CCNA 3 v6.0 Study Material – Chapter 6: EIGRP

Chapter 6 – Sections & Objectives

6.1 EIGRP Characteristics

Explain the features and characteristics of EIGRP

6.2 Implement EIGRP for IPv4

Implement EIGRP for IPv4 in a small to medium-sized business network

6.3 EIGRP Operation

Explain how EIGRP operates in a small to medium-sized business network

6.4 Implement EIGRP for IPv6

Implement EIGRP for IPv6 in a small to medium-sized business network


6.1 EIGRP Characteristics

EIGRP Basic Features

  • Features of EIGRP
  • Uses the Diffusing Update Algorithm (DUAL) to calculate paths and back-up paths.
  • Establishes Neighbor Adjacencies.
  • Uses the Reliable Transport Protocol to provide delivery of EIGRP packets to neighbors.
  • Partial and Bounded Updates. Sends updates only when there is a change and only to the routers that need the information.
  • Supports Equal and Unequal Cost Load Balancing.
  • Protocol Dependent Modules – responsible for network layer protocol-specific tasks
  • Sends and receives EIGRP packets that are encapsulated in IPv4
  • Parses EIGRP packets and informs DUAL of the new information and DUAL makes routing decisions. The results are stored in the IPv4 routing table.
  • Reliable Transport Protocol
  • Used for the delivery and reception of EIGRP packets
  • Can send EIGRP packets as unicast or multicast.
  • Reserved IPv4 multicast address
  • Reserved IPv6 multicast address FF02::A.
  • Authentication
  • Only accepts routing information  from other routers with the same  authentication information
  • Does not encrypt the routing updates


EIGRP Packet Types

  • EIGRP Hello Packets
  • Are sent as multicasts and uses RTP for unreliable delivery
  • Used to form and maintain EIGRP neighbor adjacencies

  • EIGRP Update and Acknowledgment Packets
  • Update packets propagate updated routing information when necessary to the routers that require the information using RTP
  • Acknowledgment packets are send to acknowledge the update was received.
  • EIGRP Query and Reply Packets
  • Searches for networks
  • Uses reliable delivery
  • Queries are multicast or unicast. Replies are always unicast.

EIGRP Messages

  • Encapsulating EIGRP Messages
  • The EIGRP packet headers and TLV are encapsulated in an IP packet.
  • EIGRP Packet Header and TLV
  • EIGRP Packet Header
  • EIGRP Packet Type: Update, Query, Reply, and Hello
  • Autonomous System Number is the ID for the EIGRP routing process
  • EIGRP Parameters TLV
  • K values: K1 and K3 are set to 1. Other K values are set to 0
  • Hold Time: Maximum time the router should wait for the next hello
  • Internal TLV
  • The IP internal message is used to advertise EIGRP routes within an autonomous system.
  • Important metric fields: delay, bandwidth, prefix length, and destination
  • EIGRP TLV: External Routes
  • The IP external message is used when external routes are imported into the EIGRP routing process.


6.2 Implement EIGRP for IPv4

Configure EIGRP with IPv4

  • Autonomous System Numbers
  • IANA globally assigned autonomous numbers
  • Used by ISP and other large institutions
  • Used in exterior routing protocol, such as BGP, to propagate routing information
  • router eigrp autonomous-system command
  • Autonomous system number is only significant to local EIGRP local domain
  • Autonomous system number functions as a process ID
  • All routers within the same domain must have the same autonomous system number
  • The command is used to begin the EIGRP routing protocol
  • EIGRP Router ID – uniquely identifies each router in the EIGRP routing domain
  • Determined in 3 ways using the following order:
  • The router router-id ipv4-address command
  • The highest active IPv4 address of any of the loopback address
  • The highest active IPv4 address of any of the physical interface
  • The network ipv4-network-address router configuration mode command
  • Enables any interface on the router that matches the network address in the network command to send and receive EIGRP updates
  • By default, ipv4-network-address is the classful network address for each directly connected network
  • The network command and Wildcard Mask
  • network network-address [wildcard-mask]
  • Wildcard mask is the inverse of a subnet mask
  • To calculate the wildcard mask:
  • – Subnet mask
  • —————–
  • 0. 0.  0.  3  Wildcard mask
  • Passive Interface – prevent the neighbor adjacencies
  • Suppress unnecessary update traffic
  • Increase security controls
  • passive-interface interface-type interface-number


Verify EIGRP with IPv4

  • The show commands are useful in verifying EIGRP operations and for debugging and troubleshooting purposes.
  • show ip eigrp neighbors command
  • View the neighbor table
  • Verify neighbor adjacencies have been established
  • show ip protocols Command
  • Identify the parameters and other information about the current state of any active IPv4 routing protocol processes configured on the router
  • What information can you get from this show command?
  • show ip route
  • Verify the routes are installed in the IPv4 routing table as expected
  • Check for convergence

6.3 EIGRP Operation

EIGRP Initial Route Discovery

  • Can you describe the initial route discovery process?


EIGRP Metrics

  • Composite Metric
  • EIGRP uses bandwidth and delay values in the composite metric to calculate the preferred path to a network.
  • The k values and EIGRP AS number must match to form an adjacency.
  • The show ip protocols command can be used to verify k values.
  • Bandwidth Metric (BW)

  • The bandwidth kilobits-bandwidth-value command is used to modify the bandwidth metric.
  • Use the show interfaces command to verify the bandwidth changes
  • Delay Metric (DLY)
  • Delay is the measure of the time it takes for a packet to traverse a route.
  • Use the show interfaces command to view the delay values.

  • Complete Composite Metric Formula
  • (𝐾1 ∗𝑏𝑎𝑛𝑑𝑤𝑖𝑑𝑡ℎ+ 𝐾2 ∗𝑏𝑎𝑛𝑑𝑤𝑖𝑑𝑡ℎ 256 −𝑙𝑜𝑎𝑑 +𝐾3 ∗𝑑𝑒𝑙𝑎𝑦)∗ 𝐾5 𝑟𝑒𝑙𝑖𝑎𝑏𝑖𝑙𝑖𝑡𝑦+𝐾4 ∗256
  • Using the default metric weight, the formula becomes
  • Metric = (𝐾1 ∗𝑏𝑎𝑛𝑑𝑤𝑖𝑑𝑡ℎ+ 𝐾3 ∗𝑑𝑒𝑙𝑎𝑦)∗256
  • where K1 and K3 equal to 1, K2, K4, and K5 equal to 0 when not in use, and If K5 = 0, 𝐾5 𝑟𝑒𝑙𝑖𝑎𝑏𝑖𝑙𝑖𝑡𝑦+𝐾4   becomes 1.
  • Calculate the EIGRP metric between R2 and R3
  • Metric = 256 ∗( 10 7 𝑏𝑎𝑛𝑑𝑤𝑖𝑑𝑡ℎ + sum of delay 10 )
  • What is bandwidth of the slowest link?
  • What is the sum of all delays?
  • What command is used to verify the metric?


DUAL and the Topology Table

  • Diffusing Update Algorithm (DUAL) provides
  • Loop-free paths
  • Loop-free backup paths that can be used immediately
  • Fast convergence
  • Successor and Feasible Distance
  • A successor is a neighboring router that is used for packet forwarding and is the least-cost route to the destination network.
  • Feasible Distance is the metric listed in the routing table entry
  • Feasible Successors, Feasibility Condition, and Reported Distance
  • The reported distance is an EIGRP neighbor’s feasible distance to the destination network.
  • The feasibility condition (FC) is met when a neighbor’s reported distance (RD) to a network is less than the local router’s feasible distance to the same destination network.
  • A feasible successor is a neighbor that has a loop-free backup path to the same network as the successor, and it satisfies the Feasibility Condition

  • The show ip eigrp topology command
  • Displays the Topology Table
  • Topology Table
  • Lists all successors and FSs to destination networks
  • Only successors are installed in the routing tables
  • Can you name all the highlighted parts in the topology table below?
  • Which is the successor?
  • Which is not a feasible successor?


DUAL and Convergence

  • DUAL Finite State Machine (FSM)
  • Contains all of the logic used to calculate and compare routes in an EIGRP network
  • DUAL: Feasible Successor
  • When a path to the successor goes down with FS in the topology table:
  • Router informs all EIGRP neighbors of the lost link.
  • Router updates its own routing and topology table.
  • DUAL: No Feasible Successor
  • When a path to the successor goes down with no FS in the topology table:
  • DUAL puts the route into an active state.
  • DUAL sends EIGRP queries asking other routers for a path to the network.
  • Other routers return EIGRP replies, letting the sender of the EIGRP query know that they have a path to the requested network. If there is no reply, the sender of the query does not have a route to this network.
  • If the sender receives EIGRP replies with a path to the requested network, the preferred path is added as the new successor and also added to the routing table.


6.4 Implement EIGRP for IPv6

EIGRP for IPv6

  • EIGRP for IPv6
  • Similar functionality as EIGRP for IPv4
  • Uses IPv6 for communicationwith EIGRP for IPv6 peers and advertising IPv6 routes
  • Uses DUAL
  • EIGRP for IPv6 is a separate process from EIGRP for IPv4

  • IPv6 Link-local Address
  • Packets with a source or destination link-local address cannot be routed beyond the link from where the packet originated.
  • IPv6 link-local addresses are in the FE80::/10 range.


Configure EIGRP for IPv6

  • Configuring IPv6 Link-local Addresses
  • Link-local address can be automatically created or manually configured
  • When created automatically, the router creates the link-local address using FE80::/10 prefix and the EUI-64 process. Use the ipv6 address link-local-address link-local command to manually configure the link-local address using the FE80::10 prefix
  • Link-local addresses must be unique on the same local link.

  • Configuring the EIGRP for IPv6 Routing Process
  • The ipv6 unicast-routing command enable IPv6 routing
  • The ipv6 route eigrp autonomous-system command is used to enter the router configuration mode. The process needs to be activated with the no shutdown command.
  • To configure the Router ID, use the eigrp router-id command.
  • Both the no shutdown command and a router ID are required for the router to form neighbor adjacencies.
  • The ipv6 eigrp interface Command
  • EIGRP for IPv6 is configured directly on the interface.
  • ipv6 eigrp autonomous-system
  • Configure passive interface in the router configuration mode
  • passive-interface interface


Verify EIGRP for IPv6

  • IPv6 Neighbor Table
  • The show ipv6 eigrp neighbors command is used to display neighbor adjacencies
  • The show ip protocols Command
  • Displays the parameters and other information about the state of any active IPv6 routing protocol processes currently configured on the router.
  • Displays different types of output specific to each IPv6 routing protocol.
  • The EIGRP for IPv6 Routing Table
  • The show ipv6 route command is used to view the IPv6 routing table




6.5 Chapter Summary

EIGRP (Enhanced Interior Gateway Routing Protocol) is a classless, distance vector routing protocol.

EIGRP uses the source code of “D” for DUAL in the routing table. EIGRP has a default administrative distance of 90 for internal routes and 170 for routes imported from an external source, such as default routes. These features include: Diffusing Update Algorithm (DUAL), establishing neighbor adjacencies, Reliable Transport Protocol (RTP), partial and bounded updates, and equal and unequal cost load balancing.

EIGRP uses PDMs (Protocol Dependent Modules) giving it the capability to support different Layer 3 protocols including IPv4 and IPv6. EIGRP uses reliable delivery for EIGRP updates, queries and replies; and uses unreliable delivery for EIGRP Hellos and acknowledgments. Reliable RTP means an EIGRP acknowledgment must be returned.

Before any EIGRP updates are sent, a router must first discover its neighbors using EIGRP Hello packets. The Hello and hold-down values do not need to match for two routers to become neighbors. The show ip eigrp neighbors command is used to view the neighbor table and verify that EIGRP has established an adjacency with its neighbors.

EIGRP sends partial or bounded updates, which include only route changes. Updates are sent only to those routers that are affected by the change. EIGRP composite metric uses bandwidth, delay, reliability, and load to determine the best path. By default only bandwidth and delay are used.

At the center of EIGRP is DUAL (Diffusing Update Algorithm). The DUAL Finite State Machine is used to determine best path and potential backup paths to every destination network. The successor is a neighboring router that is used to forward the packet using the least-cost route to the destination network. Feasible distance (FD) is the lowest calculated metric to reach the destination network through the successor. A feasible successor (FS) is a neighbor who has a loop-free backup path to the same network as the successor, and also meets the feasibility condition. The feasibility condition (FC) is met when a neighbor’s reported distance (RD) to a network is less than the local router’s feasible distance to the same destination network. The reported distance is simply an EIGRP neighbor’s feasible distance to the destination network.

EIGRP is configured with the router eigrp autonomous-system command. The autonomous-system value is actually a process-id and must be the same on all routers in the EIGRP routing domain. The network command is similar to that used with RIP. The network is the classful network address of the directly connected interfaces on the router. A wildcard mask is an optional parameter that can be used to include only specific interfaces.

EIGRP for IPv6 shares many similarities with EIGRP for IPv4. However, unlike the IPv4 network command, IPv6 is enabled on the interface using the ipv6 eigrp autonomous-system interface configuration command.


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