Module 7 - Implementing OSPF Flashcards
OSPF Process ID
What is it? why is it important?
router ospf 1 - creates the OSPF process on a router.
The process ID for OSPF is locally significant, generally you’d only have 1 processs of OSPF running on a router, however if there is a company merger or MPLS configuration needed then more than one process can be configured.
OSPF Router-ID
#router ospf 1
router-id 10.10.10.1
The router id is a 32 bit value but is not an IP address. is recommended to have this set as a created loopback address. This is so that diagnostic/monitoring information can be done directly from the loopback. The router-id is important as it is used when exchanging LSA’s.
OSPF
Routing Protocol
What type of routing protocol is it? what is the Admin distance of this protocol?
OSPF is a link state routing protocol. Is is an open standard as opposed to EIGRP. It was developed by multiple vendors.
It is used to provide fast convergence in large networks an is hierachical in nature.
It makes use of an Autonomous System, which is a network under the control of a single administrative owner/company.
AS and Areas (backbone ‘Area 0’ or multi area) are the foundation of OSPF.
Multi-area OSPF is used to provide failure domains and improve network efficiency by minimising the size of the Link state database.
OSPF has an Administrative distance of 110. Making this more preferred than what it was designed to replace in RIP.
OSPF Defined
It’s a ‘link-state’ routing protocol, designed to be more efficient than RIP. RIP was limited to an hop count of 15, anything beyond 15 hops was considered unreachable. It didn’t scale well for large networks. RIP was also classful adveritsing Class A,B,C networks.
OSPF is a VLSM Classless routing protocol. It’s 2 - tier hierachical, it makes use of an Autonomous System and an Area.
It calculates paths using the Dijktra Algorithm, using link speed to calaculate the best path. The best bandwidth = lowest cost = fastest path.
OSPF Continued
OSPF can be single area or multi-area, multi-area designs make use of contigous networks, they reduce the propagation of link state advertisements also.
LSA’s advertise link state changes and advertise networks. They are sent immediatley upon a state change. By default is no LSA is received then one is sent every 30 minutes by default from neighboring routers.
LSA’s are pieced together to form an OSPF Routers ‘Link State Database’ LSDB, the lsdb is responsible for providing a router with a full picture of a topology.
Which of the following statements is correct concerning Autonomous Systems in OSPF?
- An AS consists of a collection of networks under a separate administration that share a common routing strategy. An AS, which is sometimes called a domain, can be logically subdivided into multiple areas.
- An AS consists of a collection of networks under a common administration that share a common routing strategy. An AS, which is sometimes called a domain, can be logically subdivided into multiple areas.
- An AS consists of a collection of networks under a common administration that share a common routing strategy. An AS, which is sometimes called an area, can be logically subdivided into multiple AS systems.
- An AS consists of a collection of networks under a common administration that share a common routing strategy. An AS, which is sometimes called an ASBR, can be logically subdivided into multiple backbone areas.
Answer
An AS consists of a collection of networks under a common administration that share a common routing strategy. An AS, which is sometimes called a domain, can be logically subdivided into multiple areas.
What is the correct order for how the router-id is selected?
- Manually configured
- Highest IP address of an active loopback interface.
- Highest IP address of an active physical interface.
A.
Manually configured
Desginate the router-id directly within the ospf process to ensure that the router-id is set. Best practice to set to a loopback address. Router-ID must be unique.
Which three parameters must match between OSPF neighbors for an adjacency to form? (Choose three.)
- Router ID
- Hello/Dead Interval
- Area ID
- Router Priority
- DR IP Address
- BDR IP Address
- Authentication data
The answer is Hello/Dead Interval, Area ID and Authentication Data must match within an Hello packet. If they do not then neighbor adjacency will no form.
Also if the stub flag is set these also must match.
OSPF Neighbor Adjacency
This is required before database information can be exchanged. It relies on 3 things that must match within the Hello Packet, these are…
Hello/Dead timers, Area ID and Authentication data.
The hello packet is advertised out every 10 seconds to neighbors by default. The dead interval is set to 4 times the hello interval, by default this is 40 seconds. A link is considered down once this timer expires.
Authentication information within OSPF although not required to be set is recommended.
MTU size must also match otherwise database information will not be exchanged.
OSPF Hello Packet
Contains 8 pieces of information that are advertised out.
Sent Every 10 seconds by default and sent by OSPF interfaces. They are sent to the following multicast address?
The hello packets are sent out of ospf interfaces to multicast address 224.0.0.5 every 10 seconds.
The packet contains these 9 pieces of information:
hello/dead interval
area id
router id
stub flag
DR/BDR IP Address
Authentication data
router priority
nieghbors
Q.
Which OSPF packet type contains a list of LSAs that should be updated, and is often used in flooding?
- LSR
- DBD
- LSU
- LSAck
- Hello
Answer
Link State Update (LSU)
OSPF
Packet Types
There are 5 types of OSPF packets, which are sent over protocol 89.
The OSPF packets are :
hello
dbd
lsr
lsu
lsack
