OSPF Flashcards
How does an OSPF election determine the DR and BDR
First by Interface Priority (default of 1).
In the event of a tie, the highest router -Id will win.
What 6 things must match for an OSPF adjacency to become full
Hello and dead timers Area MTU size Authentication Subnet Matching stub flags
What is the default hello and dead timer values for OSPF
10 sec hello, 40 sec dead
What happens if a DR has its interface priority set to 0
It immediately relinquishes its role and will never become DR for that subnet again
What is the AD of all (inter and intra) OSPF routes
110
What are the 2 ways you can apply OSPF authentication
Interface (enabled between router peers)
Area (enabled on all interfaces in a given area)
What are the 3 types of OSPF authentication
Null (no auth)
Clear text (password sent unencrypted)
Cryptographic (MD5 hash)
What is an OSPF passive interface
A interface which will pass OSPF routes, but will not form neighbour adjacency’s
What is the difference between a link state and distance vector routing protocol
A link state maintains a database of all routers and interfaces in a given area.
Distance vector only learns routes from its adjacent neighbour router.
What are the 6 types of OSPF LSA
Type 1 - Router LSA Type 2 - Network LSA Type 3 - Summary LSA Type 4 - ASBR (summary LSA) Type 5 - AS external LSA Type 7- Not-so-stubby-area (external LSA)
What is a Type 1 LSA
Type 1 are router LSAs
They contain the IP address information for each connected OSPF interface.
Only really relevant for point to point configuration.
What are OSPF type 2 LSAs
Type 2 LSAs are Network LSAs
These contain subnet information for the routers within a given OSPF area.
Type 2 LSAs are generated by a DR from Type 1 LSAs.
What is a type 3 LSA
Type 3 LSAs are summary advertisements which include inter area prefixs from other areas.
An ABR will generate and send the summary LSA into the adjacent area.
What IP protocol number and multicast addresses are used by OSPF routers
IP protocol 89
Hello messages are sent to 224.0.0.5
In a multicast environment, all LSAs are sent to the DR on 224.0.0.6. The DR will then send the Type 2 LSA on 224.0.0.5
What are the 7 states of OSPF adjacency
Down - sending hello packets
Init - ack sent after hello received
Two-way - bi directional communication confirmed
Exstart - negotiating of sequence numbers for error recovery
Exchange - LSA header exchange
Loading - routers exchange full link state information
Full - link state database are in sync. non DR routers will never become full, they will remain 2 way.
