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CCNA interview questions -Eigrp

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.

What are the different tables in EIGRP?

EIGRP router stores routing and topology information in three tables:

  • Neighbor table – Stores information about EIGRP neighbors.
  • Topology table – Stores routing information which is learned from neighbor routers.
  • Routing table  – Stores the best paths to all networks.

Why EIGRP is called hybrid protocol?

EIGRP is also called hybrid protocol because its metric is not just plain HOP COUNT (max-

255, 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.

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 224.0.0.10.  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.
  • Refer the link for more info

Conditions for EIGRP neigbours.

Conditions for EIGRP neigbours are given below

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

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..

What are the different K-values used in EIGRP

Ans:-

  • 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.

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

 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.

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.

What debugging capabilities does EIGRP have?

Ans:-

  • show ip eigrp neighbors
  • show ip eigrp interfaces
  • show ip eigrp topology
  • show ip eigrp traffic

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.

What is Advertised distance ?

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

What is successor ?

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

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

Ans- 224.0.0.10

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.

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.

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).

What packets are RTP enabled?

RTP enabled packets are

  • Update Packet.
  • Query Packet.
  • 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 1.1.1.1

180.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 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.

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?   

Ans:-

# 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 types of Authentication is supported by EIGRP ?

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

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).

Internal and external Administrative distance in EIGRP ?

Ans:-

1.Internal – 90

2.External – 170

3.Summary – 5

Give the Formula EIGRP uses to calculate Metric?

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

What is Feasible successor ?

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

What is Graceful Shutdown and GoodBye message in EIGRP?

When an EIGRP process is shut down, 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.

190.Maximum path load balanced by EIGRP ?

up to 32 equal-cost entries can be in the routing table for the same destination. The default is 4. We can also set the maximum-path to 1 disables load balancing.

Router(config)#router eigrp 100

Router(config-router)#maximum-paths 6

Set the maximum number of parallel routes that EIGRP will support to 6

How EIGRP support unequal load balancing ?

EIGRP also support unequal cost path load balancing. Use the variance n command in order to instruct the router to include routes with a metric of less than n times the minimum metric route for that destination. The variable n can take a value between 1 and 128.

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

Ans- 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 number associated with the DRDB.

Threading is the technique used inside the router to queue items up for transmission to neighbors. The updates 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 are local to the router and are not passed with the routing update.

193.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.

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.

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.

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.

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.

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.

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.

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.

Or you can also limit Max-path to 1 for load balancing.

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

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.

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.

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.

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.

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.

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.

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.

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.

What are the various load-balancing options available in eigrp?

The offset-list can be used to modify the metrics of routes that eigrp learns through a particular interface, or pbr can be used.

What does the %dual-5-nbrchange: ip-eigrp(0) 100: neighbor 10.254.0.3 (tunnel0) is down: holding time expired error message mean?

This message indicates that the router has not heard any eigrp packets from the neighbor within the hold-time limit. Because this is a packet-loss issue, check for a layer 2 problem.

From the 16:29:14.262 poison squashed: 10.x.x.x/24 reverse message, what does poison squashed mean?

The router threads a topology table entry as a poison in reply to an update received (the router sets up for poison reverse). While the router is building the packet that contains the poison reverse, the router realizes that it does not need to send it. For example, if the router receives a query for the route from the neighbor, it is currently threaded to poison. Thus, it sends the poison squashed message.

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