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EIGRP Interview Questions and Answers CCNP

What is EIGRP?
Enhanced Interior Gateway Routing Protocol (EIGRP) is an enhanced distance vector routing protocol which uses Diffused Update Algorithm (DUAL) to calculate the shortest path. It is also considered as a Hybrid Routing Protocol because it has characteristics of both Distance Vector and Link State Routing Protocols.
EIGRP supports classless routing and VLSM, route summarization, incremental updates, load balancing and other features.

What are the requirements for neighborship in EIGRP?
The following fields in a hello packet must match for routers to become neighbors:-
1. Autonomous System number.
2. K-values.
3. Authentication.
4. The primary address should be used.
5. If static neighborship then should be defined on both sides.

What is the metric of the EIGRP protocol & its default values?
1. Bandwidth (K1=1)
2. Load (K2=0)
3. Delay (K3=1)
4. Reliability (K4=0)
5. Maximum Transmission Unit (K5=0)
By default, EIGRP only uses Bandwidth (K1) and Delay (K3) to calculate metrics.

Give the formula by which EIGRP calculates metric?
Metric = 256 * [(10^7 / lowest-bandwidth) + cumulative-delay]
The lowest bandwidth is the lowest-bandwidth link in the route, using a unit of Kilobits per second.
The cumulative-delay value used in the formula is the sum of all the delay values for all links in the route, with a unit of tens of microseconds.

What are the four basic components of EIGRP?
The four basic components of EIGRP are -
1. The Protocol Dependent Module - It supports IP, IPv6, IPX, Apple Talk.
2. The Reliable Transport Protocol - RTP is used in EIGRP for detecting packet loss and to ensure ordered delivery of the packets.
3. The Neighbor Discovery and Recovery Module - Hello messages are used for Neighbor Discovery and Recovery.
4. The Diffusing Update Algorithm - It is an algorithm used by EIGRP for selecting the lowest cost loop-free path for each possible destination.

What are the different packet types used by EIGRP?
The packet types used by EIGRP are:-
1. Hello - Neighborship is discovered and maintained by Hello Packets.
2. Acknowledgement - ACK packets are used to acknowledge the receipt of an update, query and reply packets. Acknowledgement packets are Unicast.
3. Update - EIGRP uses Update messages to send routing information to neighbors. Update packets can be sent to a single neighbor using unicast or to a group of neighbors using multicast.
4. Query -  Query packets are used when EIGRP router has lost path (Successor) to a certain network and does not have any backup paths (Feasible Successor). The router sends query packets to its neighbors asking them if they have information about this particular network. Query packets are multicast.
5. Reply -  Reply packets are used in response to the query packets. Reply packets are unicast to the originator of the query.

What is Reliable Transport Protocol?
EIGRP uses RTP (Reliable Transport Protocol) to deliver EIGRP packets between neighbors in a reliable and ordered way. If the packet with RTP enable sent, gets lost in the transit it will be sent again (resend).

What packets are RTP enabled?
1. Update Packet.
2. Query Packet.
3. Reply Packet.

Explain what will happen if the packet is not acknowledged?
If a packet is not acknowledged, EIGRP will retransmit the packet to the non-responding neighbor as a unicast. No other traffic is sent to this neighbor until it responds. After 16 unacknowledged re-transmissions, the neighbor is removed from the neighbor table.

Explain EIGRP Router ID?
In EIGRP, duplicate RIDs do not prevent routers from becoming neighbors and two EIGRP routers with the same router ID will still form a neighbor relationship. The only time the value of EIGRP RIDs consider is when injecting external (redistributed) routes into EIGRP. In this case, the routers injecting the external routes should have unique RIDs to avoid confusion.
To manually configures the router ID
R1(config)# router eigrp 10
R1(config-router)# eigrp router-id

Explain Unequal Cost Load Balancing in EIGRP?
By default, EIGRP will automatically load-balance across equal-metric routes. EIGRP also supports load-balancing across routes with an unequal metric. Unequal cost load balancing in EIGRP is the concept by which load sharing can take place on paths that do not have the equal metric. In EIGRP variance is used for Unequal cost load balancing. Variance is specified as an integer in the range of 1 through 128. The router then 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 successors FD are considered to be equal routes and can be placed into the routing table for load sharing.
Router(config)#  router eigrp 100
Router(config-router)#  variance 2

In this case, variance is 2.

Explain Split Horizon?
The Split Horizon feature prevents a route learned on one interface from being advertised back out of that same interface. It is used to prevent loop in EIGRP.

Explain Null Zero?
It is a loop avoidance mechanism entry stored in the routing table only in case of summarization (auto & manual). It terminates or flushes unwanted packets, if any traffic goes towards null0 it will be drop by EIGRP.

What is Active State and Passive State?
Routes for which the successor route fails and no feasible successor routes exist moves to an active state forcing the EIGRP to send out query packets and reconverge.
A route is in a passive state for which the router has a successor route, and no failure has yet occurred. A stable EIGRP network will have all routes in a Passive state.

Explain Stuck in Active?
When for a certain prefix, successor route fails and no feasible successor route exists then the router begins a process of finding any loop-free alternative routes to reach that prefix by sending Query messages to all of its neighbors requesting path to the lost prefix. If the neighbor routers do not have information about the lost prefix, they will forward the query message to further routers. Within a large network, particularly when routers exist several router hops away, the number of Queries might not only be large, but there also might be a string of routers that all must wait on multiple Reply messages before they can, in turn, issue a Reply. To deal with this long time problem, Cisco IOS first sets a limit on how long it should take to receive all such replies. This timer is called the active timer and is set to 3 minutes by default. Routes for which a router does not receive a Reply within the active timer are considered to be Stuck-in-Active (SIA) routes. The router sends an SIA-Query (Stuck-in-Active Query) EIGRP message to each neighbor that has yet to send back a Reply. The purpose of the message is to either get an SIA-Reply back indicating that the neighbor really is still waiting for replies to its own queries meaning the neighbor is alive and still working & there is no need to kill the neighborship or to get nothing in reply meaning neighbor was not able to reply, so the action of failing the neighborship is reasonable.

What are Graceful Shutdown and GoodBye message in EIGRP?
When an EIGRP process is shut down, the router sends out “goodbye” messages to its neighbors. The neighbors can then immediately begin recalculating paths to all the destinations that went through that shutdown router without having to wait for the hold timer to expire.

How Passive Interface command works in EIGRP?
With EIGRP running on a network, the passive-interface command stops sending outgoing hello packets, hence the router cannot form any neighbor relationship via the passive interface. This behavior stops both outgoing and incoming routing updates. However, EIGRP still advertises the connected subnets if matched with an EIGRP network command.
# router eigrp 1
# passive-interface fastethernet0/0
Command to see list of passive-interfaces
# show ip protocols

How can we change Hello and Hold time in EIGRP?    
# interface Fa0/0
# ip hello-interval eigrp 100 3
# ip hold-time eigrp 100 12
These commands will make hello interval 3 seconds and hold time 12 seconds.

# show ip eigrp interfaces detail (To verify)

What is the Feasibility Condition in EIGRP?
For any route to be a feasible successor it has to fulfil feasibility condition which is as follows:-
The advertised distance of Feasible successor should be less than Feasible distance of Successor
AD of feasible successor < FD of successor.

What is the Multicast IP address used by EIGRP?
EIGRP uses the multicast address

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