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  • Table of Contents
  • Layer 2 Technologies
    • Ethernet Switching
      • L2 Switch Operations
      • Spanning Tree
        • 802.1d – STP
        • 802.1w – RSTP
        • 802.1s – MSTP
      • VTP 101
      • Private VLANs
      • VLANs
      • EtherChannel 101
    • Layer 2 WAN Protocols
      • HDLC
        • HDLC 101
      • PPP
        • PPP 101
        • PPP Authentication - PAP
        • PPP Authentication – CHAP
        • PPP Authentication – EAP
        • PPP Multilink
        • PPPoFR – PPP over Frame Relay
        • PPPoE – PPP over Ethernet
      • Frame Relay
        • Frame Relay 101
        • Frame Relay 102
        • Frame Relay Encapsulations – IETF vs Cisco
        • Multilink Frame Relay
        • Frame Relay Switching
        • Routing over Frame Relay
      • Bridging
        • Bridging on a router
        • MTU 101
    • Wireless
      • Wireless Principles
      • Wireless Implementations
      • Wireless Roaming
      • Wireless Authentication
        • WPA2 PSK
        • WPA2 802.1X
  • IPv4
    • IPv4 Addressing
      • Backup Interfaces
      • FHRP 101
      • DHCP 101
      • DNS 101
      • ARP 101
      • IPv4 101
      • Tunnel Interfaces
        • GRE Tunnels
      • BFD – Bidirectional Forwarding Detection
    • IPv4 Routing
      • How the routing table is built
        • How CEF works
        • Routing Order of Operations
        • NSF – Non Stop Forwarding
      • RIP
        • RIP 101
      • EIGRP
        • EIGRP 101
        • EIGRP Metric
        • More EIGRP Features
      • OSPF
        • OSPF 101
        • OSPF Areas
        • OSPF LSAs
        • OSPF Mechanics
      • IS-IS
        • IS-IS 101
        • IS-IS Mechanics – CLNP
      • BGP
        • BGP 101
        • BGP Attributes
        • More BGP
      • Route Redistribution
      • Policy based Routing
      • PfR 101 – Perfromance Routing
      • ODR
  • IPv6
    • IPv6-101
    • IPv6 Routing
    • Interconnecting IPv6 and IPv4
  • MPLS
    • MPLS 101
    • MPLS L3 VPN
  • Multicast
    • Multicast 101
    • PIM 101
    • IGMP 101
    • Inter Domain Multicast
    • IPv6 Multicast
    • Multicast features on switches
  • Security
    • NAT 101
    • NAT for Overlapping Networks
    • ACLs 101
    • ACLs 102
    • Cisco IOS Firewall
    • Zone Based Firewall
    • AAA 101
    • Controlling CLI Access
    • Control Plane
    • Switch Security
      • Switchport Traffic Control
      • Switchport Port Security
      • DHCP Snooping and DAI
      • 802.1x
      • Switch ACLs
    • IPSec VPN 101
      • IKE / ISAKMP 101
      • IPSEC Crypto Maps 101
      • IPSEC VTI 101
      • DMVPN 101
    • EAP 101
  • Network Services
    • NTP 101
    • HTTP 101
    • File Transfer 101 – TFTP & FTP
    • WCCP 101
  • QoS
    • QoS 101
    • Classification and Marking
    • Congestion Management
      • Legacy Congestion Management
      • SPD – Selective Packet Discard
      • CBWFQ
      • IP RTP Priority
    • Congestion Avoidance – WRED
    • Policing and Shaping
      • CAR 101
    • Compression and LFI
      • Header and Payload Compression
      • LFI for MultiLink PPP
    • Frame Relay QoS
      • Per VC Frame Relay QoS
    • RSVP 101
    • Switching QoS
  • Network Optimization
    • NetFlow 101 – TNF – Traditional NetFlow
    • NetFlow 102 – FNF – Flexible NetFlow
    • IP SLA 101
    • IP Accounting 101
    • Logging 101
    • SNMP and RMON 101
    • Cisco CLI Tips and Tricks
    • AutoInstall
    • Enhanced Object Tracking
    • Troubleshooting 101
    • SPAN, RSPAN, ERSPAN
  • Network Architecture
    • Hierarchical Network Architecture
    • SD Access
    • SD WAN
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On this page
  • Router ID
  • DUAL
  • Feasible Distance and the Successor
  • Reported Distance and Feasible Successors
  • Going Active – Sending Queries
  • Stub Routers
  • Next Hop Self

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  1. IPv4
  2. IPv4 Routing
  3. EIGRP

More EIGRP Features

PreviousEIGRP MetricNextOSPF

Last updated 2 years ago

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Router ID

The router ID is a 32 bit number, usually represented as a dotted decimal (like an IP address). The Router ID is determined when the routing process is started by the following algorithm:

  1. Use the configured value

    R(config-router)# eigrp router-id ROUTER-ID
  2. Use the highest up/up loopback ip address

  3. Use the highest up/up non-loopback ip address

The router ID should be different between neighbors. When a router receives a routing update from a router with the same Router ID, it will ignore the update considering it is from itself.

DUAL

The following information can be found looking at the EIGRP topology table:

R# show ip eigrp topology

Feasible Distance and the Successor

A router uses the EIGRP formula to calculate the metric for each destination for all available paths. For each destination, an EIGRP router calculates the metric based on information it has from the neighbors that advertise the destination and its incoming interfaces. See . The best metric for each destination is called the Feasible Distance. This indicates the best path to reach the destination, and the router that is the next hop in this path is called the Successor.

Reported Distance and Feasible Successors

For each destination, an EIGRP router also calculates a Reported Distance (RD), aka Advertised Distance, for the neighbor advertising it. It is actually the metric to the destination seen from the point of view of the neighbor. This will be lower than the metric calculated by the router. All routers that advertise a RD lower than the FD for each destination, are called Feasible Successors. These routers, are guaranteed to have a loop free path to the destination. EIGRP achieves quick conversion by immediately using one of the FS routes as the new Successor, in case the path via the Successor is down. All other paths cannot be guaranteed to be loop free and are not used in this step.

Going Active – Sending Queries

What happens if there are no available FS in the topology table? The route will be marked as Active, and the router will send Queries to all its neighbors (except the Successor that just failed), asking if they have routes for that destination.

The neighbors will have to respond to the Query with a Reply, like this:

Query from
Route state
Action

Successor

Passive

Attempt to find a new successor in the FS list. If none found, mark the destination active, and query all neighbors, except the previous successor.

Neighbor (not current Successor)

Passive

Reply with current Successor information

Neighbor (not current Successor)

not in the topology table

Reply that the destination is unreachable

Waiting for each neighbor to reply, can put the route in a SIA (Stuck in Active) state. When a neghbor doesn’t reply in a resonable interval, a router will bring down the relationship with the neighbor. The default interval is 180 seconds and can be changed with:

R(config-router)# timers active-time {SEC|disabled}

To prevent unnecesary clearing of neighbor relationships, EIGRP sends at half the active-timer (90 sec by default) a SIA-Query message. If the router is still waiting for replies from its neighbors, it will send a SIA-Reply, so the active timer is reset. If the router does not receive a SIA-Reply, thent the neighbor relationship will be broken whent the active-timer expires. The Query domain can be limited by using route summarization and Stub Routers. The route will reply with an unreachable message if it has a route for a summary address, but not for the component route that he is beeing asked about. To see all current active routes, use:

R# show ip eigrp topology active

Stub Routers

A stub router is a router that doesn’t need to know all routes in a network. It is usually the spoke in a hub-and-spoke network. Stub routers do not advertise EIGRP learned routes to other neighbors and Query messages are not sent to stub routers. To define an EIGRP router as stub, use:

R(config-router)#eigrp stub [receive-only | [connected] [redistributed] [static] [summary] [leak-map ROUTE-MAP]]

By default, if no option is used, IOS will use the connected and summary options.

  • receive-only – does not advertise any routes

  • connected – advertises connected routes but only for interfaces matched with a network command

  • summary – advertises auto-summarized or statically configured summary routes

  • static – advertises static routes, assuming the redistribute static command is configured

  • redistributed – advertises redistributed routes, assuming redistribution is configured

  • leak-map – allows a subset of the routing table to be advertised. This subset will be matched by the ROUTE-MAP

Usually, the upstream router of the stub router is configured to only send a summary or a default route to the stub router, but this has to be manually configured.

Next Hop Self

By default, an EIGRP router will advertise routes with itself as the next hop. This can be changed per interface, with:

R(confi-if)#no ip next-hop-self eigrp AS-NUMBER

The command can be usefull in hub & spoke networks, when you want to have the spokes as the next hop instead of the hub.

EIGRP Metric