EIGRP Fundamentals

Question 1

EIGRP

Answer 1

EIGRP overcomes the deficiencies of other distance vector routing protocols like RIP with features such as unequal-cost load balancing, support for networks 255 hops away, and rapid convergence features.

EIGRP uses a diffusing update algorithm (DUAL) to identify network paths and enable fast convergence using precalculated loop-free backup paths.

Question 2

EIGRP Features

Answer 2

Fast convergence.

Partial updates conserve network bandwidth.

Runs directly over IP, using protocol number 88.

Manual summarization at any interface.

Use multicast address 224.0.0.10 for EIGRPv4 and FF02::A for EIGRPv6.

Sophisticated metric that supports load-balancing across unequal-cost paths.

Question 3

AS

Answer 3

A router can run multiple EIGRP processes.

Each process operates under the context of an autonomous system, which represents a common routing domain.

Routers within the same domain use the same metric calculation formula and exchange routes only with members of the same autonomous system.

Each EIGRP process correlates to a specific autonomous system and maintains an independent EIGRP topology table.

A router that participate in both AS by default do not transfer routes learned from one AS into a different AS.

Question 4

Topology Table

Answer 4

EIGRP’s topology table is a vital component to DUAL and contains information to identify loop-free backup routes.

The topology table contains all the network prefixes advertised within an EIGRP autonomous system.

By default only successor and feasible successor routes are displayed, but the optional all-links keyword will show the paths that did not pass the feasibility condition.

Question 5

Topology Table

example

Answer 5

Network 10.4.4.0/24

R1 calculates an FD of 3328 for the successor route.

The successor (upstream router) advertises the successor route with an RD of 3072.

2nd path has a metric of 5376 and a RD of 2816.

Because 2816 is less than 3072, the second entry passes the feasibility condition, which means the second entry is classified as the feasible successor for the prefix. (Backup route)

The 10.4.4.0/24 route is passive (P) which means the topology is stable.

Question 6

EIGRP Packet Types

Answer 6

Hello: Used for discovery of EIGRP neighbors and for detecting when a neighbor is no longer available.

Update: Used to transmit routing and reachability information with other EIGRP neighbors.

Query: Sent out to search for another path during convergence.

Reply: Sent in response to a query packet.

Question 7

EIGRP Neighbors

Answer 7

EIGRP neighbors exchange the entire routing table when forming an adjacency, and they advertise only incremental updates as topology changes occur within a network.

The neighbor adjacency table is vital for tracking neighbor status and the updates sent to each neighbor.

EIGRP uses its own IP number (88), uses multicast packets where possible and unicast packets when necessary.

Communication between routers on 224.0.0.10.

Question 8

Forming EIGRP Neighbors

Parameters that must match for two routers to become neighbors

Answer 8

Unlike other distance vector routing protocols, EIGRP requires a neighbor relationship to form before routes are processed and added to the (RIB).

Upon hearing an EIGRP hello packet, a router attempts to become neighbors.

1. Metric K values
2. Primary subnet
3. Autonomous system number (ASN)
4. Authentication parameters