Category Archives: EIGRP

Enhanced Interior Gateway Protocol

Default and Summary Routes

Today I ran into some issues with a lab I was doing involving an eigrp default route and summarisation.

Firstly the default route, basically the task was to inject the loopback interface IP into eigrp as the default route. Since the loopback is a directly connected interface a static route would not work so I used the ip default-network [address] command.

However after I entered this command I checked the RIB and found the route was not marked as the candidate default route. I checked and double checked the config (which looked fine to me) and eventually had to resort to It was there I found the simple mistake I made.

When I entered the command, I used the IP address of the loopback interface but the ip default-network command is classful… meaning I needed to enter the class C (in this case) address of the loopback interface, once I corrected this I checked the RIB again and there I could see the correct route marked as the candidate default.

On to the summarisation issue. The task was to summarise four loopback networks which were emulating a LAN network connected to the router. I worked out the correct summary address/mask and entered the ip summary-address eigrp 1 [summary address] [metric] command. When I checked the RIB there was no summary route, so again I double checked my summary address/mask and the command before I hit google.

Here I found that when you enter a summary route you need to enter the interface IP address that is part of the summary route. If you just enter the summary subnet and mask, and there is no interface with that IP address configured the command is allowed but IOS does not put the summary into the routing table. Once I entered the interface IP (which was part of the summary route) and the summarised mask the summary route was added to the RIB and all was well.

Here is a link to the chapter from Troubleshooting EIGRP CCIE Professional Development series that explains the summary route issue I described above.

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CCNP Route v1.0 EIGRP Notes

  • Fast Convergence
  • Partial triggered udpates
  • Multiple protocol support (Ipv4/6, appletalk, IPX)
  • Multicast and Unicast (communications between routers)
  • VLSM support (classless)
  • Datalink layer protocol and topologies – no extra config it just works (eg: FR, broadcast, non broadcast, LAN, WAN)

Four Key Technologies

  • Neighbour discovery/re-discovery – hello packets
  • RTP – reliable transport protocol
  • Dual finite state machine – Decision process for ALL route communications
  • PDMs – Protocol dependant modules, IPv4/6,appletalk, ipx all operate independantly of each other

EIGRP Operation

For IP routing the process is:

Routing Information -> EIGRP Neighbour Table -> Topology Table -> Routing Table

  • Neighbour table, all adjacent neighbours
  • Topology Table, all learned routes to all destinations
  • Routing Table, best route to each destination

Successor = Best route (least cost path)
Feasible Successor = 2nd best route (backup route)
Feasible Distance = The metric of the successor route

The routing table is essentially a subset of the topology table

The topology table stores the RIB of each neighbour, the AD for each distination and the FD> to reah each destination.

Eigrp Packets

Hello – multicast discovery
Update – route changes, sent only to affected routers
Query – RTP, multicast then unicast, look for a successor route
Reply – Response to query, unicast, RTP
Ack – Acknowledge update/query/reply, unicast

All EIGRP overhead traffic is generated through the primary interface address

All packets carrying routing information are sent reliably

Retransmission of a reliable packet occurs up to 16 times or until the hold timer expires. After 16 tries the router resets the neighbourship.

DUAL Feasibility Condition

If your metric is lower than mine, you are loop free

Advertised Distance (AD) = Metric from the neighbour routers point of view
Local Distance = Metric to reach the neighbour

Dual Metric Calculation

Bandwidth (lowest link in Kbps) + Delay (in 10s of microseconds)

K Values

K1 & K3 = 1
K2 & K4 & K5 = 0
Identical K values are required to for an adjacency

Planning EIGRP Implemenations

  • IP Addressing Plan
  • Network TOpology
  • EIGRP Traffic Engineering

Verifying EIGRP Operation

show ip route eigrp
show ip eigrp neighbors
show ip eigrp interfaces
show ip eigrp topology [all-links]
show ip eigrp traffic
show ip protocols

debug ip eigrp
debug eigrp packets
debug ip eigrp summary
debug ip eigrp neighbors

EIGRP Commands

ip default-network – candidate default route
ip summary-address – sumarise on an interface
ip bandwidth-percent eigrp [as] [%] – percentage of interface bandwidth used for eigrp operations (updates/queries/replies)
no ip split-horizon eigrp [as] – turn off split horizon for multi-point FR
ip authentication mode eigrp [as] md5 – enable md5 authentication on the interface
ip authentication key chain eigrp [as] [key chain name] – specify the key chain to use for eigrp authentication

General Notes

Key chain authentication is configured globally and then added per interface
The summary route will stay in the RIB as long as one of the summarised prefixes is still reachable
Summary route have a default AD = 5

All IP routing protocols on Cisco routers can perform equal cost load balancing.

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