OSPF Flashcards
What is the backbone area
Area 0 or the backbone is the area that all other OSPF areas must connect to. the backbone provides transit connectivity between the non backbone areas.
What is an OSPF area
An area provides separation in OSPF. this allows scalability to increase as a routing domain is only within the area and OSPF adjacencies need to be made within the area not with every router running OSPF.
What IP protocol number does OSPF use
IP 89
What address does OSPF use to communicate with all OSPF routers
224.0.0.5
01:00:5E:00:00:05
What address does OSPF use to communicate with DR routers
224.0.0.6
01:00:5E:00:00:06
What are the different OSPF packet types
Hello
Database description (DBD)
Link state request
Link state update
Link state Acknowledgement
What is a OSPF Hello packet
Used for discovering and maintaining neighbour relationships.
What is a DBD packet
Database descriptor: used to describe the contents of the LSDB
What is a LSR packet
Link state request: Packets used to to download specific networks from the LSDB
What is a LSU packet
Link state update: Sent in response to a LSR
What is a Link state acknowledgement
Sent as an acknowledgement of the LSA.
What are the different fields in a OSPF hello packet
RID
authentication options
Area ID
Hello & dead timers
DR/BDR IP
Interface address mask
Interface priority
OSPF neighbours
What is an OSPF neighbour
IS a router that shares a common OSPF enabled network, OSPF discovers neighbours by Hello packets. All neighbours have a synchronised LSDB
When a OSPF neighbour forms what are the different states of the adjacency
Down
Attempt
Init
2way
ExStart
Exchange
Loading
full
What are the requirements must match for OSPF neighbours to form adjacency
RID must be unique
Interfaces must share a common subnet
MTU must match
Area ID must match
DR enablement must match
OSPF hello/dead timers must match
Authentication must match
Area type flags must match
What is the purpose of the designated router (DR)
To maintain neighbour adjacencies with all OSPF routers within the network segment. All routers can not maintain adjacencies with all routers as this would be impossible with scaling.
What is the process for a OSPF router learning a new route within a broadcast domain.
- OSPF router sends the route to 224.0.0.6
- The DR send a unicast message to the sender acknowledging the route.
- DR sends route to 224.0.0.5 to distribute via all OSPF routers on network segment
What are the 2 methods of configuring OSPF
- the network statement configured under the OSPF process with a wild card mask.
- Interface specific configuration enabling OSPF under a specific interface
What is the benefit of interface specific configuration over network statement when enabling OSPF
Using the interface configuration provides explicit control over the OSPF process
In a hybrid situation where a network statement and an interface configuration are both enabled on the same OSPF router, which takes precedence
Interface specific configuration
By default how is the RID set in OSPF
The RID is allocated by:
- The highest IP of any UP loopback interface.
- The highest IP of an UP physical interface.
How does a router select a new RID
OSPF process must be restarted
What is a OSPF passive interface used for
To prevent un-authorised routers from been connected to the OSPF domain and advertising false routes to cause traffic problems.
A passive interface prevents hello packets from been sent and therefore adjacencies from being formed on that interface.
Network segments however are still added to the LSDB.
Does OSPF support default route advertisement
Yes, command: default-information originate [always][metric]
What are the main optimisations that can be performed on a OSPF router
Changing OSPF cost of an interface
Changing OSPF priority of a router to be elected for DR/BDR
Changing Hello/dead timers
What is the reference bandwidth to calculate OSPF link cost
100Mbps
What is the default OSPF cost (metric) for E1, ethernet, FA, Gbps, TenGbps
E1 - 64
ethernet - 10
FA - 1
Gbps - 1
TeGbps - 1
What is the calculation to calculate OSPF cost
Reference bandwidth (in Mbps) divided by interface bandwidth (Mbps)
Why would you change the reference bandwidth and how do you do it
To support higher interface speeds. As 1 Gbps link has the same metric as Ten Gbps link with default values.
command: auto cost reference-bandwidth
What is the important thing to do when changing the reference bandwidth
change the reference bandwidth on all OSPF devices in the routing domain to ensure the Dijkstra’s algorithm runs correctly.
What is the default priority for DR elections on a OSPF router
1
If there is a match the OSPF priority how is the winner identified
the highest RID will become the DR
how do you remove a router from the election process
set the priority to 0
what is the maximum OSPF priority value
255
Does OSPF DR priority update support pre-emption
No
What are the different OSPF network types
Broadcast
Non-Broadcast
P2P
Loopback
What are the default timers in OSPF Broadcast network
Hello: 10
Wait: 40
Dead: 40
What are the default timers in a OSPF non broadcast network
Hello: 30
Wait: 120
Dead: 120
What are the default timers in a OSPF P2P network
Hello: 10
Wait: 40
Dead: 40
What are the default timers in a OSPF P2MP network
Hello: 30
Wait: 120
Dead: 120
What are the default timers in a OSPF Loopback network
No timers for loopbacks
What is the OSPF wait timer used for
To allow newly booted routers to Identify the DR/BDR on a segment instead of immediately declaring themselves a DR
Which OSPF network types have a DR/BDR field in the ‘hello packet’
Broadcast and non broadcast OSPF networks.
How does OSPF determine which network type to use
Media plugged in.
What are the drawbacks to using a single area in OSPF
Does not scale well due to:
Neighbour adjacencies required with all router within the area.
LSDB becomes unmanageable.
No summarisation
What are the advantages of multi area OSPF model
LSDBs are smaller
Quicker SPF calculations
Decreases LSA flooding when a link flaps
All areas transmit routes to all other areas injecting them into each other.
True or False
False. Only area 0 can inject routes into non-backbone areas.
What is a ABR
Area border router, a router that has multiple areas directly connected. One of the areas must be area 0.
What is a intra-area route
Route learnt from an OSPF router within the same area.
What is a Inter-area route
A route learnt from a OSPF router in another area.
What is the code in the RIB for a intra-area route
O
What is the code in the RIB for a inter-area route
O IA
What is the purpose of LSAs
to build the LSDB and the OSPF tree
How many LSAs are used for IPv4 routing
6
What is a type 1 LSA
Router LSA - advertises LSA the originate within the area
What is a type 2 LSA
Multi access network segment LSA: Used when a DR is used.
What is a type 3 LSA
Summary, used to advertise networks to OSPF routers outside of the originating router.
What is a type 4,5,7 LSA
Type 4: ASBR Summary LSA
Type 5: AS external LSA
Type 7: NSSA external LSA
Every OSPF enabled link advertises one type 1 LSA.
True or False
True
Only some routers advertise Type 1 LSAs
True or False
False. All routers advertise type 1 LSA
On what type of router is a Type 2 LSA created
DR
Does a type 1 or 2 LSA leave the area
No both are only intra-area
How are type 3 LSAs generated
When a ABR receives a Type 1 LSA it generates a type 3 LSA and sets itself as the advertising router and sends it to another area.
What happens if a ABR receives 2 type 3 LSAs for the same network segment
the ABR only advertises the type 3 LSA with the lowest metric.
What is a discontiguous network
when inter-area traffic is trying to cross a non-backbone area.
e.g. area 12 > area 40
Not possible as it MUST transmit through area 0.
What is the priority for OSPF path selection when running Dijkstra’s algorithm
- Intra-area routes
- Inter-area routes
- External routes
What happens if there is a match of metrics with 2 routes to the same network and prefix length
ECMP is used.
What is OSPF route summarisation
Summarising the network prefix’s into a shorter subnet mask that are advertised in the type 3 LSA.
What is the benefit of OSPF summarisation
Reduces number of type 3 LSAs generated on the ABR and the amount advertised into other areas.
How is the metric calculated for a summarised network
Default is the lowest metric associated with the LSA, can be manually configured
What is OSPF route filtering
Method of selecting which identified routes are advertised or received from or to neighbours
What are the advantages of route filtering
Manipulate traffic flow
reduce memory utilisation
improve security
What are the different methods of route filtering
Summarisation filtering
Area filtering
Local OSPF router filtering
What is Summarisation filtering
Preventing type 3 LSAs from been generated on ABR of a specific summarised network.
This prevents the route to that network from been advertised outside of the area.
What is Area filtering
Allows ABR to filter routes as they leave or enter the area.
What is Local OSPF filtering
To remove routes from specific routers within a OSPF area, this is done by still installing LSA into LSDB as they must be synchronised across the area.
However the route is not installed into the RIB.
