Sunday, 30 March 2008

Some Interview Question: CCNA & Basic Networking

Basics Q & A {Switching}

1) What is unicast and how does it work?

** Unicast is a one-to-one transmission method. A single frame is sent from the source to a destination on a network. When this frame is received by the switch, the frame is sent on to the network, and the network passes the frame to its destination from the source to a specific destination on a network.

2) What is multicast and how does it work?

** Multicast is a one-to-many transmission method. A single frame is sent from the source to multiple destinations on a network using a multicast address. When this frame is received by the switch, the frame is sent on to the network and the network passes the frame to its intended destination group.

3: What is broadcast and how does it work?

** Broadcast is a one-to-all transmission method. A single frame is sent from the source to a destination on a network using a multicast address. When this frame is received by the switch, the frame is sent on to the network. The network passes the frame to all nodes in the destination network from the source to an unknown destination on a network using a broadcast address. When the switch receives this frame, the frame is sent on to all the networks, and the networks pass the frame on to all the nodes. If it reaches a router, the broadcast frame is dropped.

4: What is fragmentation?

** Fragmentation in a network is the breaking down of a data packet into smaller pieces to accommodate the maximum transmission unit (MTU) of the network.

5:What is MTU? What's the MTU for traditional Ethernet?

** MTU is the acronym for maximum transmission unit and is the largest frame size that can be transmitted over a network. Messages longer than the MTU must be divided into smaller frames. The network layer (Layer 3) protocol determines the MTU from the data link layer (Layer 2) protocol and fragments the messages into the appropriate frame size, making the frames available to the lower layer for transmission without further fragmentation. The MTU for Ethernet is 1518 bytes.

6: What is a MAC address?

** A MAC address is the physical address of a network device and is 48 bits (6 bytes) long. MAC addresses are also known as physical addresses or hardware addresses.

7) What is the difference between a runt and a giant, specific to traditional Ethernet?

** In Ethernet a runt is a frame that is less than 64 bytes in length, and a giant is a frame that is greater than 1518 bytes in length. Giants are frames that are greater than the MTU used, which might not always be 1518 bytes.

8: What is the difference between store-and-forward and cut-through switching?

** Cut-through switching examines just the frame header, determining the output switch port through which the frame will be forwarded. Store-and-forward examines the entire frame, header and data payload, for errors. If the frame is error free, it is forwarded out its destination switch port interface. If the frame has errors, the switch drops the frame from its buffers. This is also known as discarding the frame to the bit bucket.

9: What is the difference between Layer 2 switching and Layer 3 switching?

* * Layer 2 switches make their forwarding decisions based on the Layer 2 (data link) address, such as the MAC address. Layer 3 switches make their forwarding decisions based on the Layer 3 (network) address.

10: What is the difference between Layer 3 switching and routing?

** The difference between Layer 3 switching and routing is that Layer 3 switches have hardware to pass data traffic as fast as Layer 2 switches. However, Layer 3 switches make decisions regarding how to transmit traffic at Layer 3 in the same way as a router. A Layer 3 switch cannot use WAN circuits or use routing protocols; a router is still required for these functions.

Materials About CCNA


(ISO) International organization for standardization

Iso has designed a reference model called osi reference model(open system interconnection).
It has 7 layers. It says ठाट Any n/w for comunication needs 7 layers

7.Application layer:

The user uses application layer to send the data। The protocols@ this layer are ftp,http,smtp(e-mail) telenet etc.

6.Presentation layer:

Presentation layer takes the data from application layer and presentIn different formats for securing reason. The services offered @This layer areCompression – decompressionCoding – decodingEncryption - decryption

5. Session layer:

Establishing the session or the conectivity n/w n/w 1 & n/w 2 is doneBy the session layer.It 1. Establishes a session2. maintains it &3. Terminates it b/w the application

4. Transport layer:

End-end connectivity during a session b/w two application is doneBy the transport layer. It also decides the type of connection like tcpor udp i.e. connection oriented or connection less.Services:SequencingFlow ctrl, error detection & correctionTransport layer info + data is called segment

3. Netwrok layer :

Logical addressing is done at thenetwork layer i.e. source address &destination address are attached to the data.Protocols @network layerRouted protocols routing protocolsEg: ip,ipx eg: rip,igrp,ospfRouted protocols: they always carry the data along with themRouting protocol: they identify the path for routed protocol tocarry the dataAt this layer routers & layer 3 switches forms packets.

2.Data link layer:

It has two Sub layersa)MAC{Media access control } b) LLC {logical link control framing of data}Ip address is lik the pincode & MAC address is like house number.Here layer2 switches are used.Wab protocols used at this layer are PPP,HDLC,FP,X.25 etc.Here error checking CRC bits are added to the packetsDLL info+ packets --> frames

1. physical layer:

Takes care of physical connectivity i.e connector,cable etc. hereFrames are converted to bits (1’s & 0’s).The devices like hubs, repeaters,cables & connectors are used atthis layer

Important Networking Q & A

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


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.


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

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

OSPF Q & A Part -II

1: 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

2: 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.

3: 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

4: 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

5: 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

6: 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.

7: 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.

8: 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.

9: What is a virtual link?

*** A virtual link is a tunnel that extends an OSPF backbone connection through a non-backbone area.

10: 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.

11: 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.

12: Why is type 2 authentication preferable over type 1 authentication?

***Type 2 authentication uses MD5 encryption, whereas type 1 authentication uses clear-text passwords.

13: 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

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 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.

Routing Information Protocol

1:What port does RIP use?

* RIP uses UDP port 520.

2:What metric does RIP use? How is the metric used to indicate an unreachable network?

* RIP uses a hop count metric. An unreachable network is indicated by setting the hop count to 16, which RIP interprets as an infinite distance.

3:What is the update period for RIP?

* RIP sends periodic updates every 30 seconds minus a small random variable to prevent the updates of neighboring routers from becoming synchronized.

4:How many updates must be missed before a route entry will be marked as unreachable?

* A route entry is marked as unreachable if six updates are missed.

5:What is the purpose of the garbage collection timer?

* The garbage collection timer, or flush timer, is set when a route is declared unreachable. When the timer expires, the route is flushed from the route table. This process allows an unreachable route to remain in the routing table long enough for neighbors to be notified of its status


1. What is a VLAN? When is it used?

Answer: A VLAN is a group of devices on the same broadcast domain, such as a logical subnet or segment. VLANs can span switch ports, switches within a switch block, or closets and buildings. VLANs group users and devices into common workgroups across geographical areas. VLANs help provide segmentation, security, and problem isolation.

2. When a VLAN is configured on a Catalyst switch port, in how much of the campus network will the VLAN number be unique and significant?

Answer: The VLAN number will be significant in the local switch. If trunking is enabled, the VLAN number will be significant across the entire trunking domain. In other words, the VLAN will be transported to every switch that has a trunk link supporting that VLAN.

3. Name two types of VLANs in terms of spanning areas of the campus network.

Answer: Local VLAN
End-to-end VLAN

4. What switch commands configure Fast Ethernet port 4/11 for VLAN 2?

Answer: interface fastethernet 4/11
switchport mode access
switchport access vlan 2

5. Generally, what must be configured (both switch and end-user device) for a port-based VLAN?

Answer: The switch port

6. What is the default VLAN on all ports of a Catalyst switch?

Answer: VLAN 1

7. What is a trunk link?

Answer: A trunk link is a connection between two switches that transports traffic from multiple VLANs. Each frame is identified with its source VLAN during its trip across the trunk link.

8. What methods of Ethernet VLAN frame identification can be used on a Catalyst switch trunk?

Answer: 802.1Q

9. What is the difference between the two trunking methods? How many bytes are added to trunked frames for VLAN identification in each method?

Answer: ISL uses encapsulation and adds a 26-byte header and a 4-byte trailer. 802.1Q adds a 4-byte tag field within existing frames, without encapsulation.

10. What is the purpose of the Dynamic Trunking Protocol (DTP)?

Answer: DTP allows negotiation of a common trunking method between endpoints of a trunk link.

11. What commands are needed to configure a Catalyst switch trunk port Gigabit 3/1 to transport only VLANs 100, 200 through 205, and 300 using IEEE 802.1Q? (Assume that trunking is enabled and active on the port already. Also assume that the interface gigabit 3/1 command already has been entered.)

Answer: switchport trunk allowed vlan 100, 200-205, 300

12. Two neighboring switch trunk ports are set to the auto mode with ISL trunking encapsulation mode. What will the resulting trunk mode become?

Answer: Trunking will not be established. Both switches are in the passive auto state and are waiting to be asked to start the trunking mode. The link will remain an access link on both switches.

13. Complete the following command to configure the switch port to use DTP to actively ask the other end to become a trunk:
switchport mode _________________

Answer: switchport mode dynamic desirable

14. Which command can set the native VLAN of a trunk port to VLAN 100 after the interface has been selected?

Answer: switchport trunk native vlan 100

15. What command can configure a trunk port to stop sending and receiving DTP packets completely?

Answer: switchport nonegotiate

16. What command can be used on a Catalyst switch to verify exactly what VLANs will be transported over trunk link gigabitethernet 4/4?

Answer: show interface gigabitethernet 4/4 switchport
show interface gigabitethernet 4/4 switchport trunk

17. Suppose that a switch port is configured with the following commands. A PC with a nontrunking NIC card then is connected to that port. What, if any, traffic will the PC successfully send and receive?

interface fastethernet 0/12
switchport trunk encapsulation dot1q
switchport trunk native vlan 10
switchport trunk allowed vlan 1-1005
switchport mode trunk

Answer: The PC expects only a single network connection, using a single VLAN. In other words, the PC can't participate in any form of trunking. Only untagged or unencapsulated frames will be understood. Recall that an 802.1Q trunk's native VLAN is the only VLAN that has untagged frames. Therefore, the PC will be capable of exchanging frames only on VLAN 10, the native VLAN.

Networking Basics

1: What is the definition of a network?

** A network is a system of lines or channels that cross or interconnect, or a group or system of electrical components and connecting circuitry designed to function in a specific manner.

2: What are network models?

** Network models provide the guiding principles behind the development of network standards.

3: What is a network standard, and why are there network standards?

** Network standards define the rules of network communication and are like laws that must be followed for different equipment vendors to work together.

4: What is a proprietary feature?

** If a vendor implements a feature that does not adhere to any network standards, it is called a proprietary feature.

5: What are the three data transmission modes, and how do they operate?

** Simplex mode, half-duplex mode, and full-duplex mode. Simplex mode is one-way communication only. Half-duplex mode is two-way communication, but not at the same time. Full-duplex mode is simultaneous two-way communication.

6: List the major characteristics of a LAN.

** The primary characteristic of a LAN is its geographic coverage. LANs are found in a small geographic area where there is a short distance between connected computers, as in small offices or on each floor of a larger office building. LANs enable the sharing of office resources, such as file servers for file sharing among users or print servers for shared printers.

7: List the major characteristics of a MAN.

** MANs are found in a metropolitan, or citywide, geographic area, interconnecting two or more office buildings in a broader geographic region than a LAN would support, but not so broad that a WAN would be required.

8: List the major characteristics of a WAN.

** WANS are found in broad geographic areas, often spanning states and countries, and are used to connect LANs and WANs together.

9: What are the three parts of a frame? What is a function of each part?

** Header, data (or payload), trailer. The header is the beginning of the frame, significant in that the frame's source and destination are found in the frame header. The payload is the data part of the frame, the user's information. The trailer identifies the end of the frame.

10: What function in a network does cabling provide?

** Cabling provides the physical interconnection between network devices and nodes.

11: List some examples of user data.

** Examples of user data include e-mail, web-browsing traffic, word-processed documents, spreadsheets, database updates.

12: What is the best definition of network topology?

** Network topology refers to the physical or logical geometric arrangement of interconnected network devices.

13: What is the best definition of network protocol?

** A network protocol is the communication rules and formats followed by all interconnected devices on a network requiring communication with one another.

14: What is the definition of network media?

** Network media refers to the physical component of a network. Communication signals traverse network media from source to destination. Some examples of network media are copper and fiber-optic cabling.

15: What is a network origination point?

** A network connection has two ends: the origination and termination points. The origination point is the source of the data—the location from which the data is being sent.

16: What is a network termination point?

** A network connection has two ends: the origination and termination points. The termination point is the destination of the data—the location to which the data is being sent.

Layer 3 Switching

1. What might you need to implement interVLAN routing?

** One or more Layer 3 interfaces

One or more SVIs

Static routes

A dynamic routing protocol

2. Can interVLAN routing be performed over a single trunk link?

** Yes. Packets can be forwarded between the VLANs carried over the trunk.

3. To configure an SVI, what commands are needed?

** First, make sure the VLAN is defined on the switch.

interface vlan vlan-id
ip address ip-address mask
no shutdown

4. What command can verify the VLAN assignments on a Layer 2 port?

** show interface type mod/num switchport


show interface status

5. A switch has the following interface configurations in its running configuration:

interface fastethernet 0/1
switchport access vlan 5
interface vlan 5
ip address
no shutdown

What is necessary for packets to get from the FastEthernet interface to the VLAN 5 SVI?

Answer: Nothing. Both are assigned to VLAN 5, so normal Layer 2 transparent bridging will take care of all forwarding between the two.

6. What is the source of FIB information?

** The routing table, as computed by the Layer 3 engine portion of a switch.

7. How often is the FIB updated?

** As needed. It is downloaded or updated dynamically by the Layer 3 engine whenever the routing topology changes or an ARP entry changes.

8. What is meant by the term "CEF punt"?

** A packet can't be forwarded or switched by CEF directly because it needs further processing. The packet is "punted" to the Layer 3 engine, effectively bypassing CEF for a more involved resolution.

9. What happens to the FIB when distributed CEF (dCEF) is used?

** It is simply replicated to each of the independent CEF engines. The FIB itself remains intact so that each engine receives a duplicate copy.

10. What happens during a "CEF glean" process?

** The MAC address (ARP reply) for a next-hop FIB entry is not yet known. The Layer 3 engine must generate an ARP request and wait for a reply before CEF forwarding can continue to that destination.

11. What does a multilayer switch do to the IP TTL value just before a packet is forwarded?

** The TTL is decremented by one, as if a router had forwarded the packet.

12. What is fallback bridging?

** On switch platforms that cannot multilayer-switch (route) all routable protocols, those protocols can be bridged transparently between VLANs instead.

13. Is it possible for an SVI to go down? If so, for what reasons?

** Yes. The SVI can be shut down administratively with the shutdown command, as with any other interface. Also, if the VLAN associated with the SVI is not defined or active, the SVI will appear to be down


1: Which command in OSPF shows the network LSA information?

** The command show ip ospf [process-id area-id] database network displays the network link-state information.

2: What command would you use to create a totally stubby area?

** The command area area-id stub no-summary will create a totally stubby area. This is a subcommand to the router ospf process-id command. It is necessary only on the ABR, but all the other routers in the area must be configured as stub routers.

3: What is a virtual link, and what command would you use to create it?

** A virtual link is a link that creates a tunnel through an area to the backbone (Area 0). This allows an area that cannot connect directly to the backbone to do so virtually. The command to create the link is area area-id virtual-link router-id. Note that the area-id that is supplied is that of the transit area, and the router-id is that of the router at the other end of the link. The command needs to be configured at both ends of the tunnel.

4: Where would you issue the command to summarize IP subnets? State the command that is used.

** Summarization is done at area boundaries. The command to start summarization is the area range command, with the syntax area area-id range address mask. To summarize external routes, use the summary-address command on the ASBRs.

5: How would you summarize external routes before injecting them into the OSPF domain?

** The command summary-address address mask is the command that you would use.

6: When is a virtual link used?

** A virtual link is used when an area is not directly attached to the backbone area (Area 0). This may be due to poor design and a lack of understanding about the operation of OSPF, or it may be due to a link failure. The most common cause of an area separating from the backbone is link failure, which can also cause the backbone to be segmented. The virtual link is used in these instances to join the two backbone areas together. Segmented backbone areas might also be the result of two companies merging.

7: Give the command for defining the cost of a default route propagated into an area.

** The command to define the cost of a default route propagated into another area is area area-id default-cost cost.

8: Give an example of when it would be appropriate to define a default cost.

** It is appropriate to define a default cost for the default route when a stub area has more than one ABR. This command allows the ABR or exit point for the area to be determined by the network administrator. If this link or the ABR fails, the other ABR will become the exit point for the area.

9: On which router is the area default cost defined?

** The default cost for the default route is defined on the ABR. The ABR will then automatically generate and advertise the route cost along with the default route.

10: Give the command to configure a stub area and state on which router it is configured.

** The command syntax to configure a stub area is area area-id stub. This command is configured on the ABR connecting to the area and on all the routers within the area. Once the configuration is completed, the Hellos are generated with the E bit set to 0. All routers in the area will only form adjacencies with other routers that have the E bit set.

11: What is the purpose of the area range command, and why is it configured on the ABR?

** The area range command is configured on an ABR because it dictates the networks that will be advertised out of the area. It is used to consolidate and summarize the routes at an area boundary.

12: Give the commands to configure a router to place subnets through to in Area 1 and to put all other interfaces into Area 0.

** The commands are as follows:

network area 1

network area 0

13: Give the syntax to summarize the subnets to into another autonomous system.

** The syntax is as follows:


14: Explain briefly the difference between the area range command and the summary-address command.

** The area range command is used to summarize networks between areas and is configured on the ABR. The summary-address command is used to summarize networks between autonomous systems and is configured on the ASBR.

15: Explain the following syntax and what it will achieve: area 1 stub no-summary.

** The command area 1 stub no-summary creates a totally stubby area. The number after the word area indicates the area that is being defined as a totally stubby area. This is necessary because the router might be an ABR with connections to many areas. Once this command is issued, it prevents summarized and external routes from being propagated by the ABR into the area. To reach the networks and hosts outside the area, routers must use the default route advertised by the ABR into the area.

16: Why would you configure the routing process to log adjacency changes as opposed to turning on debug for the same trigger?

** The reason to configure the router process to log adjacency changes to syslog as opposed to running debug is an issue of resources. It takes fewer router and administrator resources to report on a change of state as it happens than to have the debugger running constantly. The debug process has the highest priority and thus everything waits for it.

17: Give some of the common reasons that neighbors fail to form an adjacency.

** Many OSPF problems stem from adjacency problems that propagate throughout the network. Many problems are often traced back to neighbor discrepancies.

If a router configured for OSPF routing is not seeing an OSPF neighbor on an attached network, do the following:

- Make sure that both routers are configured with the same IP mask, MTU, Interface Hello timer, OSPF Hello interval, and OSPF dead interval.

- Make sure that both neighbors are part of the same area and area type.

- Use the debug and show commands to trace the problem.

18: When configuring a virtual link, which routers are configured?

** The configuration is between the ABRs, where one of the ABRs resides in Area 0 and the other in the area that is disconnected from the backbone. Both of the ABRs are also members of the transit area. Having created the virtual link, both ABRs are now members of Area 0, the disconnected area, and the transit area.

19: What does the command area 1 default-cost 15 achieve?

** The command area 1 default-cost 15 will assign a cost of 15 to the default route that is to be propagated into the stub area. This command is configured on the ABR attached to the stub area.

20: Explain what is placed in the parameters area-id and router-id for the command area area-id virtual-link router-id.

** The parameter area-id is the area ID of the transit area. So if the ABR in Area 0 is creating a virtual link with the ABR in Area 3 through Area 2, the area ID stated in the command is Area 2. The router ID is the router ID of the router with whom the link is to be formed and a neighbor relationship and adjacency established.


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