EIGRP Questions

EIGRP Quick Summary: + EIGRP supports up to six unequal-cost paths. + EIGRP provides a mechanism to load balance over unequal cost paths (or called unequal cost load balancing) through the “variance” command. + The feasibility condition states that, the Advertised Distance (AD) of a route must be lower than the feasible distance of the current successor route. + The path that meets the feasibility condition requirement is called a feasible successor

Question 1

Explanation

By default, EIGRP load-shares over four equal-cost paths. EIGRP also support unequal-cost load balancing via the “variance” command.

Question 2

Explanation

The table below lists the default administrative distance values of popular routing protocols:

Routing Protocols	Default Administrative Distance
EIGRP	90
OSPF	110
RIP	120
eBGP	20
iBGP	200
Connected interface	0
Static route	1

Question 3

Explanation

Requirements
+ The time must be properly configured on all routers.
+ A working EIGRP configuration is recommended.

Reference: https://www.cisco.com/c/en/us/support/docs/ip/enhanced-interior-gateway-routing-protocol-eigrp/82110-eigrp-authentication.html

Question 4

Explanation

The following list of parameters must match between EIGRP neighbors in order to successfully establish neighbor relationships:

+ Autonomous System number.
+ K-Values.
+ If authentication is used both: the key number, the password, and the date/time the password is valid must match.
+ The neighbors must be on common subnet (all IGPs follow this rule).

Question 5

Explanation

When EIGRP is configured in a point-to-multipoint Frame Relay network, although the Hub can receive routing updates sent from its Spoke routers but split horizon rule forbids the Hub from relaying advertisements back out the interface on which they were received. For example in the topology below, Hub can receive routing updated from two Spokes but it cannot relay them out of S0/0 interface again (as it is the interface where it received the updates). To solve this problem we need to disable split horizon on S0/0 interface of Hub.

The command should be (suppose EIGRP 1 is running):

Hub(config)#interface serial0/0 Hub(config-if)#no ip split-horizon eigrp 1

Therefore answer A is correct.

In Non-broadcast networks (such as Frame-Relay), multicast (and broadcast) are not allowed while EIGRP (and OSPF, RIPv2) uses multicast to send Hello and Update messages. Therefore these dynamic routing protocols would not work well under Frame-Relay. To overcome this issue we usually add the keyword “broadcast” at the end of the frame-relay map statement (for example, “frame-relay map ip 10.1.1.1 403 broadcast“). This makes EIGRP to send update via unicast instead of multicast.

Another way to resolve above issue is to use the “neighbor” command. This command also make EIGRP to communicate with its neighbors via unicast -> B is correct.

Note: Although we can use the “neighbor” command to set up EIGRP neighbor relationship but the routes cannot be advertised from the Hub to the Spoke because of split horizon rule.

Question 6

Explanation

Although we can use the “neighbor” command to set up EIGRP neighbor relationship but the routes cannot be advertised from the Hub to the Spoke because of split horizon rule -> Answer D is not correct.

To overcome the split horizon rule we can use subinterface as each subinterface is treated like a separate physical interface so routing updates can be advertised back from Hub to Spokes.  -> Answer C is correct.

Note: The split horizon rule states that routes will not be advertised back out an interface in which they were received on

Question 7

Explanation

The “ip summary-address eigrp” command is used to configure interface-level address summarization. EIGRP summary routes are given an administrative distance value of 5. The administrative distance metric is used to advertise a summary without installing it in the routing table.

Reference: http://www.cisco.com/c/en/us/td/docs/ios/iproute_eigrp/command/reference/ire_book/ire_i1.html

Question 8

Question 9

Explanation

An example of how to configure EIGRP authentication on two routers that are connected to each other is shown below:

R1,R2(config)#key chain MYKEYS
R1,R2(config-keychain)#key 1
R1,R2(config-keychain-key)#key-string SecRetThing!
R1,R2(config-keychain-key)#end

R1,R2(config)#interface serial 0/0
R1,R2(config-subif)#ip authentication mode eigrp 10 md5
R1,R2(config-subif)#ip authentication key-chain eigrp 10 MYKEYS

Question 10

Explanation

When redistributing into RIP, EIGRP (and IGRP) we need to specify the metrics or the redistributed routes would never be learned. In this case we need to configure like this:

router eigrp 1 redistribute ospf 100 metric 10000 100 255 1 1500

Question 11