EIGRP Opcode 1 and the Three-Way Handshake

In the world of EIGRP, Opcode 1 is a significant player. It's part of the EIGRP three-way handshake, a crucial process in establishing and maintaining neighbor relationships. Let's break down the details:

Opcode 1: Reliable Communication

• As per RFC 7868: Section 4.1, Opcode 1 is used for EIGRP packets that require reliable communication. These packets are always sent reliably to ensure their safe delivery.

• They serve multiple purposes, including initial neighbor acquisition and functioning as acknowledgment packets (ACK). You can identify Opcode 1 packets because they have the 'init bit' set to one (1).

• When an EIGRP router sends an Opcode 1 packet, it's signaling a request for the full routing table from its neighbor. This is particularly useful during the initial phase of neighbor discovery.

• Importantly, Opcode 1 packets must be acknowledged because they fall into the category of reliable packet types. Acknowledgment ensures that the recipient received the packet successfully.

Neighbor Behavior: Poison Reverse

• Additionally, it's worth noting that each neighbor will send UPDATE packets where they employ a mechanism known as "poison reverse" for routes learned from other neighbors. This mechanism helps in preventing routing loops.

Split Horizon & Poison Reverse: Managing Routes

• Split Horizon and Poison Reverse, discussed in RFC 7868: Section 5.4.2, are strategies used in EIGRP to manage routes effectively. They contribute to the stability and loop avoidance in the network.

Startup Mode and Delay Value

• In the startup mode, detailed in RFC 7868: Section 5.4.2.1, the delay value is set to INFINITY - 1. This signifies that EIGRP is cautious when it starts up and is not immediately available for normal routing operations.

The EIGRP Three-Way Handshake:

The EIGRP Three-Way Handshake

The EIGRP three-way handshake is a precisely coordinated process aimed at establishing neighbor relationships within an EIGRP (Enhanced Interior Gateway Routing Protocol) network. This handshake is a fundamental component of efficient routing in EIGRP, involving multiple steps, with a key role played by Opcode 1 packets, which ensure reliable communication.

Step 1: Hello Messages

The initial step in the EIGRP three-way handshake involves the exchange of Hello packets between two routers. These Hello packets serve as a form of greeting and are regularly sent out by routers. Their primary purposes are:

• Identifying potential neighbor routers.
• Verifying the reachability of these potential neighbors.
• Sharing essential information about the routers themselves, including their EIGRP parameters and capabilities.

Step 2: Exchange of Topology Information (Opcode 1 Packets)

After identifying each other as potential neighbors through Hello packets, routers move on to the second step. This phase includes the exchange of EIGRP topology information, and it is here that Opcode 1 packets come into play. Opcode 1 packets are known for their reliability, as they are sent with the 'init bit' set to one (1). This setting signifies a request for the full routing table from the neighboring router.

During this step, routers share in-depth routing information. This includes details about known routes and their associated metrics. This information exchange allows routers to create a comprehensive picture of the network's topology and routing table. The reliability of Opcode 1 packets ensures the successful transmission and receipt of this crucial data.

Step 3: Acknowledgment and Neighbor Relationship Establishment

The final step of the handshake is where routers acknowledge the receipt of routing information exchanged in the previous step. Acknowledgments are vital to confirm the successful delivery of information and complete the three-way handshake.

Upon the successful completion of these three steps, routers establish a neighbor relationship. This relationship is the cornerstone of effective communication within the EIGRP network. With neighbor relationships established, routers can trust each other's routing information, work together to make informed routing decisions, adapt to network changes, and ensure efficient data forwarding.

In summary, the EIGRP three-way handshake is a meticulously coordinated process, essential for routers to establish neighbor relationships. It involves the exchange of Hello packets and Opcode 1 packets, ensuring the secure transmission of routing information. This handshake is a fundamental aspect of EIGRP's operation, contributing significantly to the stability and efficiency of EIGRP-based networks.

Configuration:

• ip hello-interval eigrp [as#] [num-seconds]: This command configures the hello interval for EIGRP, influencing the timing of hello packets exchanged with neighbors.

• ip hold-time eigrp [as#] [num-seconds]: This command configures the hold time for EIGRP, specifying how long a neighbor relationship should be maintained if hello packets are not received.

Verification:

• show ip eigrp interfaces [interface] [detail]: Use this command to display detailed information about EIGRP interfaces, including their configuration.

• show ip eigrp neighbor [detail]: This command provides detailed information about EIGRP neighbors, including their IP addresses and state.

• debug eigrp packets [update] [detail]: Enabling this command allows you to debug EIGRP update packets, including Opcode 1 updates, with detailed information.

• show ip eigrp traffic | inc Hellos: Use this command to view EIGRP traffic statistics, specifically identifying hello packets.

• show ip protocols: This command displays information about IP routing protocols configured on the router, including EIGRP.

In summary, Opcode 1 in EIGRP ensures reliable communication, especially during the initial neighbor acquisition phase. Understanding the EIGRP three-way handshake, along with its configuration and verification commands, is essential for effective EIGRP network management and stability.