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EIGRP Interview questions and Answers

Writer's picture: Techclick co_inTechclick co_in

Updated: Jan 19

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What is EIGRP?

·EIGRP (Enhanced Interior Gateway Routing Protocol) is a Cisco-proprietary routing protocol that uses a hybrid of distance-vector and link-state algorithms.


·it has faster convergence times and uses less network bandwidth.


·EIGRP uses a composite metric, called the DUAL (Diffusing Update Algorithm), to determine the best path to a destination.


Metric used in EIGRP?

The metric takes into account several factors, including:

·Bandwidth: The highest bandwidth of the path to the destination.

·Delay: The sum of the delays of all links in the path to the destination.

·Reliability: The reliability of the path to the destination.

·Load: The load on the path to the destination.

·MTU: The maximum transmission unit of the path to the destination.

By default, Bandwidth and Delay used to calculate metrics.


Can you explain the concept of EIGRP neighbors and how they are formed?


·EIGRP neighbours are routers that have established a relationship with each other by exchanging hello packets and have the same autonomous system number configured.


·They share routing information with each other and form a neighbor table.


Tables maintained by EIGRP routers?


EIGRP routers maintains below tables to store information about the network, including:


·Neighbour Table- Store information about EIGRP neighbors. 

·Topology Table- Store routing information which is learned from neighbors’ routers. 

·Routing Table- Store the best path to all networks. 


Criterion to form neighborship?


·Must be directly connected.

·Same AS number.

·Same Subnet

·They must have the same authentication configuration

·They must have the same K values: EIGRP routers use the same K-values for metric calculations. If there are different K-values, the routers will not form a neighborship.


Explain packets in EIGRP?


·Hello Packets: Sent by routers to establish and maintain a neighborship. Hello packets contain information about the router's ASN, K-values, and hold time


·Update Packets: Sent by routers to update their routing tables and share routing information with their neighbors. Update packets contain information about routes, including the destination, next hop, and metric.


·Query Packets: Sent by a router when it loses a route and needs to find an alternate path. Query packets are multicast to the router's neighbors, asking them if they have a route to the destination.


·Reply Packets: Sent by a router in response to a query packet. Reply packets contain information about routes to the destination, including the next hop and metric.


·Ack Packets: Sent by a router to acknowledge the receipt of update and query packets. Ack packets are unicast to the sender of the update or query packet to confirm that the packet was received.


What are AD values in EIGRP?


·External EIGRP-170

·Internal EIGRP – 90

·Summery EIGRP – 5


What are external routes in EIGRP?


·In EIGRP, external routes refer to routes that are learned from other routing protocols, such as OSPF or BGP. These routes are not part of the EIGRP autonomous system (AS) and are redistributed into EIGRP.


·External routes are less trusted hence AD value is 170.


·When a router receives external routes, it will add them to its routing table, but it will mark them as "external" routes


What is Feasibility condition in EIGRP?


·In EIGRP, feasibility condition is a set of rules that a router uses to determine if a route is considered "feasible" or "not feasible."


·A feasible route is a route that is considered to be a valid alternative path to a destination.


EIGRP consist of two terms:


·Successor: Best route to destination.


·Feasible Successor: Alternative or Backup path to destination. A route is considered feasible if its reported distance (RD) is less than the feasible distance (FD) of current successor route is called Feasible condition.


Timers in EIGRP?


·Hello Timer: The hello timer is used to establish and maintain neighborship between routers. The default hello timer is 5 seconds.

·Hold Timer: This timer controls the maximum amount of time a router will wait to receive a hello packet from a neighbor before considering the neighborship to be down

The default hold timer is 15 seconds.

·Update Timer: Update packets contain information about routes and are used to update the routing tables of the router's neighbors.

The default update timer is 90 seconds.

·Invalid Timer: This timer controls how long a route will remain in the routing table after it is marked as "invalid." An invalid route is a route that is no longer considered feasible.

The default invalid timer is 180 seconds.

·Flush Timer: The flush timer ensures that the route is removed from the routing table even if the router does not receive an update packet.

The default flush timer is 240 seconds.

·SIA-Query Timer: This timer controls how long a route will remain in the stuck-in-active state before a SIA-Query packet is sent by a router The default SIA-Query Timer is 3 minutes.


What is Passive Interface?


Don’t send and ] receive update on that interface.


Authentication type supported by EIGRP?

MD5


EIGRP stub router?


·A stub router in EIGRP (Enhanced Interior Gateway Routing Protocol) is a router that is configured to advertise only a default route to its neighboring routers, rather than the full routing table.


·This is useful in situations where a router has a limited number of directly connected networks, and it is not necessary to advertise all of the routes in the network.


·The main goals of the EIGRP stub routing function are to increase network stability and save memory and CPU on local routers


What is stuck-in-active?


·When EIGRP loses a route, and there is no feasible successor the route will go from passive to active, and the router starts sending queries to its neighbors. To get to know about any route to reach destination network.


·Until router receives Reply for Query msg no successor selected.


·EIGRP router will wait for 3 min for the reply of the queries sent by it and if don’t receives reply the route become stuck-in-active


·And router reset neighbor relationship with neighbor that did received reply

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