What type of routing protocol is OSPF? - OSPF is a link state routing protocol.
POP QUIZ: How frequently does OSPF send out routing updates? - OSPF sends out routing updates every
30 minutes, and refreshes the LSDB for neighbor adjacencies.
POP QUIZ: How long does OSPF wait before considering a route previously learned, dead? - The MaxAge
timer is set to 60 minutes.
MaxAge is always 1 hour, and it has to be 1 hour on all platforms. The reason why is that MaxAge is a
special value that's used to flush a route out of the OSPF database.
For example if I have a connected link that's advertised into OSPF, and the link goes down, to withdraw
the link from other routers I re-flood it with MaxAge.
This is why the value has to be the same everywhere otherwise the routers couldn't properly remove a
route from the database.
What are the three main branches that can be broken down from link state routing protocol logic? - You
have
1. Neighbor discovery (neighbor table)
2. Topology database exchange (topology table)
3. Route computation (routing table)
What multicast address are OSPF hello packets sent to? - 224.0.0.5
POP QUIZ: OSPF uses TCP/UDP for its transport protocol. T/F? - F.
,It uses IP protocol 89.
What concept does OSPF use to segment LSDB flooding? - OSPF uses the concept of "Area's" to segment
LSDB flooding between peers.
How does OSPF decide upon its RID? - It's the exact same as EIGRP.
It checks to see if you:
1. Manually configured (first)
2. Highest IP address of an up/up Loopback interface
3. The highest IP address of any up/up non-loopback (physical) inteface
What do you have to do after changing the OSPF RID to take effect? - You need to either run the `reload`
or `clear ip ospf process` command.
It's an intrusive change, and will prompt you with a warning message before proceeding.
This is because it will tear down the OSPF neighbor relationship, and cause all links within that area to
recalculate their SPF tree.
How many OSPF neighbor states are there? - 8
What are those 8 OSPF neighbor states? - Down
Attempt
Init
2-Way
Exstart
Exchange
Loading
,Full
What is the purpose of each OSPF neighbor state? - Down:
This is the first OSPF neighbor state. It means that no information (hellos) has been received from this
neighbor, but hello packets can still be sent to the neighbor in this state.
During the fully adjacent neighbor state, if a router doesn't receive hello packet from a neighbor within
the RouterDeadInterval time (RouterDeadInterval = 4*HelloInterval by default) or if the manually
configured neighbor is being removed from the configuration, then the neighbor state changes from Full
to Down.
Attempt:
This state is only valid for manually configured neighbors in an NBMA environment. In Attempt state,
the router sends unicast hello packets every poll interval to the neighbor, from which hellos have not
been received within the dead interval.
Init:
This state specifies that the router has received a hello packet from its neighbor, but the receiving
router's ID was not included in the hello packet. When a router receives a hello packet from a neighbor,
it should list the sender's router ID in its hello packet as an acknowledgment that it received a valid hello
packet.
2-Way:
This state designates that bi-directional communication has been established between two routers. Bi-
directional means that each router has seen the other's hello packet. This state is attained when the
router receiving the hello packet sees its own Router ID within the received hello packet's neighbor field.
At this state, a router decides whether to become adjacent with this neighbor. On broadcast media and
non-broadcast multiaccess networks, a router becomes full only with the designated router (DR) and the
backup designated router (BDR); it stays in the 2-way state with all other neighbors. On Point-to-point
and Point-to-multipoint networks, a router becomes full with all connected routers.
, At the end of this stage, the DR and BDR
What does OSPF use to describe each neighbor state? - OSPF uses it's underlying finite state machine
(FSM) to represent its state changes.
What IP address does OSPF send multicast traffic to? - 224.0.0.5
POP QUIZ: OSPF sends multicast traffic (hello packets) on a LAN when what two requirements are met? -
1. OSPF has been enabled on the interface, using either the `network` statement in the OSPF instance,
or you use the `ip ospf area` interface-level sub-command.
2. The interface has *not* been made passive using the `passive-interface` command from the `router
ospf <#> instance.
POP QUIZ: What type of OSPF enabled routers will stay in a 2-way state by default? - DROTHERS
POP QUIZ: What routers will become fully adjacent (FULL state)? - Router's that are part of a point-to-
point link, or DR's and BDR's of their respective NBMA segment.
What DR and BDR multicast address are OSPF hello packets sent to? - 224.0.0.6
What similar concept in relation to EIGRP does OSPF do with passive interfaces? - If an interface is set to
a passive state it won't send hello messages, but, it will still advertise that subnet or prefix into OSPF.
What parameters must match in the hello packet to form an OSPF neighbor adjacency? - 1. Area
2. Hello and Dead Interval
3. Prefix/Subnet
4. Authentication
5. Stub Area Flag
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