EIGRP Frequently Asked Questions

Cisco Interview Questions, EIGRP Frequently asked questions or EIGRP FAQ

  1. Explain EIGRP, why it is called Hybrid routing protocol? What metric does it use

Ans:- EIGRP is an enhanced version of IGRP. It adds features of link state protocol in to IGRP. It is classless in nature, supports multicast, uses incremental multicast updates, supports PDM i.e. maintains separate table for IP, IPX, Appletalk. Provides support for IPv6. Since it has features of both Distance and Link state – therefore, known as Hybrid. It uses 32 bit composite metric. Formula is same as that of IGRP’s but it has additional multiplier 256.

{(10000000/bandwidth in kbps)+ (sum of delay of exit interface  till destination in Micro second)/10}*256

  1. How EIGRP’s metric is different than of IGRP’s metric? How is it represented in routing table?

Ans:- Its metric is 32 bit while IGRP is 24 bit. Both use composite metric. EIGRP’s metric is 256 times than of IGRP’s. EIGRP is represented by Keyword “D” , External route of EIGRP as “D Ex”

  1. What is summary route and external route?

Ans:- when we advertise a route by summarizing multiple subnets – it’s known as summary route. External route means the route which is redistributed in EIGRP or route of other routing protocol or static redistributed in EIGRP.

  1. What do you mean by backward compatibility of IGRP with EIGRP? Do we need to do redistribution between IGRP and EIGRP? Explain either case

Ans:- Since EIGRP is enhanced version of IGRP so it has inherited many of its functionalities. It is also backward compatible – means we do not have to redistribute IGRP in to EIGRP or vice versa. IGRP and EIGRP understand so adjusts metric accordingly.

  1. Which routing protocol has the fastest convergence or Rapid convergence?

Ans:- EIGRP has the rapid convergence among dynamic routing protocols. Reason is that it keeps feasible successor or Backup route in ready state and installs in routing table when successor fails.

  1. What is the formula of metric calculation of EIGRP? What is K value? Where do we use K values?

Ans:- {(10000000/bandwidth in kbps)+ (sum of delay of exit interface  till destination in Micro second)/10}*256

By default it is using Bandwidth and Delay while calculating metric. Else we can also use Load, Reliability and MTU but that will make it very complex and metric will become variable due to variable nature of Load and Reliability. K values are 1 or 0 for each of these five elements. 0 means not in use. 1 means in use. So Bandwidth and Delay will have K value as 1. But if all K values are in use than it must be same on neighbor else It won’t form neighborship. Recommendation is avoid using K Values else it will cause huge stability issues.

  1. What is Autonomous Number? Does it need to be same on routers participating in neighborship?

Ans:- AS means a group of router working on a single protocol. It also defines EIGRP Boundaries. It has to be same on participating neighbors.

  1. What algorithm does EIGRP use for best path calculation?

Ans:- It uses DUAL Diffusing update algorithm while calculating the best path.

  1. What do you mean by Link State Protocol? Explain its differences with Distance vector Protocol?

Ans:- Link state knows about the complete network topology and gives information about the link. Link state does not have any hop limit – so that’s the difference with Distance vector.

  1. What are the Hello , Hold timers in EIGRP in case of Bandwidth >1.544Mbps and Bandwidth<=1.544Mbps

Ans:- Hello= 5sec, Hold 15Sec  when Link Bandwidth is >1.544 Mbps . On slow circuits like ISDN or FrameRelay it will have Hello as 60 seconds and Hold as 180 seconds just to reduce the traffic.

  1. Do the timers need to be same on both ends while forming eigrp neighborship?

Ans:- Timers do not need to be same in case of EIGRP while forming neighbors.

  1. What are various packet Types in EIGRP? Explain Hello, Hold , Reply , Query Packets

Ans:- Hello is just a normal handshake packet which verifies that router is alive. Hold is the time till it will wait before flagging that neighbor is down or no response. It is by default set to 3 times Hello unless defined otherwise. Query is a probe packet sent by eigrp router when there is no feasible successor or backup route is available. Reply is response packet to query.

  1. What is SIA (Struck in Active State) in EIGRP? Why it occurs? How we can avoid this? Can we use summarization to address this issue?

Ans:- SIA is struck in active state means that there is no response to query packet sent for finding the route for feasible successor. It is a very bad state occurs due to low speed links or high utilization or memory issues or loops in the circuit. We can avoid it by having loop free network, or by having good memory in devices and cpu. Also we can configure EIGRP STUB on routers which do not act as transit paths and we can also use summarization to suppress the unnecessary routing convergence.

  1. What do you mean by PDM (Protocol Dependency Module)? Does EIGRP support IPv6?

Ans:- EIGRP provides support of PDM which means that IP, IPX, Appletalk routed protocols can be used . EIGRP also maintains different tables of each routed protocol. It also supports IPv6.

  1. What is Successor, Feasible successor in EIGRP? Does feasible successor mean Backup route?

Ans:- Successor means Primary Route to a destination. Feasible successor means alternate or Backup route

  1. What is Feasible Distance, Advertised distance and Reported Distance?

Ans:- Best metric or cost is known as Feasible Distance. Advertised Distance is the Best metric/cost to destination from Neighbor router running EIGRP.  Reported Distance is other name of Advertised Distance.

  1. What is the concept of feasibility condition in EIGRP?

Ans:- For any route to be a backup route in eigrp, there is a rule known as feasibility condition. It states that the advertised distance or reported distance should be less than the best cost or feasible distance. If we are trying to find Backup path for and its feasible distance thru one path is 20 and advertised distance ( i.e. feasible distance from neighbor for is 21,  then there would not be Backup route for because 21 is not less than 20.

  1. Does EIGRP do equal cost balancing by default? Does it do it default for unequal as well?

Ans:- It does equal cost path load balancing by default. So, if we want load balancing happen for unequal path too,  then we need to use variance and multiplier.

  1. How can you configure EIGRP to entertain unequal cost path routes for load balancing? What is “variance” and “multiplier”?

Ans:-  for unequal cost path load balancing, we need to define the boundary of routes  that will fall in unequal path load balancing which will happen by using variance command. Variance will have multiplier “x”. If Feasible distance is 20 and variance multiplier is 3 .  So all route with cost or metric till 20*3= 60 will come i.e. if there are 3 paths apart from 20  with costs as 21 , 34, 59, so it will do load balancing of 4 routes . By default unequal load balancing happens for 4 paths and maximum 6.

  1. If in EIGRP- the feasible distance of Network “A” thru first path is 10, Reported Distance thru second path is 9, Reported Distance for third path is 15. Then how many feasible successors will be there in this case?

Ans:- Feasible condition says that for a route to have a backup path , the Reported Distance of the peer should be less than the feasible distance from local router. In this case , FD from local router to Network “A” is 10. So Reported Distance of the second path is 9, which is valid. Reported distance for  third path is 15 which does not meet condition. So it will have only one feasible distance or backup route i.e. the one thru peer whose Reported Distance from local router is 9.

  1. What are the common causes of eigrp neighborship down/failures?

Ans:-    Show ip eigrp neighbor

              Show IP Eigrp topology x.x.x.x/x

              Show ip eigrp topology all

              Show ip protocols

              Show run | section eigrp

  We may also check the interfaces which are specifically advertised in eigrp.

  1. What are the commands we use to check eigrp neighborship and eigrp topology table?

Ans:-    Show ip eigrp neighbor

              Show IP Eigrp topology x.x.x.x/x

              Show ip eigrp topology all

  1. What do you mean by Active, Passive and SIA states in Topology table?

Ans:-  EIGRP Topology shows three states. “Active state” means that route can’t be used as it is still waiting for replies for query packets sent by eigrp router. “ Passive State” means route is installed in Topology table and is available. “SIA state” means Struck in Active which shows that there has been problems receiving  reply packets of the queries made earlier.  It will reset the complete topology table. SIA can occur due to oversubscribed links, errors in links, High CPU or memory or loops.

  1. How can we check routes in topology table that don’t meet feasible condition?

Ans:- You need to use command “ show ip eigrp topology all-links” to view all routes including the one which don’t meet feasible condition.

  1. What is “Passive Interface”? How it is used in routing protocols especially in EIGRP
  2. What is the max hop limit in eigrp? What’s the default limit?

Ans:- By default its 100. Max is 224. However, some say 255.

  1. How many paths can be load balanced in eigrp? What is the default limit?

Ans:- Default is 4. However, it can be configured till 6 using max-paths command.


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