EIGRP Packet Types and RTP Protocol

This tutorial provides an overview of RTP (Reliable Transport Protocol) and the EIGRP (Enhanced Interior Gateway Routing Protocol) packet types: Update, Query, Reply, Hello, and Acknowledgment. It examines the functions of each EIGRP packet type and describes how EIGRP utilizes RTP to facilitate communication between EIGRP-enabled routers.

Unlike other routing protocols, EIGRP does not rely on TCP or UDP for communication. Instead, it uses the Reliable Transport Protocol (RTP) to exchange information. RTP is a proprietary protocol developed by Cisco to ensure reliable communication between its devices. It supports both multicast and unicast transmission to ensure reliable packet delivery. It transmits a single packet at a time, which eliminates the need for windowing or congestion control mechanisms.

EIGRP and RTP

An EIGRP router uses RTP to communicate with other EIGRP-enabled routers within the network. RTP ensures the reliable, ordered delivery of EIGRP packets by using sequence and acknowledgment numbers. When transmitting a multicast, it maintains a list of neighbors that have responded. If a neighbor does not reply to the multicast, the router retransmits the data using unicast. After 16 unsuccessful unicast attempts, the neighbor is considered unreachable. This mechanism is referred to as reliable multicast.

It assigns a unique sequence number to each packet, enabling routers to identify and discard outdated, redundant, or out-of-sequence packets. It maintains a retransmission table for each neighbor to track reliable packets that have not been acknowledged within the Retransmission Time Out (RTO) period. When a reliable packet is sent, an entry is created in the table with the associated RTO timer. If the timer expires before an acknowledgment is received, RTP retransmits the packet. This process continues until the neighbor's hold time expires, at which point it removes the neighbor from the neighbor table.

EIGRP packet types

EIGRP uses five types of packets. These types are Update, Query, Reply, Hello, and Acknowledgment.

Update

An Update packet contains the routing update or route information. When two EIGRP routers establish a neighbor relationship, they exchange routing information using update packets. After building a neighbor relation, they use update packets solely to exchange information about changes.

EIGRP transmits Update packets using either reliable multicast or unicast. Multicast distributes the same information to all neighbors, while unicast targets a specific neighbor. Regardless of the transmission method, EIGRP requires an acknowledgment for every Update packet sent.

Query

EIGRP uses a query packet to find an alternate path to a destination when it has lost the existing path. EIGRP always uses reliable multicast to send query packets and requires an acknowledgment for each query.

Reply

EIGRP uses a reply packet to respond to a query packet. EIGRP uses the reliable unicast method to send a reply packet. A reply packet either includes information about a specific route or a message indicating that no known route exists to that route. EIGRP also needs an acknowledgment for a reply packet.

Hello

EIGRP uses hello packets to discover potential EIGRP neighbors. EIGRP sends hello packets via unreliable multicast. It does not need an acknowledgment for a hello packet.

Acknowledgment (Ack)

EIGRP sends an ack packet in response to an update, query, or reply packet. An ack packet confirms that the destination device received the packet. ACK packets are always sent via unicast and do not require an acknowledgment. An ack packet is itself sent as acknowledgment. An acknowledgment for an acknowledgment packet makes no sense.

Key Points

• EIGRP utilizes RTP to exchange packets with other EIGRP-enabled routers.
• EIGRP exchanges five types of packets. These packet types are Update, query, reply, hello, and acknowledgment.
• Update, Query, and Reply packets require acknowledgment.
• Hello, and ack packets do not require acknowledgment. Since they don't require acknowledgment, they do not have sequence numbers.
• The EIGRP multicast address is 224.0.0.10. Packets sent to this address are received by all connected EIGRP routers.
• EIGRP Hello packets are sent every 5 seconds on LANs and point-to-point links. On T1 or low-speed interfaces, they are sent every 60 seconds.

Conclusion

EIGRP relies on the Reliable Transport Protocol (RTP) to ensure the efficient and secure delivery of routing information between routers. By using five distinct packet types: Update, Query, Reply, Hello, and Acknowledgment, it maintains robust communication, quickly adapts to network changes, and ensures reliable data exchange. Understanding how these packet types function and how RTP manages their delivery is essential for anyone working with EIGRP networks.