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OSPF Interview Questions

Routing Information Protocol Version 2

1: Which three fields are new to the RIPv2 message format?

*** The Route Tag field, the Subnet Mask field, and the Next Hop field are RIPv2
extensions that do not exist in RIPv1 messages. The basic format of the RIP
message remains unchanged between the two versions; version 2 merely uses
fields that are unused in version 1.

2: Besides the extensions defined by the three fields of question 1, what
are the other two major changes from RIPv1?

***In addition to the functions that use the new fields, RIPv2 supports
authentication and multicast updates.

3: What is the multicast address used by RIPv2? What is the advantage of
multicasting messages over broadcasting them?

***RIPv2 uses the multicast address 224.0.0.9. Multicasting of routing messages
is better than broadcasting because hosts and non-RIPv2 routers will ignore the
multicast messages .

4: What is the purpose of the Route Tag field in the RIPv2 message?

*** When another routing protocol uses the RIPv2 domain as a transit domain,
the protocol external to RIPv2 can use the Route Tag field to communicate
information to its peers on the other side of the RIPv2 domain.

5: What is the purpose of the Next Hop field?

*** The Next Hop field is used to inform other routers of a next-hop address on
the same multiaccess network that is metrically closer to the destination than the
originating router.

6: What is the UDP port number used by RIPv2?

***RIPv2 uses the same UDP port number as RIPv1, port number 520.

7: Which one feature must a routing protocol have to be a classless routing
protocol?

***A classless routing protocol does not consider the major network address in its
route lookups, but just looks for the longest match.

8: Which one feature must a routing protocol have to use VLSM?

*** To support VLSM, a routing protocol must be able to include the subnet mask
of each destination address in its updates.

9: Which two types of authentication are available with Cisco’s RIPv2? Are
they both defined in RFC 1723?

*** Cisco’s implementation of RIPv2 supports clear-text authentication and MD5
authentication. Only clear-text authentication is defined in RFC 1723.

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OSPF Interview Questions

1: What is an OSPF neighbor?

*** From the perspective of an OSPF router, a neighbor is another OSPF router
that is attached to one of the first router’s directly connected links.

2: What is an OSPF adjacency?

***An OSPF adjacency is a conceptual link to a neighbor over which LSAs can
be sent.

3: What is an LSA? How does an LSA differ from an OSPF Update packet?

***A router originates a link state advertisement to describe one or more
destinations. An OSPF Update packet transports LSAs from one neighbor to
another. Although LSAs are flooded throughout an area or OSPF domain,
Update packets never leave a data link.

4: What is a link state database? What is link state database
synchronization?

***The link state database is where a router stores all the OSPF LSAs it knows
of, including its own. Database synchronization is the process of ensuring that all
routers within an area have identical link state databases.

5: What is the default HelloInterval?

***The default OSPF HelloInterval is 10 seconds.

6: What is the default RouterDeadInterval?

***The default RouterDeadInterval is four times the HelloInterval.

7: What is a Router ID? How is a Router ID determined?

***A Router ID is an address by which an OSPF router identifies itself. It is either
the numerically highest IP address of all the router’s loopback interfaces, or if no
loopback interfaces are configured, it is the numerically highest IP address of all
the router’s LAN interfaces.

8: What is an area?

***An area is an OSPF sub-domain, within which all routers have an identical link
state database.

9: What is the significance of area 0?

***Area 0 is the backbone area. All other areas must send their inter-area traffic
through the backbone.

10: What is MaxAge?

***MaxAge, 1 hour, is the age at which an LSA is considered to be obsolete.

11: What are the five OSPF packet types? What is the purpose of each type?

*** The five OSPF packet types, and their purposes, are:

Hellos, which are used to discover neighbors, and to establish and maintain
adjacencies

Updates, which are used to send LSAs between neighbors

Database Description packets, which a router uses to describe its link state
database to a neighbor during database synchronization

Link State Requests, which a router uses to request one or more LSAs from a
neighbor’s link state database

Link State Acknowledgments, used to ensure reliable delivery of LSAs

12:What are LSA types 1 to 5 and LSA type 7? What is the purpose of each
type?

*** The most common LSA types and their purposes are:

Type 1 (Router LSAs) are originated by every router and describe the originating
router, the router’s directly connected links and their states, and the router\xd5 s
neighbors.

Type 2 (Network LSAs) are originated by Designated Routers on multiaccess
links and describe the link and all attached neighbors.

Type 3 (Network Summary LSAs) are originated by Area Border Routers and
describe inter-area destinations.

Type 4 LSAs (ASBR Summary LSAs) are originated by Area Border Routers to
describe Autonomous System Boundary Routers outside the area.

Type 5 (AS External LSAs) are originated by Autonomous System Boundary
Routers to describe destinations external to the OSPF domain.

Type 7 (NSSA External LSAs) are originated by Autonomous System Boundary
Routers within not-so-stubby areas.

13: What are the four OSPF router types?

***The four OSPF router types are:

# Internal Routers, whose OSPF interfaces all belong to the same area

# Backbone Routers, which are Internal Routers in Area 0

# Area Border Routers, which have OSPF interfaces in more than one area

# Autonomous System Boundary Routers, which advertise external routes into
the OSPF domain
14: What are the four OSPF path types?

***The four OSPF path types are:

Intra-area paths

Inter-area paths

Type 1 external paths

Type 2 external paths
15: What are the five OSPF network types?

*** The five OSPF network types are:

i)Point-to-point networks

ii) Broadcast networks

iii) Non-broadcast multi-access (NBMA) networks

iv) Point-to-multipoint networks

v) Virtual links
16: What is a Designated Router?

***A Designated Router is a router that represents a multiaccess network, and
the routers connected to the network, to the rest of the OSFP domain.
17: How does a Cisco router calculate the outgoing cost of an interface?

***Cisco IOS calculates the outgoing cost of an interface as 108/BW, where BW
is the configured bandwidth of the interface.
18: What is a partitioned area?

***An area is partitioned if one or more of its routers cannot send a packet to the
area’s other routers without sending the packet out of the area.
19: What is a virtual link?

*** A virtual link is a tunnel that extends an OSPF backbone connection through
a non-backbone area.
20: What is the difference between a stub area, a totally stubby area, and a
not-so-stubby area?

***A stub area is an area into which no type 5 LSAs are flooded. A totally stubby
area is an area into which no type 3, 4, or 5 LSAs are flooded, with the exception
of type 3 LSAs to advertise a default route. Not-so-stubby areas are areas
through which external destinations are advertised into the OSPF domain, but
into which no type 5 LSAs are sent by the ABR.
21: What is the difference between OSPF network entries and OSPF router
entries?

*** OSPF network entries are entries in the route table, describing IP
destinations. OSPF router entries are entries in a separate route table that record
only routes to ABRs and ASBRs.
22: Why is type 2 authentication preferable over type 1 authentication?

***Type 2 authentication uses MD5 encryption, whereas type 1 authentication
uses clear-text passwords.
23: Which three fields in the LSA header distinguish different LSAs? Which
three fields in the LSA header distinguish different instances of the same
LSA?

***The three fields in the LSA header that distinguish different LSAs are the
Type, Advertising Router, and the Link State ID fields. The three fields in the LSA
header that distinguish different instances of the same LSA are the Sequence
Number, Age, and Checksum fields

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EIGRP Interview Questions

1: Is EIGRP a distance vector or a link state routing protocol?

*** EIGRP is a Hybrid routing protocol,it have features of both distance vector
and link state routing protocol.

2: What is the maximum configured bandwidth EIGRP will use on a link?
Can this percentage be changed?

*** By default, EIGRP uses no more than 50% of the link’s bandwidth, based on
the bandwidth configured on the router’s interface. This percentage to be
changed with the command ip bandwidth-percent eigrp.

3: How do EIGRP and IGRP differ in the way they calculate the composite
metric?

*** EIGRP and IGRP use the same formula to calculate their composite metrics, but EIGRP scales the metric by a factor of 256.

4: In the context of EIGRP, what does the term reliable delivery mean?
Which two methods ensure reliable delivery of EIGRP packets?

*** Reliable delivery means EIGRP packets are guaranteed to be delivered, and
they are delivered in order. RTP uses a reliable multicast, in which received
packets are acknowledged, to guarantee delivery; sequence numbers are used
to ensure that they are delivered in order.

5: Which mechanism ensures that a router is accepting the most recent
route entry?

*** Sequence numbers ensure that a router is receiving the most recent route
entry.

6: What is the multicast IP address used by EIGRP?

*** EIGRP uses the multicast address 224.0.0.10.

7: At what interval, by default, are EIGRP Hello packets sent?

*** The default EIGRP Hello interval is 5 seconds, except on some slow-speed
(T1 and below) interfaces, where the default is 60 seconds.

8: What is the default hold time?

*** The EIGRP default hold time is three times the Hello interval.

9: What is the difference between the neighbor table and the topology
table?

*** The neighbor table stores information about EIGRP-speaking neighbors; the
topology table lists all known routes that have feasible successors.

10: 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 receiving router’s feasible distance to
the destination. In other words, a router’s neighbor meets the feasibility condition
if the neighbor is metrically closer to the destination than the router. Another way
to describe this is that the neighbor is “downstream” relative to the destination

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Networking Question and Answer

1: What information must be stored in the route table?

* At a minimum, each entry of the routing table must include a destination
address and the address of a next-hop router or an indication that the destination
address is directly connected.

2: What does it mean when a route table says that an address is variably

subnetted?

**Variably subnetted means that the router knows of more than one subnet mask
for subnets of the same major IP address.

3: What are discontiguous subnets?

** Discontiguous subnets are two or more subnets of a major IP network address
that are separated by a different major IP address.

4: What command is used to examine the route table in a Cisco router?

** show ip route is used to examine the routing table of a Cisco router.

5: What are the two bracketed numbers associated with the non-directly
connected routes in the route table?

**The first bracketed number is the administrative distance of the routing protocol
by which the route was learned. The second number is the metric of the route.

6: When static routes are configured to reference an exit interface instead
of a next-hop address, in what way will the route table be different?

**When a static route is configured to reference an exit interface instead of a
next-hop address, the destination address will be entered into the routing table
as directly connected.

7: What is a summary route? In the context of static routing, how are
summary routes useful?

**A summary route is a single route entry that points to multiple subnets or major
IP addresses. In the context of static routes, summary routes can reduce the
number of static routes that must be configured.

8: What is an administrative distance?

**An administrative distance is a rating of preference for a routing protocol or a
static route. Every routing protocol and every static route has an administrative
distance associated with it. When a router learns of a destination via more than
one routing protocol or static route, it will use the route with the lowest
administrative distance.

9: What is a floating static route?

** A floating static route is an alternative route to a destination. The
administrative distance is set high enough that the floating static route is used
only if a more-preferred route becomes unavailable.

10: What is the difference between equal-cost and unequal-cost load
sharing?

**Equal-cost load sharing distributes traffic equally among multiple paths with
equal metrics. Unequal-cost load sharing distributes packets among multiple
paths with different metrics. The traffic will be distributed inversely proportional to
the cost of the routes