CCNP-Studies
Routing
1-EIGRP
2 Eigrp Main Concepts

EIGRP Main Concepts

EIGRP (Enhanced Interior Gateway Routing Protocol) is a unique routing protocol known for its efficient and advanced features. Contrary to common classification, EIGRP is not a hybrid routing protocol but rather an Advanced Distance Vector (DV) routing protocol. The term "Advanced" is attributed to several distinctive characteristics:

  • Partial and Bounded Updates: EIGRP reduces network traffic by sending only partial and bounded updates when there are changes in the routing table, minimizing the impact on network resources.

  • Multicast vs. Broadcast: EIGRP uses multicast communication instead of broadcast, which enhances scalability and reduces unnecessary traffic.

  • Support for Variable Length Subnet Masking (VLSM): EIGRP is compatible with VLSM, allowing for more efficient address allocation.

  • Faster Convergence: EIGRP boasts faster convergence times compared to other Distance Vector (DV) routing protocols, enabling quicker response to network changes.

  • Loop Freedom at All Times: EIGRP ensures loop-free routing paths, enhancing network stability.

EIGRP operates on the principle that "Distance" equals "Cost," which is the metric used to reach a destination. The "Vector" in EIGRP refers to the path or direction packets take to reach their intended destination. Unlike some other routing protocols like OSPF, EIGRP does not impose hierarchical boundaries.

Diffusing Update Algorithm (DUAL) Finite State Machine (FSM):

  • DUAL, as defined in RFC 7868 Section 3, is at the core of EIGRP's metric calculation and ensures loop-free routing decisions. It utilizes information from standard distance vector routing to determine route validity.

  • DUAL responds to various input events such as interface status changes (up/down), route advertisements or withdrawals, and computes important values like Computed Distance (CD), Feasible Distance (FD), Feasible Successor (FS), Reported Distance (RD), and Successor (S) for each route.

  • The information provided to DUAL is critical for EIGRP's routing decisions, but the question arises, who supplies this information?

Protocol-Dependent Modules (PDM):

  • PDM, described in RFC 7868 Section 5.4, plays a vital role in conveying new and changing routing information to DUAL.

  • PDM allows EIGRP to support multiple network layer (L3) protocols, such as IPv4 and IPv6. It is particularly significant during the process of redistributing routes into or out of EIGRP.

  • These modules maintain topology and neighbor tables for each supported protocol and act as intermediaries between DUAL and protocol-specific routing tables.

  • PDMs are also instrumental in implementing Access Control Lists (ACLs) and filters in EIGRP, ensuring reliable communication.

Reliable Transport Protocol (RTP):

  • As detailed in RFC 7868 Section 5.2, RTP is employed to ensure the delivery of certain "reliable" EIGRP packet types.

  • Some EIGRP packet types, like Hellos, are considered unreliable and do not require acknowledgment.

  • However, for reliable packet types such as Replies, Updates, Queries, SIA-Replies, and SIA-Queries, acknowledgments (ACKs) are necessary to guarantee successful delivery.

  • RTP is essential for handling these reliable packets, and it takes action when expected acknowledgments are not received. It can retransmit packets up to 16 times if needed, ensuring reliability.

  • Notably, EIGRP does not use UDP or TCP for its communication; instead, it relies on RTP for this purpose.

  • RTP introduces "reliable multicast" to facilitate communication through specific multicast group addresses, such as 224.0.0.10 and FF02::A.

In conclusion, EIGRP is an advanced Distance Vector routing protocol with unique features, such as efficient updates, multicast communication, support for VLSM, rapid convergence, and loop-free routing. DUAL, PDMs, and RTP are essential components that contribute to EIGRP's reliability and efficient operation in diverse network environments.

1 Eigrp Packet Header3 Hello Messages