OSPF Basics Flashcards
What does OSPF packet header include? (9)
- Version
- Type
- Packet Length
- Router ID
- Area ID
- Checksum
- Authentication Type
- Authentication Data
- Data
Does OSPF send periodic routing updates?
no
What are the 4 basic characteristics of OSPF?
- open standard
- classless
- interior
- link-state
What algorithm does OSPF use for topology calculation?
Dijkstra SPF algorithm
What is the role of the Dijkstra algorithm?
build the overall map of the network topology represented by state of the links between the routers
Do routers know about every single interface packet needs to go through to reach the destination?
yes
What is the IP protocol number of OSPF?
89
Does OSPF use TCP or UDP for transport?
neither
What is the OSPF version for IPv4 and IPv6?
IPv4: OSPFv2
IPv6: OSPFv3
How many tables are constructed by OSPF?
3
What are the three tables constructed by OSPF?
- Neighbor Database
- Link State Database
- Routing Table
What is the purpose of the Neighbor Table (Neighbor Database)?
keep track of its neighboring routers with which it has established OSPF adjacencies
What does the Neighbor Table (Neighbor Database) contain? (11)
- Neighbor Router ID
- Neighbor IP Address
- Neighbor Priority
- Neighbor State
- Neighbor Interface
- Neighbor Area
- Neighbor DR and BDR
- Neighbor Router Dead Timer
- Neighbor Authentication Type
- Next Hop Address (the neighbor’s IP on the link)
- Exit interface (what interface to use to send the packet to the neighbor)
What is Router ID?
- IP address-like ID of the OSPF device used to identify a device
- each OSPF device requires a Router Identifier (Router ID)
What happens if the same Router ID shows up in a multiple places of a tree?
the overall SPF calculations will be broken and routing loops may occur
How is the Router ID represented?
32bit dotted-decimal value that looks like an IPv4 IP address (also applicable to OSPFv3)
Which Router ID value is reserved and cannot be used?
0.0.0.0
Is it true that multiple OSPF process on same node need unique RID values?
yes, also includes VRFs
What is the Router ID (RID) selection order of precedence?
- Manually configured under the global OSPF configuration section
- Highest numeric IPv4 on a loopback interface (the interface cannot be shutdown)
- Highest numeric IPv4 address on any non-loopback interface (can be in down/down state but cannot be shutdown)
How is a routing table calculated in OSPF?
on each router, LSDB is provided as an input to SPF algorithm, which then generates a routing table
What is a passive interface?
router prevents sending Hello messages through interfaces configured as passive
What type of updates does OSPF use?
Event Driven Incremental Updates
How do Event Driven Incremental Updates work?
- no need to wait for a timer to expire so update can be triggered
- uses multicast and unicast addresses
What is the time period for which advertised OSPF routes are valid?
60 minutes - the router that advertised the network periodically refreshes the route each 30 minutes
