EIGRP Quick and Important Facts

Today, i will discuss about the one of the important routing protocol in many of the enterprise domain network. The Protocol is initially Cisco Propriety  protocol but later on Cisco announces this protocol as open standard protocol and is now used in any of the routers ( It can be Juniper, Huawei, HP or any other ). 

There are lot of interesting facts around the EIGRP protocol, Some says it is Link state routing protocol and some says it is distance vector routing protocol. Well it is a Hybrid routing protocol. I am expecting you guys know about the fact of Distance Vector routing protocol and also know about the features and working of the Link State routing protocols.

Whenever there is discussion on Distance vector routing protocol then there are two protocols and they are RIP ( Routing Information Protocol ) and IGRP ( Interior Gateway Routing Protocol)

Whenever there is another discussion where we thought about Link state routing protocol, there we have IS-IS routing protocol ( Intermediate system to Intermediate System) and OSPF  ( Open shortest Path First) routing protocols.

Now we have the query, If we have two kinds of the routing protocols and EIGRP which we are going to discuss today is not the part of any of these kinds, then where this EIGRP routing protocol lies actually. Right, i am expecting the same question which you guys have. 

Starting from EIGRP, It is an enhanced version of IGRP protocol which lies under distance vector category.  EIGRP ( Enhanced Interior Gateway Routing Protocol ) is a Hybrid protocol which has both the features of Distance Vector routing Protocol and Link state routing protocol.

Quick Facts of EIGRP


  • EIGRP : Hybrid Routing Protocol ( Link State and Distance Vector features)
  • EIGRP uses DUAL algorithm ( Diffusion update Algorithm )
  • EIGRP are of two types : iEIGRP and eEIGRP
  • AD: iEIGRP have the AD value of 90 and eEIGRP have the AD value of 170
  • EIGRP is a layer 3 routing protocol and it operates on IP protocol number 88.
  • EIGRP supports maximum Hop count of 255 and by default it is 100 Hop counts.
  • EIGRP uses multicast address of 224.0.0.10
  • EIGRP supports IP, IPX and Appletalk.
  • EIGRP uses metric for path selection. These metrics are Bandwidth, Load, delay, Reliability and MTU
  • EIGRP uses formula for the path selection and is called as K values. These K values are K1=Bandwidth, K2=Load, K3=Delay, K4=Reliability, K5=MTU.
  • By default Bandwidth and delay metrics are taken for the calculation of the path from source to destination.
  • There are some of the important terms used in the EIGRP and they are Advertised distance, Feassible distance, Successor, Feassible successor and Feasibility Condition.
  • EIGRP uses MD5 Authentication.
  • EIGRP supports equal-cost load balancing and also unequal-cost load balancing.
  • It builds 3 different tables and these tables are Routing Table, Topology Table and Neighbor table.
  • EIGRP packet types are Hello, update, Query, Reply and Acknowledgement packets.
  • EIGRP has two route states, they are Active and Passive.
Fig 1.1 EIGRP Tables


Now lets talk about the other important facts and some of the quick facts in details:

Important Facts of EIGRP:

EIGRP is basically Cisco Propriety Protocol when started but now it is open standard protocol. It is Hybrid Protocol and hardly used in the Live or real Networks. People are using OSPF, IS-IS protocol generally in their Environment.
  • The IP header of an EIGRP packet specifies protocol number 88.
  • To establish neighbor relationship, the neighbors must be in the same IP subnet. While EIGRP supports secondary IP addresses and subnets, EIGRP sources its messages always from the address in the primary subnet, so the IP addresses of neighbors must be in the subnet of the primary subnets. Routers will not form EIGRP neighbors over secondary networks.
  • Make a note of it that both sides must match metric weights (K values) in order to form EIGRP neighbor adjacency.
  • Same as OSPF, In EIGRP the hello and hold time parameters do not need to match to form EIGRP neighbor relationships.
  • In EIGRP the default the hold time is three times the Hello interval; 180 seconds for low-speed non-broadcast multiaccess (NBMA) networks and 15 seconds for all other networks.
  • If you are talking about the EIGRP auto-summarizes connected, internal routes across classful network boundaries.
  • There is one command in EIGRP and that command is “passive-interface” command for an interface does not stop advertising of that interface in the EIGRP updates.
  • In EIGRP, the default route can be advertised in the EIGRP domain several ways via static route to 0.0.0.0, with the “redistribute static” command or you can have ip summary-address 0.0.0.0 0.0.0.0 command, and you can use it as “ip default-network ” command.
  • Same as RIP( Routing Information Protocol )in EIGRP to propagate the default route, the network specified by the “ip default-network ” command must be advertised into EIGRP.
  • When we are generating default route in EIGRP, "ip summary-address eigrp 100 0.0.0.0 0.0.0.0 250" should be used along with higher administrative distance (floating route) if this router already has a default route in its routing table leaned via any other means. Otherwise the default route (to null interface) generated by this command, may black hole the traffic.
  • leak-map option ("ip summary-address eigrp leak-map ") is only available under physical and virtual-template interfaces. Again, if the leak-map keyword is configured but the access-list does not exist or the route map does not reference the access list, the summary address and all component routes are sent.
  • In EIGRP leak-map option with “eigrp stub ...” command has the same functionality as leak-map option with “ip summary-address ” command.
  • Same as RIP, EIGRP only needs “neighbor” command to send unicast updates. “passive-interface ” command should not be used along with it; otherwise it will stop sending EIGRP hello packets.
  • In“passive interface ” command in the frame-relay physical interface does not inherited by the subinterfaces. So configuring this command on the frame-relay physical interface does not affect EIGRP process at all.
  • Same as RIP, with EIGRP split horizon is enabled on all frame-relay multipoint interfaces (physical or subinterface). The “show ip interface” command doesn’t verify split horizon for EIGRP, the only way to verify it, is by checking running configuration.
  • If you are talking about the authentication, In EIGRP authentication, key number must match along with key string on both sides. If configured with multiple keys, EIGRP only sends the lowest numbered valid key but accepts any valid key.
  • The administrative distance filtering technique only works for EIGRP internal routes, doesn’t work for EIGRP external routes. The distance of external EIGRP cannot be changed on a per prefix basis.
  • The “default-metric ” command does not affect in EIGRP-to-EIGRP redistribution.
  • To change the EIGRP metric, it’s better to use “delay”, so it will not affect other protocols (OSPF) dependent on “bandwidth”.
  • The “ip bandwidth-percent ” command can have values greater than 100 percent if the bandwidth is configured (by the “bandwidth ” interface configuration command) artificially low due to policy reasons.
  • A route becomes active when no feasible successor exists in its topology table. An active route becomes passive when a reply has been received from every queried neighbor.
  • A route map may be configured with both the “redistribute ” and the “distribute-list ” commands in the same routing process.
  • gateway” option in “distribute-list ” command is only available with prefix-list, but not with ACL.
  • EIGRP does not automatically summarize external routes.
  • The router originating the external route inserts its EIGRP router-id in the update. If an update is received back in with the router-id matching the local router-id, the update is dropped.
These are the important facts in EIGRP, we will discuss all these in detail as per. Also we will discuss the configurational part soon.
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