CCNP-Studies
Routing
1-EIGRP
6 Reply Messages

EIGRP Opcode 4: Reply Packets

In the world of EIGRP (Enhanced Interior Gateway Routing Protocol), Opcode 4 serves a specific purpose. Opcode 4 packets, also known as "Reply" packets, play a crucial role in EIGRP communication. Let's dive into the details:

Opcode 4: Reply Packets

  • According to RFC 7868: Section 4.3, Opcode 4 is assigned to a distinct type of EIGRP packet. These packets are always sent reliably, ensuring their safe and guaranteed delivery.

  • The primary function of Opcode 4 packets is to respond to EIGRP Query or SIA-Query packets. When a router in the EIGRP network receives a Query or SIA-Query packet from a neighboring router, it sends back a Reply packet in response.

  • An important aspect to note is that when the receiving node does NOT have an alternate path to the queried destination, it responds to the Query with values of infinity in its Reply. This signifies that the router does not possess an alternative route to suggest to the querying router.

Verification:

To effectively manage and troubleshoot EIGRP networks involving Reply packets, the following verification commands are valuable:

  • show ip eigrp interfaces [interface] [detail]: This command provides a detailed view of EIGRP interfaces, including their configuration and status. It's a useful tool for assessing the state of interfaces involved in the EIGRP process.

  • show ip eigrp neighbor [detail]: Use this command to access comprehensive information about EIGRP neighbors. It offers insights into neighbor relationships, including details such as IP addresses, hold times, and state.

  • debug eigrp packets [reply] [detail]: Enabling this debug command allows for close monitoring of EIGRP Reply packets and their associated details. This can be instrumental in diagnosing network issues related to Reply packets.

  • show ip eigrp traffic | inc Rep: With this command, you can obtain an overview of EIGRP traffic statistics, specifically focusing on Reply packets. It helps in assessing the frequency and volume of Reply packets in the network.

  • show ip protocols: When you need a comprehensive view of IP routing protocols in use, including EIGRP, this command provides detailed information. It offers insights into routing protocol settings and parameters.

In summary, Opcode 4 in EIGRP, represented by Reply packets, plays a pivotal role in EIGRP communication by responding to Query or SIA-Query packets. These Reply packets are always sent reliably and may contain information about alternate routes, or if none exist, they convey values of infinity. For effective management and troubleshooting of EIGRP networks, the provided verification commands offer valuable tools and insights.

5 Query Messages7 Sia Query and Reply