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EIGRP Interview Questions - Part 1


Something about EIGRP..





  • Enhanced Interior Gateway Routing protocol
  • It is an enhanced distance vector routing protocol based on Diffused Update Algorithm (DUAL) to calculate shortest path.
  • Classless routing protocol
  • Support for VLSM, route-summarization
  • Supports unequal cost load balancing, incremental updates




What parameters should match for EIGRP formation?





  • Primary address should be used for forming EIGRP neighborship
  • Autonomous System number
  • K-values
  • In case a static neighborship is configured, it must be done on both the sides




What is the multicast address of EIGRP?





224.0.0.10





What are the packet types used by EIGRP?





  • Hello
  • Acknowledgement
  • Update
  • Query
  • Reply




What are the basic components of EIGRP?





  • Diffused Update Algorithm - It is used for selecting the lowest cost loop-free path for a given destination
  • Reliable Transport Protocol - RTP is used in EIGRP for detecting packet loss and to ensure ordered delivery of packets
  • Protocol Independent Module - Support for IP, IPX, AppleTalk, IPv6
  • Neighbor Discovery (ND) and Recovery Module - Hello packets are used for ND and Recovery




What is RTP?





  • RTP - Reliable Transport Protocol
  • EIGRP uses RTP to deliver packets between neighbors in an ordered and reliable manner. In case of packet loss, RTP ensures re-transmission of the packet to ensure information transfer consistency.




Which EIGRP packets are RTP enabled?





  • Update Packet
  • Query Packet
  • Reply Packet




Do duplicate Router-IDs in EIGRP prevent neighborship? If yes, under what conditions?





Duplicate Router-IDs do not prevent routers from forming neighborships. The only instance when the value of EIGRP router ID is considered is during route redistribution in EIGRP. Manually EIGRP router id can be configured using the command:
eigrp router id x.x.x.x





What is unequal cost load balancing in EIGRP?





Like other protocols, EIGRP supports load-balancing the routers with equal metrics. However, in addition, EIGRP also supports unequal cost load balancing thereby utilizing 'slightly' less preferred links. (The degree of "slightly" depends on the "variance" command). Variance (an integer between 1 and 128, both inclusive) is used for Unequal cost load balancing.
The router multiplies the variance by the successor route's FD (metric of the best route to reach that subnet). Any feasible successor route whose metric is less than or equal to the product of the variance by the successor's FD is considered to be "worthy" of being placed in the routing table (along with the best route).





Command:
router eigrp 100
variance 2





What is a split horizon?





Loop prevention concept used in both EIGRP and RIP, split horizon ensures a route learned on one interface is not advertised back out of that same interface.





What is null zero route?





Another loop prevention mechanism stored in the routing table (in case of summarization) to terminate or flush (black hole) unwanted packets that do not match any other route (don't consider default route)





What is Active state and Passive state?





In EIGRP, "Active" is bad!
In a normal stable network, all EIGRP routes should exist in Passive state with the router having a successor for every EIGRP route. However, in case of a route being unreachable due to link failure or any other reason, it transitions to "Active" state.
Routes where the successor route fails and no feasible successor route exists move to Active state forcing the EIGRP to send out query packets and to reconverge.


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