OSPF Theory Flashcards
What is OSPF?
Open Shortest Path First - a non-proprietary link-state routing protocol
What is an LSA?
Link State Advertisement - information about an individual routers links aka networks.
What is an ABR?
Area Border Router - a router that has interfaces on networks that are in different OSPF areas.
What is an ASBR?
Autonomous System Border Router - a router that is running OSPF and also redistributing non-OSPF routes.
Describe a Type 1 LSA
Router LSA - Advertises the LSA that originate within an area
Describe a Type 2 LSA
Network LSA - Advertises multi-access network segment attached to a DR
Describe a Type 3 LSA
Network Summary (OSPFv3: Inter-Area Prefix)- Advertises network prefixes that originated from a different area. (ABR)
Describe a Type 4 LSA
ASBR Summary (OSPFv3: Inter-Area Router) - Advertises a summary LSA for a specific ASBR
Describe a Type 5 LSA
AS External LSA - Advertises LSAs for routes that have been redistributed.
Describe a Type 7 LSA
NSSA External LSA - Advertises redistributed routes in Not So Stubby Areas
Describe a Type 8 LSA
OSPFv3 Link - IPv6 only. These are for IPv6 Intra-Area link local addresses.
Describe a Type 9 LSA
IPv6 Network Link - IPv6 prefix advertisements that are only flooded within that area.
When do routers exchange their entire Link State DB with each other?
When those neighbors are fully adjacent.
What is effectively happening when you issue the ‘network’ command within OSPF config mode?
You are telling the router to enable all interfaces within the specified network range for OSPF and to advertise their networks.
What is a stub network?
A network where this is only one router. In other words, there are no other networks beyond that network.
What is a stub area?
An OSPF area that does not accept redistributed routes from an ASBR.
What is the formula for the number of neighbor adjacencies formed in an OSPF broadcast segment?
Where N is the number of OSPF routers: N(N-1)/2
What is a DR in OSPF?
Designated Router - The router that is responsible for advertising route updates. Only the DR forms full OSPF adjacencies with the other routers in the segment.
How is a DR elected in a network segment?
1.) First to come online
2.) Highest OSPF priority
3.) Highest Router ID (RID)
4.) Highest IP on Loopback
5.) Highest IP on Non-loopback
What is the BDR?
Backup Designated Router - takes over if the DR goes offline
Name the OSPF packet types that are sent when OSPF routers are performing neighbor relationships.
Hello - type 1
Database Description (DBD) - type 2
Link State Request (LSR) - type 3
Link State Update (LSU) - type 4
Link State Ack - type 5
What is the function of OSPF Hello packets
These packets are for discovering and maintaining neighbors.
What is the function of OSPF DBD packets
These packets are for summarizing database contents. Packets are exchanged when an OSPF adjacency is first being formed. These packets are used to describe the contents of the LSDB.
What is the function of OSPF LSR packet
These packets are sent from one OSPF router to request full LSA data. This is typically in response to a DBD or when part of the Link State Database is stale.
What is the function of OSPF LSU packet
These packets are for database updates. This is an explicit LSA for a specific network link and normally is sent in direct response to an LSR.
What is the function of OSPF LS Ack packet
These packets are acknowledge the receipt of an LSU. These packets can also be flooded across a network segment.
What states do routers go through in the process of forming an OSPF full adjacency?
Down
Attempt
Init
2-Way
Exstart
Exchange
Loading
Full
What happens in the OSPF Down state?
This state indicates that no information has been received recently, but the router is sending hello packets
What happens in the OSPF Attempt state?
This state only occurs when neighbors are manually configured on a non-broadcast multi-access network (multicast disabled) link and hellos are sent via unicast.
What happens in the OSPF Init state?
This state indicates that a hello packet has been received from another router and the routers in a segment are aware of each other (hello packets have seen each router in the segment).
What happens in the OSPF 2-way state?
- The router has received a hello packet with its RID inside of it, indicating that the neighbor now knows that it exists.
- Bidirectional communication has been established.
- If a DR or BDR is needed, the election occurs during this state.
What happens in the OSPF Exstart state?
This is the first state in forming an adjacency. Database Descriptor Packets (DBD) are sent from both sides and the routers negotiate Master/Slave.
What happens in the OSPF Exchange state?
During this state, the Master/Slave election is complete and DBD packets are sent with Link State DB summaries.
What happens in the OSPF Loading state?
LSAs are exchanged via the LSR, LSU, LS Ack process.
What happens in the OSPF Full state?
Neighboring routers are fully adjacent.
What effect does the ‘passive-interface’ command have?
It tells OSPF to advertise the network for the interface, but the interface will not send or receive OSPF hello packets. This is to prevent a rogue device from making an OSPF adjacency and inserting routes into the network.
What are the types of OSPF Networks?
Broadcast
Non-broadcast
Point-to-Point
Point-to-Multipoint
Point-to-Multipoint Non-Broadcast
What are the OSPF network types that use a DR?
Broadcast
Non-broadcast
What are the default OSPF timers?
It depends on the network type…
Type; Hello; Wait; Dead
Broadcast & Point-to-Point; 10; 40; 40
Non-Broadcast & Point-to-Multipoint; 30; 120; 120
What is the default OSPF network type for Ethernet?
Broadcast
What types of network connections use Point-to-Point OSPF network types by default?
Serial (HDLC, PPP), GRE Tunnels, P2P Frame Relay.
NOTE: This is because only 2 endpoints can exist and OSPF does not want to waste CPU cycles.
When dealing with point-to-point or point-to-multipoint networks, what needs to be configured properly to ensure adjacency?
Timers need to match on both ends (Hello, Dead, Wait, ReTx)
When can neighboring OSPF interfaces have different types and still be adjacent?
When the timers match on both ends
What are the different types of OSPF authentication?
Type 0 = None
Type 1 = Plain text
Type 2 = MD5
What are the default OSPF link costs for T1, Ethernet, FastEth, GigEth,10-GigEth?
T1: 64
Eth: 10
Fast, Gig, & 10-Gig: 1
In OSPF, what is area 0 called?
The backbone area where all other areas must connect. This could also be referred to as the transit area.
For Inter-Router communication, OSPF uses what two multicast addresses (IP and MAC) for AllSPFRouters?
224.0.0.5
01:00:5e:00:00:05
For Inter-Router communication, OSPF uses what two multicast addresses (IP and MAC) for AllDRouters?
224.0.0.6
01:00:5e:00:00:06
OSPF does not use TCP or UDP as a transport protocol. Instead it uses its own called what?
Protocol 89
What does it mean to say that OSPF is a two-tier hierarchy?
All areas must connect to the backbone area (area 0) via an ABR.
What is a passive interface? Why might you need it?
A passive interface does not try to form OSPF adjacencies. This may be needed for security reasons, so a rogue device cannot be plugged in and introduce false routes.
What is the IPv6 multicast address (group) for OSPFv6?
FF02::5
In OSPFv3, what is the new name for the LSA Type 3?
Inter-Area Prefix
In OSPFv3, what is the new name for the LSA Type 4?
Inter-Area Router
