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BSCI - Building Scaleable Cisco Internetworks : 642-901 Exam

Exam Number/Code: 642-901
Exam Name: BSCI - Building Scaleable Cisco Internetworks

QUESTION 1:
Certkiller uses EIGRP as the primary routing protocol in their network. How does
EIGRP advertise subnet masks for each destination network?
A. EIGRP advertises a fixed length subnet mask for each destination network.
B. EIGRP advertises only a classful subnet mask for each destination network.
C. EIGRP, like IGRP and RIP, does not advertise a subnet mask for each destination
network.
D. EIGRP advertises a prefix length for each destination network.
E. None of the above
Answer: D
Explanation:
Enhanced Interior Gateway Routing Protocol (EIGRP) is a Cisco-proprietary routing
protocol based on IGRP. Unlike IGRP, which is a classful routing protocol, EIGRP
supports CIDR, allowing network designers to maximize address space by using CIDR
and VLSM. Compared to IGRP, EIGRP boasts faster convergence times, improved
scalability, and superior handling of routing loops.
The prefix length field signifies the subnet mask to be associated with the network
number specified in the destination field. Thus, if an EIGRP router is configured as
follows:
1. ip address 172.16.1.1 255.255.255.0
it will advertise 172.16.1.0 with a prefix length of 24.
Likewise, if the router is configured as follows:
1. ip address 172.16.250.1 255.255.255.252
it will advertise 172.16.250.0 with a prefix length of 30.
QUESTION 2:
The Certkiller network uses EIGRP. Identify three characteristics of EIGRP feasible
successors? (Select three)
A. Traffic will be load balanced between feasible successors with the same advertised
distance.
B. If the advertised distance of the non-successor route is less than the feasible distance
of best route, then that route is identified as a feasible successor.
C. If the successor becomes unavailable, then the feasible successor can be used
immediately without recalculating for a lost route.
D. The feasible successor can be found in the routing table.
E. A feasible successor is selected by comparing the advertised distance of a
non-successor route to the feasible distance of the best route.
Answer: B, C, E
642-901
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Explanation:
Once a neighbor relationship has been formed, called an Adjacency, the routers exchange
routing update information and each router builds its own topology table. The Updates
contain all the routes known by the sender. For each route, the receiving router calculates
a distance for that route based on the distance that is conveyed and the cost to that
neighbor that advertised the particular route. If the receiving router sees several routes to
a particular network with different metrics, then the route with the lowest metric becomes
the Feasible Distance (FD) to that network. The Feasible Distance is the metric of a
network advertised by the connected neighbor plus the cost of reaching that neighbor.
This path with the best metric is entered into the routing table because this is the quickest
way to get to that network.
With the other possible routes to a particular network with larger metrics, the receiving
router also receives the Reported Distance (RD) to this network via other routers. The
Reported Distance being the total metric along a path to a destination network as
advertised by an upstream neighbor. The Reported Distance for a particular route is
compared with the Feasible Distance that it already has for that route. If the Reported
Distance is larger than the Feasible Distance then this route is not entered into the
Topology Table as a Feasible Successor. This prevents loops from occurring. If the
Reported Distance is smaller than the Feasible Distance, then this path is considered to be
a Feasible Successor and is entered into the Topology table. The Successor for a
particular route is the neighbor/peer with the lowest metric/distance to that network.
If the receiving router has a Feasible Distance to a particular network and it receives an
update from a neighbor with a lower advertised distance (Reported Distance) to that
network, then there is a Feasibility Condition. In this instance, the neighbor becomes a
Feasible Successor for that route because it is one hop closer to the destination network.
There may be a number of Feasible Successors in a meshed network environment, up to 6
of them are entered into the Topology table thereby giving a number of next hop choices
for the local router should the neighbor with the lowest metric fail. What you should note
here, is that the metric for a neighbor to reach a particular network (i.e. the Reported
Distance) must always be less than the metric (Feasible Distance) for the local router to
reach that same network. This way routing loops are avoided. This is why routes that
have Reported Distances larger than the Feasible Distance are not entered into the
Topology table, so that they can never be considered as successors, since the route is
likely to loop back through that local router.
Incorrect Answers:
D: The feasible successors are found in the topology table, but not the active routing
table.
E: With EIGRP, traffic is load balanced across equal cost links in the routing table, but
not between feasible successors.
Reference: http://www.rhyshaden.com/eigrp.htm
QUESTION 3:
Two Certkiller EIGRP routers are attempting to establish themselves as neighbors.
Which EIGRP multicast packet type is responsible for neighbor discovery?
642-901
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A. Update
B. Query
C. Acknowledgment
D. Reply
E. Hello
F. None of the above
Answer: E

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