07 - Implementing OSPF Flashcards
Do routers have to have the same process ID?
No
What is the router ID election process?
- Manually set RID
- Highest loopback
- Highest set active IPv4
Hello packets are periodically sent to the multicast address _____.
224.0.0.5
Neighbour relationship is formed if two routers agree on:
- Area ID
- Hello and dead intervals
- Authentication
- Same subnet
What are the OSPF packet types?
- Hello
- DBD
- LSR
- LSU
- LSAck
OSPF packet type - discovers and maintains neighbours
Hello
OSPF packet type - contain the LSA headers that help build the database
DBD
OSPF packet type - After DBDs are exchanged, router checks the LSA headers again its own database. If it does not have current information for any LSA, it generates a ___ packet and sends it to its neighbour
LSR
OSPF packet type - contains a list of LSAs that should be updated. Often used in flooding
LSU
OSPF packet type - help to ensure a reliable transmission of LSAs. Each LSA is explicitly acknowledged
LSAck
OSPF has protocol number __.
89
What are the LSA types?
- 1 Router LSA
- 2 Network LSA
- 3 ABR Summary
- 4 ASBR summary
- 5 Autonomous system
- 6-11 other types
LSA type: describes the state of the router links to the area and flooded within that particular area
1 - router LSA
LSA type: describe the set of routers that are attached to a particular multiaccess network
2 - Network LSA
LSA type: takes the information it learned in one area and summarised it for another area
3 - ABR summary
LSA type: informs the rest of the OSPF domain how to get to the ASBR.
4 - ASBR summary
LSA type: generated by ASBRs, describes routes to destinations that are external to the AS. They get flooded everywhere except special areas.
5 - Autonomous system
In the configuration of the OSPF router if either of the following commands are used, the route becomes an ASBR:
- Resdistribute….
* Default-information originate
What are the OSPF neighbour states?
- Down – no neighbour detected
- Init – hello packet received from neighbour
- Two way – own router id in received hello
- Exstart – master and slave roles determined
- Exchange – DBD packets sent
- Loading – exchange of LSRs and LSUs
- Full – fully adjacent
How do you observe routes in a routing table?
show ip route ospf
How do you observe all ospf routes?
show ip ospf route
How do you observe ospf neighbour adjacency formation?
- Debug ip ospf adj
2. Clear ip ospf process
What are the benefits of having a DR/BDR?
- Reduces routing update traffic – central point of contact for link-state information exchange
- Manages link-state synchronisation – DR & BDR ensures other routes have same link-state information about common segment
What is the multicast IP to communicate with the DR ?
224.0.0.6
OSPF ___ is used to elect DR/BDR
priority - a value between 0-255
If there is a tie where routers have same priority value, the __is the tie breaker. Highest __ becomes the DR. 2nd highest BDR
router ID
Router with a priority of _ cannot be DR/BDR.
0
What is the default priority?
1
Router that is not the DR/BDR is called the ____.
DROTHER
The IP __ determines the maximum size of the packet that can be forwarded without fragmentation
MTU
Mistmatched MTUs can lead to …
Inconsistent OSPF databases
OSPF neighbours with mismatches MTUs are stuck in the ___ adjacency state
EXSTART
How do you change the MTU?
ip mtu 1500
What is the default value of the OSPF hello interval on broadcast multiaccess and p2p links?
10 seconds
What is the default value of the OSPF dead interval on broadcast multiaccess and p2p links?
40 seconds - 4 times the hello interval
What is the value of the OSPF hello/dead interval on all other OSPF network types including NBMA such as frame relay?
30/120 seconds
How do you change the hello an dead timers?
- Ip ospf hello-interval X
* Ip ospf dead-interval x
You can view detailed states such as how long a neighbour has been up with what command?
show ip ospf neighbour detail
How do you change the the OSPF network type?
Using interface command – ip ospf network network type
