OSPF COPY Flashcards
What are LSA used for?
Build a LSDB
Who will OSPF routers form a FULL adjacency with?
Designated Router and Backup Designated Router.
Who does a OSPF router send its LSA to?
Designated Router only.
DR distributes to all other routers.
What tpye of network would you have DR/BDR and what network would you not have a DR/BDR?
Multi-access Network requiers a DR/BDR. Point-to-Point does not requier a DR/BDR.
What is used to calculate the best path of the LSDB?
Shortest-path first Algorithm SPF
- ID ABR
- ID ASBR
- Why is it a ABR?
- Why is it a ASBR?
ABR are between two area’s and can summarize routes from one area to another. ASBR are routers that run OSPF and connect to another network that runs another routing protocol.
What is in an OSPF Hello Packet?
- Router ID
- Hello/Dead Interval *
- Neighbors
- Area ID *
- Router Priority
- DR IP
- BDR IP
- Authentication
- Password*
- Sub Area Flag
What metric does OSPF use?
Cost=Reference BW/Interface BW
How to change the Reference B/W?
(config)#router ospf 1
(config-router)# auto-cost reference-bandwidth 1000
What is the default Reference Bandwith for OSPF
100 Mbits
What does OSPF do if it has 2 path with equal cost?
Load-Balance
What are the Max Load Balance paths of IOS 12
- 16 Paths
(32 for IOS 15)
What does the network command do while under the router ospf 1 process.
(conf-router)# network 10.10.10.0 0.0.0.255 area 0
- Advertise the networks that fall within this range in OSPF.
- Activate OSPF on the interface(s) that fall within this range. This means that OSPF will send hello packets on the interface.
What is used as the router ID in OSPF?
- Loopback interface
- highest active IP
or set manualy.
(conf-router)#router-id x.x.x.x
How do you change the COST of a OSFP link?
ip ospf cost 50
within the interface use the command.
How would you make all routers send there unknow traffic to you.
- Advertise a default router.
- (config-router)# default-information originate [always]
How would you configure clear text Authentication in OSPF?
R1(config)#interface fastEthernet 1/0
R1(config-if)#ip ospf authentication
R1(config-if)#ip ospf authentication-key password
How would you configure MD5 Authentication in OSPF?
R3(config)#interface fastEthernet 1/0
R3(config-if)#ip ospf authentication message-digest
R3(config-if)#ip ospf message-digest-key 1 md5 0 password
What does this output of the show ip route mean.
O IA 4.4.4.4 [110/3] via 192.168.12.2, 00:00:54, GigabitEthernet0/1
How many States does OFPF go through to become Neighbors?
7
What are the 7 States that OSPF goes through to become neighbors?
- Down: no OSPF neighbors detected at this moment.
- Init: Hello packet received.
- Two-way: own router ID found in received hello packet.
- Exstart: master and slave roles determined.
- Exchange: database description packets (DBD) are sent.
- Loading: exchange of LSRs (Link state request) and LSUs (Link state update) packets.
- Full: OSPF routers now have an adjacency.
What are the OSPF Packet Types?
What Two Multicast Address does OSPF Send to?
- 0.0.5 - goes to All routers
- 0.0.6 - goes to DR & BDR
When a Router in OSPF is not a DR or BDR, what is it called?
DROTHER
True or False
DR/BDR election is per multi-access segment…not per area!
True
Explain in the picture why you would not want to advertise OSPF out of interface FA0/1?
It is a security risk. anyone could become neighbors and inject routes.
What is a LSA Type 1 ?
What is a LSA Type 2?
The network LSA or type 2 is created for each multi-access network. Remember the OSPF network types? The broadcast and non-broadcast network types require a DR/BDR. If this is the case you will see these network LSAs being generated by the DR. In this LSA we will find all the routers that are connected to the multi-access network, the DR and of course the prefix and subnet mask.
What is a type 3 LSA?
R2 will take the Type 1 message and create a type 3 summary LSA and flood it to Area 0
What is a type 4 LSA?
In this example we have R1 that is redistributing information from the RIP router into OSPF. This makes R1 an ASBR (Autonomous System Border Router). What happens is that R1 will flip a bit in the router LSA to identify itself as an ASBR. When R2 who is an ABR receives this router LSA it will create a type 4 summary ASBR LSA and flood it into area 0. This LSA will also be flooded in all other areas and is required so all OSPF routers know where to find the ASBR.
What is a Type 5 LSA?
Same topology but I’ve added a prefix (5.5.5.0 /24) at our RIP router. This prefix will be redistributed into OSPF. R1 (our ASBR) will take care of this and create a type 5 external LSA for this. Don’t forget we still need type 4 summary ASBR LSA to locate R1. If you ever tried redistribution with OSPF you might have seen O E1 or E2 entries. Those are the external prefixes and our type 5 LSAs.
What are type 7 LSA?
NSSA areas do not allow type 5 external LSAs. In my picture R1 is still our ASBR redistributing information from RIP into OSPF.
Since type 5 is not allowed we have to think of something else. That’s why we have a type 7 external LSA that carries the exact same information but is not blocked within the NSSA area. R2 will translate this type 7 into a type 5 and flood it into the other areas.
