Section 11. Implementing OSPF Flashcards
Q1-What is the routing metric that OSPF is based on?
Bandwidth is the routing metric that OSPF is based on.
OSPF’s metric is a cost value based on bandwidth. The default formula used to calculate OSPF cost is as follows:
Cost = 100,000,000 / Bandwidth in bps
For example, OSPF assigns the cost of 10 to a 10-MB Ethernet line (100,000,000 / 10,000,000 = 10).
For the exam, you might see multiple correct answers. If a question asks what the OSPF metric is based on, bandwidth is the correct answer. If the question asks what the OSPF metric is, the correct answer is cost.
Q2-What is the OSPF neighbor table?
The OSPF neighbor table is a list of all neighbors discovered by OSPF that an adjacency is formed with.
OSPF routers must have certain parameters matching before forming adjacencies and appearing in the neighbor table.
The show ip ospf neighbor command is used to display OSPF neighbors.
Q3-Provide four reasons why you would use OSPF instead of RIPv2.
Four reasons why you would use OSPF instead of RIPv2 are as follows:
Fast convergence.
No reachability limitations.
More efficient use of bandwidth.
Path selection is based on bandwidth rather than hops.
Q4-Before OSPF-enabled routers can exchange link-state information, what must be established?
Neighbor adjacencies must be established.
Before OSPF-enabled routers can exchange link-state information on directly connected routes, they must know about each other and form an adjacency with routers in their area.
Q5-How do OSPF-enabled routers learn and recognize other OSPF-enabled routers?
OSPF-enabled routers learn and recognize other OSPF-enabled routers through Hello packets. OSPF-enabled routers send Hello packets out all OSPF-enabled interfaces to determine whether there are any neighbor routers on those links.
Q6-What information does each Hello packet contain?
Each Hello packet contains the following information:
Router ID of the originating router
Area ID of the originating router interface
IP address and mask of the originating router interface
Authentication type and authentication password if required of the originating router interface
HelloInterval
DeadInterval
Interface router priority
Designated router (DR) and backup designated router (BDR)
Neighbor field
Q7-What fields in Hello packets must match for OSPF-enabled routers to form adjacencies?
Area ID, subnet mask and subnet number, Hello Interval, Dead Interval, and Authentication fields must match.
Q8-How do OSPF-speaking routers build adjacencies and exchange advertisements?
OSPF-speaking routers build adjacencies based on OSPF network types and exchange link-state information by sending Hello packets out all OSPF-enabled interfaces.
If the routers share a common network type and agree on certain parameters set in their Hello packets, they become neighbors. If these parameters are different, they do not become neighbors and no adjacency is formed. OSPF routers can then form adjacencies with certain routers. The routers that OSPF-speaking routers build adjacencies with are determined by the data-link media type. After adjacencies have been formed, each router sends link-state advertisements (LSA) to all adjacent routers. These LSAs describe the state of each of the router’s links. Because of the varying types of link-state information, OSPF defines multiple LSA types. Finally, a router receiving LSAs from its neighbors records the LSA in a link-state database and floods a copy of the LSA to all its other adjacent neighbors in most situations. When all databases are complete, each router uses the SPF algorithm to calculate a loop-free topology and builds its routing table based on this topology.
Q9-What are link-state advertisements?
Link-state advertisements (LSA) are what OSPF-enabled routers send out all OSPF-enabled interfaces to describe the state of the routers’ links. LSAs are also packets that OSPF uses to advertise changes in the condition of a specific link to other OSPF routers.
Q10-How many LSAs exist in OSPF?
Eleven distinct link-state packet formats are used in OSPF; each is used for a different purpose. However, Cisco only supports eight: Types 1–5 and 7–9.
The ICND1 exam will most likely test you on two LSA types: Type 1 and Type 2.
Type 1 LSAs are Router LSAs and are generated by each router for each area to which it belongs. These LSAs describe the states of the router’s links to the area and are flooded within a single area.
Type 2 LSAs are Network LSAs and are generated by the DR and BDR. They describe the set of routers attached to a particular network. They are flooded within a single area.
Q11-What is the OSFP router ID, and how does an OSPF router derive its router ID?
For OSPF to initialize, it must be able to define a router ID for the entire OSPF process. A router ID is derived from the manual configuration through the router-id command and from the numerically highest IP address set on the loopback interface. The loopback interface is a logical interface that never goes down. If no loopback address is defined, an OSPF-enabled router selects the numerically highest IP address on all its interfaces as its router ID.
Q12-What IOS commands shows the router ID configured on an OSPF-enabled router?
The show ip protocols, show ip ospf neighbor, show ip ospf, and show ip ospf interface commands all display the router ID configured for OSPF.
Q13-How do you enable OSPF on a Cisco router?
Basic OSPF is configured in two steps: Enable the OSPF routing process and then identify the networks that you want to advertise.
The router ospf process-id command enables the OSPF process, and the network address wildcard-mask area area-id command assigns networks to a specific OSPF area. Consider the following example:
RouterA(config)# router ospf 10
RouterA(config-router)# network 192.168.10.0 0.0.0.255 area 0
These commands enable OSPF process 10 and advertise any network assigned to any interface within 192.168.10.0/24 to possibly be part of area 0. Notice that you must specify the wildcard mask instead of the subnet mask.
Q14-What does the show ip protocols command display?
The show ip protocols command displays a summary of configured routing protocol information. It is useful for a quick verification of how routing protocols are configured. It allows you to verify which protocols are enabled and which networks they are routing to.
Q15-What IOS commands display the OSPF neighbor information on a per-interface basis?
The show ip ospf neighbor and show ip ospf interface interface-type interface-number commands display OSPF neighbor information on a per-interface basis.
RouterB# show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
172.16.0.1 1 FULL/ - 00:00:31 10.1.1.1 Serial0
RouterB# show ip ospf interfaces
Serial00 is up, line protocol is up
Internet Address 10.1.1.1, Area 0
Process ID 1, Router ID 1.1.1.1, Network Type POINT_TO_POINT, Cost: 64
Transmit Delay is 1 sec, State POINT_TO_POINT
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
oob-resync timeout 40
Hello due in 00:00:00
Supports Link-local Signaling (LLS)
Cisco NSF helper support enabled
IETF NSF helper support enabled
Index 1/1, flood queue length 0
Next 0x0(0)/0x0(0)
Last flood scan length is 1, maximum is 1
Last flood scan time is 0 msec, maximum is 0 msec
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 172.16.0.1
Suppress hello for 0 neighbor(s)
