Fundamental EIGRP Concepts

EIGRP Fundamentals

  • What will the network command advertise without wildcard mask?
    • The whole class A network (10.0.0./8)
  • What is the high end range of this network command “network”?
  • How to check EIGRP active interfaces?
    • sho ip eigrp interfaces
  • What are the general steps of EIGRP (forming neigbhors, adding routes)?
    • neighbor discovery: hello messages
    • topology exchange: first it send the full topology updates; after this only partial updates
    • choosing routes: each router analyzes the topology table and put the lowest metric route into the routing table
  • Which address do EIGRP uses for routing updates? What protocol is used?
    • multicast
    • RTP (reliable transport protocol), resends routing updates in case it lost in transit
  • What does EIGRP uses for calculating metrics?
    • by default: bandwidth, delay
    • optional: load, interface reliability
  • How can you impact the (default) metric?
    • bandwidth Kbps and delay interface subcommands
  • What are the default EIGRP hello and hold timer?
    • hello: 5 sec
    • hold: 15 sec
  • What type of authentication does EIGRP use?
    • MD5
  • How do you turn on automatic summarization?
    • it is turned on by default
    • to turn off: no auto-summary

EIGRP neighborships

  • What if EIGRP AS numbers do not match?
    • neighborship wont be formed
  • What happens if the EIGRP active interface goes up/down state?
    • EIGRP knows that the neighbor failed and converges
  • What happens if the interfaces stays up/up but the line is not working?
    • the hold timer will
  • How do you configure hello and hold timers for specific interfaces?
    • under interface:
      ip hello-interval eigrp asn-number seconds
      ip hold-time eigrp asn-number seconds
  • How the above hold-time command effects the neighbor?
    • The neighbors use each other configured hold-time for countdown.
      ie.: R1 configured 2 sec hello and 6 sec hold
      R2 configured 5 sec hello and 15 sec hold
      In this case if R2 doesn’t receive hello message within 6 seconds it will declare R1 failed and converge
      R1 will do the same if no hello message received within 15 seconds
  • How do you verify timers?
    • sho ip eigrp interfaces detail
      (older IOS will not show hold time, try sho run in this case)
  • What happens when you change hello timer in EIGRP and what happens in OSPF?
    • EIGRP: hello timer changes
    • OSPF: hello timer and dead time changes (3* hello)
  • How do you advertise an interface subnet without allowing to form EIGRP neighborships?
    • Use the network command as always, but implement “passive-interface interface-name” command. (prefered)
      • An alternative of this is to use “passive-interface default” and use “no passive-interface interface-name” to allow EIGRP communication on an interface.
    • Another way is using redistribution instead of network command (redistribute connected).
  • What is the point of the static EIGRP neighbor configuration?
    • After using the neighbor command there wont be hello messages sent to (on that interface). The hello messages are going to be sent to the neighbor via unicast (also useful in NBMA)
  • How do you configure static neighbor?
    • neighbor IP outgoing-interface
      The IP need to be in a connected subnet.
      Also this need to be implemented on both sides.
    • network command is still needed to advertise the subnets, but no hello messages sent anymore
  • How do you recognize if the neighbor is static or dynamic?
    • It is marked in “show ip eigrp neighbors detail” (only in detail).
  • What is the danger of the static EIGRP neighbor config?
    • In case there are multiple EIGRP neighbor connected dynamically then neighbor command is used, the router will lose all existing (or new) dynamic neighbors.
  • What are the neighbor requirements of EIGRP and OSPF ?
    • neighboring-eigrp-ospf.PNG
      (The primary address means the case of an interface with secondary IP configured)
  • What happens if the neighbors K values don’t match?
    • The neighborship won’t be formed and a log message will be made.
  • How do you configure metrics?
    • metric weights TOS K1 K2 K3 K4 K5
    • !Default K values
      R1#sho ip protocols
      Routing Protocol is "eigrp 1"
       Outgoing update filter list for all interfaces is not set
       Incoming update filter list for all interfaces is not set
       Default networks flagged in outgoing updates
       Default networks accepted from incoming updates
       EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
      • Beth Loves Deep Relaxing Massages 
      • Bandwidth (K1)
      • Load (K2)
      • Delay (K3)
      • Reliability (K4)
      • MTU (K5)
  • EIGRP and OSPF have the same rules to create RID. What are those?
    • 1. Use the configured ID ( eigrp router-id a.b.c.d )
    • 2. Use the highest IPv4 address on an up/up loopback interface.
    • 3. Use the highest IPv4 address on an up/up non-loopback interface.
  • How do you verify EIGRP router id?
    • R2#show ip eigrp topology
      IP-EIGRP Topology Table for AS(12)/ID(
  • How strict / important the presence of RID in EIGRP?
    • It is necessary to have an RID and best is to have it unique, but duplicated RID do not prevent routers from becoming neighbors.

Neighborship over WAN

  • Describe Frame Relay service!
    • Frame Relay provides a Layer 2 WAN service. Each router connects to the service using a physical serial link, called a Frame Relay access link. The provider then creates logical connections, called permanent virtual circuits (PVC) , which are logical paths between pairs of routers connected to a Frame Relay service. Any pair of routers that connect to the ends of a Frame Relay PVC can send Frame Relay frames to each other. Therefore, they can send IP packets.
  • Describe MPLS service!
    • Multiprotocol Label Switching (MPLS) Virtual Private Networks (VPN) create a WAN service that has some similarities but many differences when compared to Frame Relay. The customer routers connect to the service, often with serial links but at other times with Frame Relay PVCs or with Ethernet. The service itself is a Layer 3 service, forwarding IP packets through a cloud. As a result, no predefined PVCs need to exist between the customer routers.
  • Describe MetroEthernet service!
    • The term Metropolitan Ethernet (MetroE) represents a range of Layer 2 WAN services in which the CE device connects to the WAN service using some form of Ethernet. Because MetroE provides a Layer 2 Ethernet service, the service delivers an Ethernet frame sent by one customer router to another customer router (for unicast frames), or to many other routers (for multicast or broadcast frames).
  • Which of the above services can run EIGRP?
    • all

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