Top CCNA Interview Questions and Answers -6

1. What is EIGRP?

Enhanced Interior Gateway Routing Protocol (EIGRP Protocol) 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 balacing and other features.

2. What are the different tables in EIGRP?

EIGRP router stores routing and topology information in three tables:
1. Neighbor table – Stores information about EIGRP neighbors.
2. Topology table – Stores routing information which is learned from neighbor routers.
3. Routing table – Stores the best paths to all networks.

3. Why EIGRP is called hybrid protocol?

EIGRP is also called hybrid protocol because its metric is not just plain HOP COUNT (max255, included in pure distance vector protocol) rather includes the links bandwidth, delay, reliability and Load parameter into the calculation. Thats why called Advanced or Hybrid protocol.

4. What are the different packets or message in EIGRP?

There are Six packets in EIGRP

1-Hello , 2-Update, 3-Query, 4-Reply, 5-Acknoladgment, 6.Request

EIGRP will use six different packet types when communicating with its neighboring EIGRP routers,

– Hello Packets – EIGRP sends Hello packets once it has been enabled on a router for a particular network. These messages are used to identify neighbors and once identified, serve or function as a keepalive mechanism between neighboring devices. EIGRP Hello packets are sent to the link local Multicast group address Hello packets sent by EIGRP do not require an Acknowledgment to be sent confirming that they were received. Because they require no explicit acknowledgment, Hello packets are classified as unreliable EIGRP packets. EIGRP Hello packets have an OPCode of 5.

– Update Packets – EIGRP Update packets are used to convey reachability of destinations. Update packets contain EIGRP routing updates. When a new neighbor is discovered, Update packets are sent via Unicast to the neighbor which the can build up its EIGRP Topology Table. It is important to know that Update packets are always transmitted reliably and always require explicit acknowledgement. Update packets are assigned an OPCode of 1.

– Query Packet – EIGRP Query packets are Multicast and are used to reliably request routing information. EIGRP Query packets are sent to neighbors when a route is not available and the router needs to ask about the status of the route for fast convergence. If the router that sends out a Query does not receive a response from any of its neighbors, it resends the Query as a Unicast packet to the non-responsive neighbor(s). If no response is received in 16 attempts, the EIGRP neighbor relationship is reset. EIGRP Query packets are assigned an OPCode of 3.

– Reply Packets – EIGRP Reply packets are sent in response to Query packets. The Reply packets are used to reliably respond to a Query packet. Reply packets are Unicast to the originator of the Query. The EIGRP Reply packets are assigned an OPCode of 4.

– Acknowledgement Packets – An EIGRP Acknowledgment (ACK) packet is simply an EIGRP
Hello packet that contains no data. Acknowledgement packets are used by EIGRP to confirm reliable delivery of EIGRP packets. ACKs are always sent to a Unicast address, which is the source address of the sender of the reliable packet, and not to the EIGRP Multicast group address. In addition, Acknowledgement packets will always contain a non-zero acknowledgment number. The ACK uses the same OPCode as the Hello Packet because it is essentially just a Hello that contains no information. The OPCode is 5.

– Request Packets – Request packets are used to get specific information from one or more neighbors and are used in route server applications. These packet types can be sent either via Multicast or Unicast, but are always transmitted unreliably.

5. Conditions for EIGRP neigbours.

1. The routers must be able to send/receive IP packets to one another.
2. Interfaces’ primary IP addresses must be in same subnet.
3. Must not be passive on the connected interface.
4. Must use the same ASN (EIGRP) in the router configuration command.
5. Must pass neighbor authentication (if configured).
6. K-values (used in metric calculation) must match

6. What is meant by active and passive states in EIGRP?

Active 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.

Passive State: A route is in 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.

7. What are the different K-values used in EIGRP

– Bandwidth (K1=1)
– Load (K2=0)
– Delay (K3=1)
– Reliability (K4=0)
– Maximum Transmission Unit (K5=0)
By default, EIGRP only uses bandwidth (K1) and delay (K3) to calculate metric.

8. Does EIGRP require an ip default-network command to propagate a default route?

Although eigrp can propagate a default route using the default network method, it is not required. Eigrp redistributes default routes directly

9. Should I always use the EIGRP log-neighbor-changes command when I configure EIGRP?

Yes, this command makes it easy to determine why an EIGRP neighbor was reset. This Reduces troubleshooting time.

10. Does EIGRP support secondary addresses?

Yes, EIGRP supports secondary addresses. Since EIGRP always sources data packets from the primary address, Cisco recommends that you configure all routers on a particular subnet with primary addresses that belong to the same subnet. Routers do not form EIGRP neighbors over secondary networks.

11. What debugging capabilities does EIGRP have?

– show ip eigrp neighbors
– show ip eigrp interfaces
– show ip eigrp topology
– show ip eigrp traffic

12. What are the advantages of EIGRP other routing protocol?

EIGRP is mix of distance vector and link state feature oriented routing protocol that uses DUAL for route calculation. It was Cisco proprietary but since it is been declared open source. It uses5 K values to calculate shortest path and is the only protocol that can provide unequal load balancing. Also provides encryption for security and can be used with iBGP for WAN routing.

13. What is Advertised distance?

The Advertised Distance (AD) is the distance from a given neighbor to the destination router also known as Reported Distance.

14. What is successor?

Successor is considered as the best path to distance from many paths.

15. What is the muticast address used by EIGRP to send Hello packets?

16. What does stuck-in-active mean?

If a router does not receive a reply from a queried neighbor within the active time (3 minutes, by default), the route is declared stuck-in-active. A response with an infinite metric is entered on the neighbor’s behalf to satisfy DUAL, and the neighbor is deleted from the neighbor table.

17. What is the feasibility condition?

The feasibility condition is the rule by which feasible successors are chosen for a destination. The feasibility condition is satisfied if a neighbor’s advertised distance to a destination is lower than the current successors feasible distance to the destination.

18. 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 send again (resend)

19. What packets are RTP enabled?

1. Update Packet.
2.Query Packet.
3.Reply Packet.

20. 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 retransmissions, the neighbor is removed from the neighbor table.

21. 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

22. 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.

23. Explain Null Zero?

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

24. 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

25. 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)

26. What types of Authentication is supported by EIGRP?

1. Null , 2.Plain text , 3. MD5

27. What is the use of “variance” Command in EIGRP?

EIGRP provides a mechanism to load balance over unequal cost paths throungh Variance Command. Variance is a number (1 to 128).

28. Internal and external Administrative distance in EIGRP?

1.Internal – 90
2.External – 170
3.Summary – 5

29. Give the Formula EIGRP uses to calculate Metric?

((10^7/least bandwidth of link) + cumulative delay)*256

30. What is Feasible successor?

A feasible successor to a destination is a neighbor that satisfies the feasibility condition for that destination.