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EIGRP Questions

February 6th, 2017 Go to comments

Question 1

Explanation

Below is an example of the “show ip eigrp neighbors” output.

EIGRP_show_ip_eigrp_neighbors_2.jpg

Let’s analyze these columns:

+ H: lists the neighbors in the order this router was learned
+ Address: the IP address of the neighbors
+ Interface: the interface of the local router on which this Hello packet was received
+ Hold (sec): the amount of time left before neighbor is considered in “down” status
+ Uptime: amount of time since the adjacency was established
+ SRTT (Smooth Round Trip Timer): the average time in milliseconds between the transmission of a packet to a neighbor and the receipt of an acknowledgement.
+ RTO (Retransmission Timeout): if a multicast has failed, then a unicast is sent to that particular router, the RTO is the time in milliseconds that the router waits for an acknowledgement of that unicast.
+ Queue count (Q Cnt): shows the number of queued EIGRP packets. It is usually 0.
+ Sequence Number (Seq Num): the sequence number of the last update EIGRP packet received. Each update message is given a sequence number, and the received ACK should have the same sequence number. The next update message to that neighbor will use Seq Num + 1.

In this question we have to check the RTO and Q cnt fields.

Question 2

Explanation

To configure EIGRP for IPv6 we must explicitly specify a router ID before it can start running. For example:

ipv6 router eigrp 1
eigrp router-id 2.2.2.2
no shutdown

Notice that EIGRP for IPv6 router-id must be an IPv4 address. EIGRP for IPv4 can automatically pick-up an IPv4 to use as its EIGRP router-id with this rule:
+ The highest IP address assigned to a loopback interface is selected as the router ID.
+ If there are not any loopback addresses configured, the highest IP address assigned to any other active interface is chosen as the router ID

EIGRPv3 also uses the AS number (for example: ipv6 eigrp 1 under interface mode).

Question 3

Explanation

The “show ipv6 eigrp neighbors” command displays the neighbors discovered by the EIGRPv6. Notice that the neighbors are displayed by their link-local addresses.

show_ipv6_eigrp_neighbors.jpg

Question 4

Explanation

The goodbye message is a feature designed to improve EIGRP network convergence. The goodbye message is broadcast when an EIGRP routing process is shutdown 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

Reference: http://www.cisco.com/c/en/us/td/docs/ios/12_2/ip/configuration/guide/fipr_c/1cfeigrp.html

Question 5

Explanation

With auto-summary feature is turned on, EIGRP will summary these networks to their classful networks automatically. For example:
+ 172.15.4.0 belongs to class B so it will be summarized to 172.15.0.0
+ 10.4.3.0 belongs to class A so it will be summarized to 10.0.0.0
+ 192.168.4.0 belongs to class C so it will be summarized to 192.168.4.0 (same)

Question 6

Explanation

Split horizon is used in distance vector routing protocols (like RIP, EIGRP) to prevent routing loops by prohibiting a router from advertising a route back to the interface from which it was learned.

Question 7

Explanation

EIGRP provides a mechanism to load balance over unequal cost paths (or called unequal cost load balancing) through the “variance” command. In other words, EIGRP will install all paths with metric < variance * best_metric into the local routing table, provided that it meets the feasibility condition (to prevent routing loop). The feasibility condition states that, the Advertised Distance (AD) of a route must be lower than the feasible distance of the current successor route.

Question 8

Explanation

EIGRP provides a mechanism to load balance over unequal cost paths (or called unequal cost load balancing) through the “variance” command. In other words, EIGRP will install all paths with metric < variance * best_metric into the local routing table, provided that it meets the feasibility condition (to prevent routing loop). The feasibility condition states that, the Advertised Distance (AD) of a route must be lower than the feasible distance of the current successor route.

Comments (7) Comments
  1. Tepi
    February 8th, 2017

    I think Q4 should read EIGRP goodbye messages give notice of “impending” topology changes.

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    February 16th, 2017

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