Term
| Which organization is responsible for the OSPF specification, and when was the current specification for use with IPv4 published? |
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Definition
-IETF
-OSPFv2 published in RFC 1247 (1991) |
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Term
| What type of routing protocol is OSPF? |
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Definition
| Link-state routing protocol |
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Term
| How is OSPF encapsulated? |
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Definition
| The data portion of an OSPF message is encapsulated in a packet. The data field can include one of five OSPF packet types. |
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Term
| What destination IP addresses are used for OSPF packets? |
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Definition
-Multicast addresses
-224.0.0.5 (01-00-5E-00-00-05)
-224.0.0.6 (01-00-5E-00-00-06) |
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Term
| What are the five types of OSPF packets? |
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Definition
-0x01:Hell0
-0x02:Database Description
-0x03:Link state request
-0x04:Link state update
-0x05:Link state ACK |
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Term
| For what purpose are Hello packets used? |
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Definition
| To establish and maintain adjacency with other OSPF routers. |
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Term
| What multicast address is used as the destination IP address for a packet that should be processed by all OSPF routers? |
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Definition
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Term
| What values must be the same on two OSPF routers if they are to become adjacent? |
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Definition
-Hello interval
-Dead interval
-Network type |
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Term
| By default, what is the hello interval on the interface of an OSPF router? |
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Definition
-10 seconds:Multiaccess and poit-to-point
-30 seconds:NBMA |
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Term
| How long will a router wait without receiving hello packets before regarding a neighbor on an Ethernet link as down? |
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Definition
-Four times the Hello interval
-40 seconds:Multiaccess and point-to-point
-120 seconds:NBMA networks |
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Term
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Definition
| Broadcast packet used by link-state protocols that contains information about neighbors and path costs. LSAs are used by the receiving routers to maintain their routing tables. |
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Term
| Where does an OSPF router store the information that it learns from link state advertisements? |
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Definition
| Each OSPF router maintains a link-state database containing the LSAs from all routers. |
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Term
| How is the information used? |
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Definition
| OSPF uses Dijkstras shortest path first (SPF)algorithm to create an SPF tree. The SPF tree is then used to populate the IP routing table with the best paths to each network. |
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Term
| Of the commonly used interior protocols, which produce routes that are preferred to OSPF by Cisco routers, and which produce routes that are not preferred? |
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Definition
-OSPF has a default AD of 110
-OSPF is preferred over IS-IS and RIP. |
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Term
| What should be done about discontiguous subnets where OSPF routing is used? |
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Definition
| Configure the MASK as part of the OSPF configuration. |
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Term
RA(config)#router ospf 1
RB(config)#router ospf 2
What are the numbers 1 and 2, and can these routers become adjacent? |
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Definition
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Term
| What are the two purposes of the network command? |
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Definition
-Any interfaces on a router that match th network address in the network command will be enabled to send and receive OSPF packets.
-The network(or subnet)will be included in OSPF routing updates. |
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Term
| How does an OSPF network command differ from an EIGRP network command? |
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Definition
| OSPF requires the wild card mask. |
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Term
| What happens if you put the subnet mask in an OSPF network command? |
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Definition
| The IOS converts the subnet mask to the wildcard mask format. |
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Term
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Definition
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Term
| What wildcard mask would be equivalent to a subnet mask of 255.255.240.0? |
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Definition
255.255.255.255
- 255.255.240.0
= 0.0.15.255 |
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Term
| What wildcard mask would be equivalent to a subnet mask of 255.255.255.192 |
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Definition
255.255.255.255
- 255.255.255.192
= 0.0.0.63 |
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Term
| What wilcard mask would be used for point to point network 192.168.6.192/30? |
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Definition
255.255.255.255
- 255.255.255.252
= 0.0.0.3 |
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Term
| What are the three ways of determining a router ID, in order of preference? |
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Definition
-Use IP address configured with OSPF router-id command.
-If the router-id is not configured the router chooses highest IP address of any of its loopback interfaces.
-If no loopback interfaces are configured, the router chooses the highest active IP address of any of its physical interfaces. |
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Term
| What is the disadvantage of having a router ID based on an active physical interface? |
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Definition
| It must be in the up state. |
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Term
| Why do adminstrators use loopback addresses when they could configure an ID using the router-id command? |
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Definition
| Using a loopback address for the router ID provides stability to the OSPF process. It cannot fail. |
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Term
| What state are neighbouring OSPF routers in when their link state databases are identical? |
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Definition
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Term
| What is the purpose of the SPF schedule delay and the hold time? |
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Definition
| To prevent the router from constantly running the SPF algorithm. |
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Term
| What is the default reference bandwidth for a Cisco OSPF router, and how is this reference bandwidth used? |
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Definition
| 100,000,000bps or 100Mbps. The reference bandwidth can be modified to accommodate networks with links faster than(100Mbps). |
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Term
| How does OSPF work out the metric for a route that has 3 hops? |
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Definition
100,000,000/Bandwidth
-Then finds cumulative cost for all links on a path. |
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Term
| A serial link is configured to operate at 64 kbps using the clock rate 64000 command on the DCE interface. The bandwidth and cost commandscare not used. What will be the calculated OSPF cost for the link? |
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Definition
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Term
| Why should you use the bandwidth command when configuring an interface of an OSPF router? |
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Definition
| To ensure both sides of the link are configured to have the same value. |
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Term
| What command could you use as an alternative to the bandwidth command? When would it be better to use this alternative command? |
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Definition
-R1(config-if)#ip ospf cost
-It is useful in multi-vendor environments where non-Cisco routers use a metric other than bandwidth to calculate the OSPF costs. |
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Term
| How many routers can you have on a point-to-point network? |
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Definition
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Term
| How many routers can you have on a multi-access network? |
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Definition
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Term
| What is a potential problem of using OSPF on multi-access networks? |
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Definition
-Creation of multiple adjacencies,one adjacency for every pair of routers.
-Extensive flooding of LSAs |
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Term
| How is this problem solved? |
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Definition
| OSPF elects a Designated Router(DR)to be the collection and distribution point for LSAs sent and received. |
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Term
| What IP destination address is used by a router sending packets to a DR or BDR router? |
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Definition
| Multicast address 224.0.0.6 |
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Term
| What name is given to a router that takes part in an election for DR and BDR, but is not chosen? |
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Definition
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Term
| What criteria are used to choose a DR and BDR? |
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Definition
-DR:router with the highest OSPF interface priority
-BDR:router with the second highest OSPF interface priority
-If OSPF interface priorities are equal, the highest router ID is used to break the tie. |
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Term
| What is the default priority value for a router? |
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Definition
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Term
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Definition
| When two DROther routers form a neighbor adjacency. |
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Term
| What should you do to the configuration of a router that you want to win election for DR on a multi-access network? |
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Definition
| Change the OSPF interface priority. |
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Term
| Why should you switch a router on first if you want it to win the election for DR? |
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Definition
| It is possible that a router with a lower router ID could become the DR because it took less time to boot. |
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Term
| What happens if the DR(or its interface)fails? |
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Definition
| BDR assumes the role of the DR and an election takes place for a new BDR. |
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Term
| A router has a default route configured. Which command will make OSPF propagate this default route to other routers? |
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Definition
| R1(config-router)#default-information originate |
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Term
| If a router has learned a default route through OSPF, how will this route be shown in the routing table? |
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Definition
| O*E2 0.0.0.0/0[110/1]via 192.168.10.10 00:00:13 serial 0/0/1 |
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Term
| How does a type 2 external OSPF route differ from a type 1? |
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Definition
| The difference between the two is in the way the OSPF cost of the route is being calculated at each router. |
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Term
| What are the advantages and disadvantages of making the hello and dead intervals smaller? |
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Definition
-Advantages:routers will detect network failures in less time.
-Disadvantages:adjacencies with other routers could be broken if the intervals are not configured correctly. |
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