EIGRP QUESTIONS

Q. A network administrator needs to configure a router with a distance-vector protocol that allows classless routing?

EIGRP

Q. Which command will display all the EIGRP feasible successor routes known to a router?

# show ip eigrp topology

Q. Where are EIGRP successor routes stored?

The routing table and the topology table

Q. How to see EIGRP neighbors address?

# show ip eigrp neighbors

Q. What are the EIGRP metrics?

EIGRP metric: Bandwidth, Load, Delay, Reliability and MTU

Q. What are the Hello and Hold timer?

Hello time : 5 sec

Hold time : 15 sec, no reply then link dead

Q. What is the EIGRP multicast address?

Eigrp multicast address: 224.0.0.10

Q. How does the passive interface command works in EIGRP?

Usually passive interface command is used in routing protocols to accept information from neighbors but prevent it from sending information to neighbors

But in case of EIGRP (Enhanced Interior Gateway Routing Protocol) it is exactly opposite. EIGRP discovers neighbours using the hello packets, before accepting routes and installing them in the routing table. Hello messages are usually sent and received on the interface on which EIGRP is configured. Passive interface command stops the sending and receiving hello packets. This breaks all existing neighbourship through that interface and stops learning of any new routes through that interface.

Q. How to configure EIGRP bandwidth usage across low speed links?

In order to prevent Enhanced Interior Gateway Routing Protocol (EIGRP) updates from overwhelming the interface, Cisco IOS provides controls how much bandwidth it can consume on the physical interface. By default, EIGRP limits itself to using no more than 50 percent of the interface bandwidth. Although, this is not very significance on high speeds like Ethernet, it becomes critical on low speed links.

Sometimes the interface bandwidth configuration cannot be changed because of routing policy considerations, or for any other reason, in that case bandwidth-percent command should be used to control the EIGRP bandwidth. On low-speed interfaces, raising the available bandwidth for EIGRP above the default of 50 percent is advisable in order to improve convergence.

# ip bandwidth-percent eigrp ip bandwidth-percent eigrp 100 25

Q. What are the EIGRP metrics to used determine route to destination?

EIGRP can utilize 5 separate metrics to determine the best route to a destination:

Bandwidth (K1)
Load (K2
Delay of the Line (K3
Reliability (K4)
MTU (K5)

By default, only Bandwidth and Delay of the Line are used

Q. What are the advantages of EIGRP?

EIGRP uses AS (Autonomous system) number ranging from 1-65535 to identify collection of routers that share same information.
EIGRP have less convergent time and is more efficient
EIGRP supports both auto and manual route summarization
Supports multiple routed protocols like IP ,IPX and apple talks
EIGRP converges rapidly in the event of link failure
EIGRP can load balance equal and unequal cost path.By default
EIGRP supports 4 load balancing path.It can be extended to 6 paths
When are periodic routing updates sent / How EIGRP form neighbors

crazzyeddy / 2 years ago

EIGRP uses non-periodic incremental routing updates. That is, EIGRP only sends routing updates about paths that have changed when those paths change. The basic problem with sending only routing updates is that you may not know when a path through a neighboring router is no longer available. You can not time out routes, expecting to receive a new routing table from your neighbors. EIGRP relies on neighbor relationships to reliably propagate routing table changes throughout the network; two routers become neighbors when they see each other’s hello packets on a common network.

Q. What is DUAL in EIGRP?

DUAL (Diffusing Update ALgorithm) is the algorithm used by Cisco’s EIGRP routing protocol to ensure that a given route is recalculated globally whenever it might cause a routing loop

Q. What are different types of packets in EIGRP?

There are 5 types of packets in EIGRP

-Hello

-Update

-Query

-Reply

-ACK (Acknowledgement)

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.

Acknowledgement Packets

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.

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.

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.

Q. What is Stuck in Active?

If the successor path is lost & there is no feasible successor path is available, router sends out query messages on all EIGRP enable interfaces & tries to find out an alternative path to the network. It is active state for that route.Now Router is waiting for reply from its neighbors. If reply is missing for 3 min, that means router didn’t get any reply from neighbors, then it becomes stuck in active.

Q. What are two values used by default to calculate the EIGRP metric?

-Bandwidth
-Delay

Q. Can you explain the concept of successor & feasible successor?

A Successor is the best path to reach a destination network in the topology table.

The Feasible successor is the 2nd best path to reach a destination network after successor. It stands as a backup for the successor.

What is the Advertised Distance and Feasible distance?

Advertised distance (AD) is the metric distance (cost) of a neighbor router to reach the destination network.

Feasible Distance (FD) is the metric distance (cost) to reach the destination network. It is always AD + metric distance to the neighbor router.

What Authentication method does EIGRP support?

EIGRP supports the Only MD5 authentication method.

Q. What is the concept of sucessor in EIGRP?

Successor is the neighbouring router with the best cost path too a destination network, will be marked as the successor for that network and will be used as the next hop to forward traffic to it.

Q. How does EIGRP protocol update route information to it’s neighbours?

A router discovers a neighbor when it receives its first hello packet on a directly connected network. The router requests DUAL to send a full route update to the new neighbor. In response, the neighbor sends its full route update. Thus, a new neighbor relationship is established in the following steps:

When a router A receives a hello packet from a new neighbor B, A sends its topology table to router B in unicast updates with the initialization bit turned on.

When router B receives a packet with the initialization bit on, it sends its topology table to router A.
The interval between hello packets from any EIGRP-speaking router on a network is five seconds (by default) on most media types. Each hello packet advertises hold-time–the length of time the neighbor should consider the sender up. The default hold-time is 15 seconds. If no hellos are received for the duration of the hold-time, DUAL is informed that the neighbor is down. Thus, in addition to detecting a new neighbor, hello packets are also used to detect the loss of a neighbor.

Q. In EIGRP, what are the requirements for forming neighborship?

to become neighbours in EIGRP, the following fields in a hello packet must match for routers: –

Autonomous System (AS) number.

Authentication.

K-values.

Q.Which tables do EIGRP routers create?

EIGRP router creates the following 3 tables to stores routing and topology information:

Routing table – Stores the best paths to all the destination networks.

Neighbour table – Stores information about all EIGRP neighbor routers.

Topology table – Stores routing information which is collected from neighbor routers.

Q. What is the importance of the “#no auto-summary” command in EIGRP?

By default, EIGRP has the automatic route summarization feature enabled. Due to this, EIGRP aggregates the routes to Classful address at network boundaries during the routing updates.

The automatic route summarization feature will cause issues with dis-contiguous sub-networks. Hence, this feature is usually turned off. To turned off this feature, the “#no auto-summary” command is used.

Q. Your company is running IGRP using an AS of 10. You want to configure EIGRP on the network but want to migrate slowly to EIGRP and don't want to configure redistribution. What command would allow you to migrate over time to EIGRP without configuring redistribution?

A. router eigrp 11

B. router eigrp 10

C. router eigrp 10 redistribute igrp

D. router igrp combine eigrp 10

Answer: B

If you enable EIGRP on a router with the same autonomous system (AS) number, EIGRP will automatically redistribute IGRP into EIGRP. You will see the IGRP injected routes as external (EX) routes with an EIGRP AD of 170. This is a nice feature that lets you migrate slowly to EIGRP with no extra configuration.

Q. Which command will display all the EIGRP feasible successor routes known to a router?

A. show ip routes *

B. show ip eigrp summary

C. show ip eigrp topology

D. show ip eigrp adjacencies

Answer: Option C

Any secondary route to a remote network is considered a feasible successor, and those routes are only found in the topology table and used as backup routes in case of primary route failure. You can see the topology table with the show ip eigrp topology command.

Q. Which is true regarding EIGRP successor routes?

A successor route is used by EIGRP to forward traffic to a destination.
Successor routes are saved in the topology table to be used if the primary route fails.
Successor routes are flagged as "active" in the routing table.
A successor route may be backed up by a feasible successor route.
Successor routes are stored in the neighbor table following the discovery process.

A.1 and 3

B.2 and 3

C.1 and 4

D.3, 4 and 5

Answer: Option C

Successor routes are the routes picked from the topology table as the best route to a remote network, so these are the routes that IP uses in the routing table to forward traffic to a remote destination. The topology table contains any route that is not as good as the successor route and is considered a feasible successor, or backup route. Remember that all routes are in the topology table, even successor routes.

Q. Which EIGRP information is held in RAM and maintained through the use of Hello and update packets?

Neighbor table
STP table
Topology table
DUAL table

A. 2 only

B. 4 only

C. 1 and 3

D. All of the above

Answer: Option C

EIGRP holds three tables in RAM: neighbor, topology, and routing. The neighbor and topology tables are built and maintained with the use of Hello packets.

Q. You need the IP address of the devices with which the router has established an adjacency. Also, the retransmit interval and the queue counts for the adjacent routers need to be checked. What command will display the required information?

A. show ip eigrp adjacency

B. show ip eigrp topology

C. show ip eigrp interfaces

D. show ip eigrp neighbors

Answer: Option D

The show ip eigrp neighbors command allows you to check the IP addresses as well as the retransmit interval and queue counts for the neighbors that have established an adjacency.

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