Default Hello timer is 5 seconds on high-speed links (T1 or faster). Hold timer is 15 seconds.

Which EIGRP packet type does NOT require acknowledgment?

• A

  Update

• B

  Query

• C

  Reply

• D

  Hello

Explanation:

Hello and ACK packets are unreliable (no acknowledgment required). Update, Query, and Reply are reliable.

For two routers to form an EIGRP neighbor adjacency, which of the following is NOT mandatory?

• A

  Same AS number

• B

  Matching K-values

• C

  Same subnet

• D

  Same MTU

Explanation:

MTU is NOT checked during neighbor formation (but affects packet size). AS, K-values, and primary subnet must match.

Which EIGRP route type has the highest (worst) Administrative Distance?

• A

  Summary route

• B

  Internal route

• C

  External route

• D

  All have same AD

Explanation:

External EIGRP routes (redistributed) have AD = 170.

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