EIGRP Interview Questions and Answer

Route table: the routes of particular destinations are stored in the routing tables. The information

contains the network topology that is immediately around it. The primary goal of routing protocols and routes is the construction of routing tables. Network id, cost of the packet path and next hop are the details are available in the routing table.

3. Why eigrp is called hybrid protocol?
Eigrp can be referred to as a hybrid protocol. It combines most of the characteristics of traditional distance vector protocols with some characteristics of link-state protocols. Specifically, eigrp is "enhanced" by using four routing technologies: neighbor discovery/recovery.

4. What are the different packets in eigrp?

Explain the different types of packets in eigrp

- hello packets: eigrp neighbor ship is discovered and maintained by hello packets. If the router fails to receive a hello packet within the hold timer, the corresponding router will be declared dead.

- update packets: at the time of discovering new neighbor, update packets are sent, so that the topology table can be built by the neighbor router. Update packets are unicast and always transmitted reliably.

- query packets: when the destination goes into active state, the query packets are sent. Query packets are multicast and replies are always sent in reply to the queries for indicating the originator that it does not need to go into active state.

- reply packets: when the destination goes into active state, the reply packets are sent. Reply packets are unicast to the originator of the query and transmission of reply packets are reliable.

- ack packets: ack packets use to know the transmission status. If a hello packet sent without data is also recognized as acknowledgement. Unicast address with non-zero acknowledgement number is always sent by acks.

5. Conditions for eigrp neighbors.

Both routers must be in the same primary subnet both routers must be configured to use the same k-values both routers must in the same as both routers must have the same authentication configuration (within reason) the interfaces facing each other must not be passive.

6. What settings must match between two routers in order to become eigrp neighbors?

Both routers must be in the same primary subnet

both routers must be configured to use the same k-values

both routers must in the same as

both routers must have the same authentication configuration (within reason)

the interfaces facing each other must not be passive

7. What is meant by active and passive states in eigrp?

Eigrp route states

An eigrp route can exist in one of two states, in the topology table:

• active state

• passive state

A passive state indicates that a route is reachable, and that eigrp is fully Converged. A stable eigrp network will have all routes in a passive state. A route is placed in an active state when the successor and any feasible Successors fail, forcing the eigrp to send out query packets and reconverge. Multiple routes in an active state indicate an unstable eigrp Network. If a feasible successor exists, a route should never enter an active State. Routes will become stuck-in-active (sia) when a router sends out a query Packet, but does not receive a reply packet within three minutes. In other Words, a route will become sia if eigrp fails to re-converge. The local Router will clear the neighbor adjacency with any router(s) that has failed to Reply, and will place all routes from that neighbor(s) in an active state.

8. What are the different k-values used in eigrp?
Eigrp uses different k values to determine the best path to each destination. If you studied ccna you might have seen and/or learned the following list:
Bandwidth (k1) Load (k2)

Delay (k3)

Reliability (k4) Mtu (k5)

9. 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.

10. 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.

11. Does eigrp support secondary addresses?

Eigrp does support 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. Therefore, if all of the primary ip addresses of routers do not agree, problems can arise with neighbor adjacencies.

12. What debugging capabilities does eigrp have?

. There are protocol-independent and -dependent debug commands. There is also a suite of show commands that display neighbor table status, topology table status, and eigrp traffic statistics. Some of these commands are:

Show ip eigrp neighbors 

Show ip eigrp interfaces 

Show ip eigrp topology 

Show ip eigrp traffic

13. What are the advantages of eigrp other routing protocol?

Advanced distance vector

Routes ip, ipx, decnet, appletalk

Routing advertisements: partial when route changes occur Metrics: bandwidth, delay, reliability, load, mtu size Hop count: 255 Variable length subnet masks Summarization on network class address or subnet boundary Load balancing across 6 equal or unequal cost paths (ios 11.0)

Hello timer: 1 second on ethernet / 60 seconds on non-broadcast

Holddown timer: 3 seconds on ethernet / 180 seconds on non-broadcast Metric calculation = destination path minimum bw * delay (msec) * 25 Bidirectional forwarding detection (bfd) support

Split horizon

Lsa multicast address: 224.0.0.10

14. What is advertised distance?

Advertised distance or reported distance. The advertised distance (ad) is the distance from a given neighbor to the destination router. Feasible distance. The feasible distance (fd) is the distance from the current router to the destination

15. What is successor?

A feasible successor is a path whose reported distance is less than the feasible distance, and it is considered a backup route. Eigrp will keep up to six feasible successors in the topology table. Only the one with the best metric (the successor) is placed in the routing table.

16. What is the multicast address used by eigrp to send hello packets?

224.0.0.10


17. What is “stuck in active”?

When eigrp returns a stuck in active (sia) message, it means that it has not received a reply to a query. Eigrp sends a query when a route is lost and another feasible route does not exist in the topology table. The sia is caused by two sequential events: * the route reported by the sia has gone away.

18. What types of authentication is supported by eigrp?

Eigrp route authentication provides md5 authentication of routing updates from the eigrp routing protocol. The md5 keyed digest in each eigrp packet prevents the introduction of unauthorized or false routing messages from unapproved sources.

19. What is the use of “variance” command in eigrp?

Eigrp provides a mechanism to load balance over unequal cost paths through variance command. Variance is a number (1 to 128), multiplied by the local best metric then includes the routes with the lesser or equal metric. The default variance value is 1, which means equal-cost load balancing.

20. Internal and external administrative distance in eigrp?

system (is-is)
Routing information protocol (rip)	120
Exterior gateway protocol (egp)	140
On demand routing (odr)	160
External eigrp	170
Internal bgp	200
Unknown*	255

21. What is feasible successor?

A feasible successor is a path whose reported distance is less than the feasible distance, and it is considered a backup route. Eigrp will keep up to six feasible successors in the topology table. Only the one with the best metric (the successor) is placed in the routing table

22. What is “graceful shutdown”?

This feature helps in faster convergence of networks. With graceful shutdown, a goodbye message is broadcast when an eigrp routing process is shutdown, to inform adjacent peers about the impending topology change

23. Maximum path load balanced by eigrp?

In general, load balancing is the capability of a router to distribute traffic over all the router network ports that are the same distance from the destination address. Load balancing increases the utilization of network segments, and so increases effective network bandwidth. There are two types of load balancing:

Equal cost path – applicable when different paths to a destination network report the same routing metric value. The maximum-paths command determines the maximum number of routes that the

routing protocol can use. Maximum-path 6

Router(config-router)#maximum-paths 6

Unequal cost path – applicable when different paths to a destination network report are of different routing metric values. The variance command determines which of these routes is used by the router.

24. How eigrp support unequal load balancing?

Eigrp supports up to six unequal-cost paths.

Router(config-router)#variance n

25. What happen when we enable passive interface in eigrp?

With eigrp running on a network, the passive-interface command stops both outgoing and incoming routing updates, since the effect of the command causes the router to stop sending and receiving hello packets over an interface.

26. What does the word serno mean on the end of an eigrp topology entry when you issue the show ip eigrp topology command?

For example:

Show ip eigrp topology

P 172.22.71.208/29, 2 successors, fd is 46163456

Via 172.30.1.42 (46163456/45651456), serial0.2, serno 7539273

Via 172.30.2.49 (46163456/45651456), serial2.6, serno 7539266

Serno stands for serial number. When drdbs are threaded to be sent, they are assigned a serial number.if you display the topology table at the time an entry is threaded, it shows you the serial numberassociated with the drdb.threading is the technique used inside the router to queue items up for transmission to neighbors. Theupdates are not created until it is time for them to go out the interface. Before that, a linked list of pointers to items to send is created (for example, the thread).these sernos is local to the router and arenot passed with the routing update

27. What percent of bandwidth and processor resources does eigrp use?
Eigrp version 1 introduced a feature that prevents any single eigrp process from using more than fifty percent of the configured bandwidth on any link during periods of network convergence. Each as or protocol (for instance, ip, ipx, or appletalk) serviced by eigrp is a separate process. You can use the ip bandwidth-percent eigrp interface configuration command in order to properly configure the bandwidth percentage on each wan interface. Refer to the eigrp white paper for more information on how this feature works.

In addition, the implementation of partial and incremental updates means that eigrp sends routing information only when a topology change occurs. This feature significantly reduces bandwidth use.

The feasible successor feature of eigrp reduces the amount of processor resources used by an autonomous system (as). It requires only the routers affected by a topology change to perform route re-computation. The route re-computation only occurs for routes that were affected, which reduces search time in complex data structures

28. Does eigrp support aggregation and variable length subnet masks?

Yes, eigrp supports aggregation and variable length subnet masks (vlsm). Unlike open shortest path first (ospf), eigrp allows summarization and aggregation at any point in the network. Eigrp supports aggregation to any bit. This allows properly designed eigrp networks to scale exceptionally well without the use of areas. Eigrp also supports automatic summarization of network addresses at major network borders.

29. Can i configure more than one eigrp autonomous system on the same router?

Yes, you can configure more than one eigrp autonomous system on the same router. This is typically done at a redistribution point where two eigrp autonomous systems are interconnected. Individual router interfaces should only be included within a single eigrp autonomous system.

Cisco does not recommend running multiple eigrp autonomous systems on the same set of interfaces on the router. If multiple eigrp autonomous systems are used with multiple points of mutual redistribution, it can cause discrepancies in the eigrp topology table if correct filtering is not performed at the redistribution points. If possible, cisco recommends you configure only one eigrp autonomous system in any single autonomous system. You can also use another protocol, such as border gateway protocol (bgp), in order to connect the two eigrp autonomous systems.

30. If there are two eigrp processes that run and two equal paths are learned, one by each eigrp process, do both routes get installed?

No, only one route is installed. The router installs the route that was learned through the eigrp process with the lower autonomous system (as) number. In cisco ios software releases earlier than 12.2(7)t, the router installed the path with the latest timestamp received from either of the eigrp processes. The change in behavior is tracked by cisco bug id cscdm47037.

31. When i configure eigrp, how can i configure a network statement with a mask?

The optional network-mask argument was first added to the network statement in cisco ios software release 12.0(4)t. The mask argument can be configured in any format (such as in a network mask or in wild card bits). For example, you can use network 10.10.10.0 255.255.255.252 or network 10.10.10.0 0.0.0.3.

32. What is “goodbye” message received in eigrp?

Goodbye message

The goodbye message is a feature designed to improve eigrp network convergence. The goodbye message is broadcast when an eigrp routing process is shut down to inform adjacent peers about the impending topology change. This feature allows supporting eigrp peers to synchronize and recalculate neighbor relationships more efficiently than would occur if the peers discovered the topology change after the hold timer expired.

The following message is displayed by routers that run a supported release when a goodbye message is received: Apr 26 13:48:42.523: %dual-5-nbrchange: ip-eigrp(0) 1: neighbor 10.1.1.1 (ethernet0/0) is down: interface goodbye received

A cisco router that runs a software release that does not support the goodbye message can misinterpret the message as a k-value mismatch and display the following message:

Apr 26 13:48:41.811: %dual-5-nbrchange: ip-eigrp(0) 1: neighbor 10.1.1.1 (ethernet0/0) is down: k-value mismatch Obviously, the signalling to a neighbor that a protocol has been gracefully shutdown means good things for protocol convergence and loop prevention in a distance vector protocol. The point that i think is important is that a network that has some ios 15.1m and more mainstream sofware might see error messages about k-value mismatch and think that something is broken. In this case, the error message is exactly correct, and can be safely ignored.

As always, it depends™ on your exact configuration, its possible that someone has actually configured k-values (but it’s unlikely these days) and the message is telling you.

33. Who does load-balancing when there are multiple links to a destination?

Load balancing is a standard functionality of the cisco ios® router software, and is available across all router platforms. It is inherent to the forwarding process in the router and is automatically activated if the routing table has multiple paths to a destination. It is based on standard routing protocols, such as routing information protocol (rip), ripv2, enhanced interior gateway routing protocol (eigrp), open shortest path first (ospf), and interior gateway routing protocol (igrp), or derived from statically configured routes and packet forwarding mechanisms. It allows a router to use multiple paths to a destination when forwarding packets.

34. How can i use only one path when a router has two equal cost paths?

Configure the bandwidth value on the interfaces to default, and increase the delay on the backup interface so that the router does not see two equal cost paths.

35. What is the difference in metric calculation between eigrp and igrp?

Eigrp has totally replaced the obsolete igrp

2. Eigrp is a classless routing protocol while igrp is a classful routing protocol

3. Eigrp uses the dual while igrp does not

4. Eigrp consumes much less bandwidth compared to igrp

5. Eigrp expresses the metric as a 32 bit value while igrp uses a 24 bit value

36. What is the eigrp stub routing feature?

The enhanced interior gateway routing protocol (eigrp) stub routing feature improves network stability, reduces resource utilization, and simplifies stub router configuration. Stub routing is commonly used in a hub and spoke network topology.

37. How can i send a default route to the stub router from the hub?

Do this under the outbound interface on the hub router with the ip summary-address eigrp x 0.0.0.0 0.0.0.0command. This command suppresses all the more specific routes and only sends the summary route. In the case of the 0.0.0.0 0.0.0.0, it means it suppresses everything, and the only route that is in the outbound update is 0.0.0.0/0. One drawback to this method is that eigrp installs a 0.0.0.0/0 route to null0 is the local routing table with an admin distance of 5.

38. What are different route types in eigrp?

Internal route—routes that are originated within the autonomous system (as).

Summary route—routes that are summarized in the router (for example, internal paths that have been summarized).

External route—routes that are redistributed to eigrp.

39. What is an offset-list, and how is it useful?

The offset-list is an feature used to modify the composite metrics in eigrp. The value configured in the offset-list command is added to the delay value calculated by the router for the route matched by an access-list. An offset-list is the preferred method to influence a particular path that is advertised and/or chosen.

40. What does the neighbor statement in the eigrp configuration section do?

The neighbor command is used in eigrp in order to define a neighboring router with which to exchange routing information. Due to the current behavior of this command, eigrp exchanges routing information with the neighbors in the form of unicast packets whenever the neighbor command is configured for an interface.

41. Why does the eigrp passive-interface command remove all neighbors for an interface?

The passive-interface command disables the transmission and receipt of eigrp hello packets on an interface. Unlike igrp or rip, eigrp sends hello packets in order to form and sustain neighbor adjacencies. Without a neighbor adjacency, eigrp cannot exchange routes with a neighbor. Therefore, the passive-interface command prevents the exchange of routes on the interface. Although eigrp does not send or receive routing updates on an interface configured with the passive-interface command, it still includes the address of the interface in routing updates sent out of other non-passive interfaces.

42. Why are routes received from one neighbor on a point-to-multipoint interface that runs eigrp not propagated to another neighbor on the same point-to-multipoint interface?

The split horizon rule prohibits a router from advertising a route through an interface that the router itself uses to reach the destination. In order to disable the split horizon behavior, use the no ip split-horizon eigrp as-numberinterface command. Some important points to remember about eigrp split horizon are:

Split horizon behavior is turned on by default.

When you change the eigrp split horizon setting on an interface, it resets all adjacencies with eigrp neighbors reachable over that interface.

Split horizon should only be disabled on a hub site in a hub-and-spoke network.

Disabling split horizon on the spokes radically increases eigrp memory consumption on the hub router, as well as the amount of traffic generated on the spoke routers.

The eigrp split horizon behavior is not controlled or influenced by the ip split-horizon command.

43. What are the primary functions of the pdm?

Eigrp supports 3 protocol suites: ip, ipv6, and ipx. Each of them has its own pdm. These are the primary functions of pdm:

Maintaining the neighbor and topology tables of eigrp routers that belong to that protocol suite Building and translating protocol specific packets for dual Interfacing dual to the protocol specific routing table Computing the metric and passing this information to dual; dual handles only the picking of the feasible successors (fss) Implement filtering and access lists.

Perform redistribution functions to/from other routing protocols.