Chap 9 - Advanced OSPF Flashcards
What is an Area Border Router (ABR)
- It has interfaces in area 0 and another area.
- ABRs take routes from one area and advertise them into another area
- Every ABR needs to participate in Area 0.
When advertising an area ID what format does OSPF use?
Dotted decimal format (Looks like an IP address)
What are routes to subnets in the same area?
Intra-Area Routes
What are routes to subnets in a different area?
Inter-Area Routes
How are Intra-Area routes displayed in a routing table?
O
How are Inter-Area routes displayed in a routing table?
O IA
What are routes called that are a different OSPF domain or routes learned from a different routing protocol?
External Routes
What are the 6 LSA types called?
- Type 1 - Router LSA
- Type 2 - Network LSA
- Type 3 - Summary LSA
- Type 4 - ASBR Summary LSA
- Type 5 - AS External LSA
- Type 7 - NSSA External LSA
Which LSA types are used for building the SPF Tree for Inter and Intra-area routes?
Types 1, 2, and 3
When does a router send out a new LSA?
When the last LSA was sent 30 minutes ago.
When does a router purge an LSA?
When its age exceeds 60 minutes.
What is a Type 1 LSA?
- Router LSA
- Sent by every router
- Not advertised outside its area
- exists for every interface and attached network
Why is the underlying topology of an area invisible to other areas?
Because Type 1 Router LSAs are not advertised outside its area.
What is a Type 2 LSA?
- Network LSA
- Sent by the DR
- Represents a multi-access network
- Identifies all routers attached to the network segment
- Not forwarded outside the area
If a DR hasn’t been elected yet on a multi-access network will there be a Type 2 LSA present?
No
What is a pseudo-node?
Represents the DR on a mult-access network.
What is a Type 3 LSA?
- Summary LSA
- Sent by ABR to Area 0
- Represents networks from other areas
When does an ABR create a Type 3 LSA?
- When it receives Type 1 LSAs from Non-Backbone Area to send to Backbone Area
- When it receives a Type 1 LSA from the Backbone Area to send to Non-Backbone Area
- When it receives a Type 3 LSA from Area 0 it creates a new Type 3 LSA to send to the Non-Backbone area
What does an ABR use a received Type 2 LSA for?
It uses the Type 2 LSA to obtain the network mask of the multi-access network.
What does an ABR do when it receives a Type 3 LSA from the backbone area?
- Creates a New Type 3 LSA for the Non-Backbone Area
- Lists itself as the Sender
- Adds the additional Cost metric
What Metric is advertised by an ABR when it creates a Type 3 LSA from a Type 1 LSA?
It advertises the Total Path Metric to reach the originating router.
What Metric is advertised by an ABR when it creates a Type 3 LSA from a Type 3 LSA?
It advertises the Total Path Metric to reach the Advertising ABR plus the metric contained in the original Type 3 LSA.
What does a Type 3 LSA contain?
- Link-state ID (IP address of interface advertising the subnet)
- subnet mask
- IP address of Advertising ABR
- Metric for the network prefix
If an ABR is aware of multiple paths to a network how many Type 3 LSAs are created?
Only 1 Type 3 LSA per prefix using the best metric out of all the paths.
What are 3 fundamental rules used by ABRs when creating Type 3 LSAs?
- Type 1 LSAs create Type 3 LSAs into the backbone and non-backbone areas
- Type 3 LSAs from the backbone create Type 3 LSAs for the non-backbone area
- Type 3 LSAs from a non-backbone area only insert into the LSDB of the Source Area.
In order of priority how are routes selected?
- Choose the Intra-area route first
- Second choose the Inter-area route
- Choose an External route last
What happens if there is are two paths to a destination - one is inter-area and the other is intra-area?
Intra-area routes from a Type 1 LSA are always preferred over routes from a Type 3 LSA even if the Inter-area route has a better metric.
What happens if there is a tie in metrics between 2 Inter-Area routes?
They both go into the RIB.
What is ECMP?
Equal Cost Multi-pathing.
What is the default for maximum paths?
4
If summarization is configured by default what does the ABR assign as a metric to a Type 3 LSA?
The best metric out of all the Type 1 LSAs routes being summarized.
When summarization is configured what is the default behavior - to Advertise or not to Advertise?
Default is to Advertise.
When summarization is configured what does the ABR do?
Creates a Discard Route to Null0 for the summary.
What is the Administrative Distance for a summary route that shows up as an External Route?
254
What are 3 methods of route filtering?
- Filtering with Summarization
- Area Filtering
- Local OSPF Filtering
What happens when an LSA Age reaches 30 minutes (1800 seconds)?
The originating router sends out a new one.
What happens when an LSA Age reaches 60 minutes (3600 seconds)?
The receiving router deems the LSA to be invalid and purges it from its LSDB
What happens when each router forwards an LSA?
The LSA Age is incremented with a calculated minimal delay that reflects that of the link.
Display the Type 3 Summary LSA.
show ip ospf database summary
