Link-State Advertisement (LSA) and Link-State Database (LSD) Flashcards
What does a Link State Advertisement (LSA) contain?
fundamental information about topology
What is formed by a collection of LSAs?
collection of LSAs forms the Link-State Database
How are the LSAs flooded?
flooded to all the routers, so they can build the topology diagram in their Link State Database (LSDb)
What are the first five LSA Types?
- LSA Type 1: Router LSA
- LSA Type 2: Network LSA
- LSA Type 3: Summary LSA
- LSA Type 4: Autonomous System Boundary Router (ASBR) Summary
- LSA Type 5: Autonomous System Boundary Router (ASBR) External LSA
What are the 6th - 11th LSA Types?
- LSA Type 6: OSPF Group Membership LSA
- LSA Type 7: Not-so-stubby area (NSSA) External LSA
- LSA Type 8: OSPF External Attributes LSA (OSPFv2) / Link Local LSA (OSPFv3)
- LSA Type 9: OSPF Link Scope Opaque (OSPFv2) / Intra Area Prefix LSA (OSPFv3)
- LSA Type 10: OSPF Area Scope Opaque LSA3
- LSA Type 11: OSPF AS (Autonomous System) Scope Opaque LSA
What does LSA Type 1: Router LSA describe?
the state and configuration of an OSPF router’s interfaces within a single OSPF area
Who generates and floods LSA Type 1: Router LSA?
generated and flooded by every OSPF node
How are LSA Type 1: Router LSA flooded and where do these packets stop?
flooded within an Area and stops when it reaches the Area Border Router
What is the primary function of LSA Type 1: Router LSA?
to help to construct the SPF tree
In which case is the LSA Type 2: Network LSA used?
- when there is a DR/BDR election on a segment
- network types that don’t support election of DR/BDR have no Type 2 LSAs
What does LSA Type 2: Network LSA describe?
all routers connected to an Ethernet subnet
Why is LSA Type 2: Network LSA required? (5)
- Representation of Multi-Access Networks
- DR/BDR Election
- Efficient Flooding of LSAs
- Reduction of LSDb Size
- Synchronization and Consistency
Is it true that anywhere in the topology, in the same area, routers have the same copy of the Link State Database?
yes
When are prefixes started to be advertised?
only after OSPF builds the SPF structure (routing tables are calculated) - then it extracts best path to given destination and puts it in routing table
What does LSA Header contain?
essential information about the LSA itself
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* LSA type
* LSA age
* LSA sequence number
* LSA checksum
* Router ID of the advertising router
What is the Advertising Router?
Router ID of the router that originated the LSA
What is the LSA Age?
- field that indicates the age of the LSA, which represents the time since the LSA was originated or last refreshed by the advertising router
- helps routers determine the freshness of the LSA and decide whether to accept it or not
What is the purpose of the LSA Sequence Number?
- track changes and updates to the LSA
- routers can determine whether a new LSA is more recent than the one they have in their Link-State Database (LSDB) by comparing sequence numbers
What happens to the SPF tree when a new LSA is received?
current SPF tree is destroyed and newly built (might advertise new link in the topology or change related to other links)
Why are Database Descriptor (DBD) packets exchanged between neighboring routers?
to compare the contents of their respective link-state databases and ensure that they have consistent and up-to-date information about the network topology
What are Database Descriptor (DBD) packets used for?
to allow routers to exchange summaries of their LSDBs without sending the complete database contents
Database Descriptor (DBD) packets are exchanged during which OSPF state?
Exchange
What is the content of the DBD packet?
- Interface MTU
- Options
- Sequence Number
- LSA Header(s)
What information does LSA Type 1: Router LSA provide?
- router’s own interfaces
- IP addresses
- neighboring routers to which it is directly connected within the same OSPF area
What is the content of the LSA Type 1: Router LSA? (4)
- Router ID
- Links and Interfaces for each of the router’s interfaces within the OSPF area
- Neighbor Information
- Optional Information (authentication, OSPF options)
Are Router LSAs are specific to the OSPF area in which they are originated?
yes - in multi-area OSPF networks, each OSPF area has its own set of Router LSAs that describe the routers and links within that area
What is the purpose of the Link State ID (LSA ID)?
- distinguish one LSA from another and help routers determine the specific information contained in each LSA
- it is a field within an OSPF LSA packet that uniquely identifies the LSA within the scope of the advertising router
The format and interpretation of the Link-State ID field depends on what?
type of LSA
What does the LSA ID contain in case of Router LSA (LSA Type 1)?
- the Router ID of the advertising router
- allows routers to differentiate the LSAs of different routers
What does the LSA ID contain in case of Network LSA (LSA Type 2)?
the IP address of the Designated Router (DR) for a multi-access network segment
What does the LSA ID contain in case of Summary LSA (LSA Type 3)?
the IP address of the destination network
What does the LSA ID contain in case of ASBR Summary LSA (LSA Type 4)?
the Router ID of the ASBR
What is the purpose of the ASBR Summary LSAs
to advertise the presence of Autonomous System Boundary Routers (ASBRs) and how to reach them
What is the purpose of External LSA (LSA Type 5)
advertise routes from external routing domains or other autonomous systems
What does the LSA ID contain in case of External LSA (LSA Type 5)?
external network’s destination IP address
What helps OSPF routers uniquely identify and process LSAs?
Link-State ID, in conjunction with the LSA type and the advertising router’s Router ID
Who generates the Type 2 Network LSAs?
Designated Router
Do LSA Type 1 and Type 2 always stay within nthe area they originated from?
yes, but the information they contain can be sent to other areas as a different LSA type, depending on the area type (the information they contain is very detailed so it needs to be truncated)
How does ABR generate the Type 3 Summary LSA?
- ABR takes information from LSA types 1 and 2 and converts it to a type 3 LSA that it sends to adjoining area
- the ABR summarizes the Type 1 and Type 2 LSAs into a Type 3 LSA (hence the term Summary LSA)
Which router type generates Type 5 AS External LSA?
ASBR
To which areas are Type 5 AS External LSA flooded?
to all normal areas
What does Type 5 AS External LSA describe?
external prefix and the gateway to reach it
Which router type generates Type 4 ASBR Summary
generated by an ABR that borders an area containing an ASBR
Is the LSDb the same on all OSPF routers within the same area?
yes
How many LSA types exist for OSPFv2?
7
