Cisco – OSPF

Describe the use of Type 1 LSAs.
Type 1 LSAs are used for floods to all other routers in the area. Each type 1 LSA is identified by the originating router’s ID in the link-state ID field. In addition, the type 1 LSA indicates the OSPF cost for each link, and whether the router is an ABR or ASBR.
What are the different link types described in a type 1 LSA?

– Link type 1 = Point-to-pont connection to another router, whose link ID is the neighbor router ID

;

– Link type 2 = Connection to a transit network, whose link ID is the DR’s interface address

 

– Link type 3 – Connection to a stub network, whose link ID is the IP network/subnet number

 

– Link type 4 = Virtual link, whose link ID is the neighbor router ID. 

Describe how type 2 LSAs are used in an OSPF network.
A type 2 LSA is generated for every transit broadcast or nonbroadcast multiaccess (NBMA) network within an area. The area’s DR is responsible for advertising the network LSA. The link-state ID for a network LSA is the IP interface address of the DR that advertises it.
Describe the use of type 3 LSAs in an OSPF network.
Type 3 LSAs are summary LSAs and are sent by ABRs. They advertise any networks owned by an area to the rest of the areas in the OSPF AS. OSPF doesn’t summarize by default, so a summary LSA contains every subnet defined in the originating area.
Describe the use of type 4 LSAs in an OSPF network.
A type 4 summary LSA is used only when an ASBR exists within an area. It identifies the ASBR and provides a router to it. When an ABR receives a type 1 LSA from an ASBR, it sends out a type 4 summary LSA to advertise the presence of the ASBR to other areas.
Describe how type 5 LSAs are used in an OSPF network.
Type 5 external LSAs describe routers to networks outside the OSPF AS. They are originated by the ASBR and are flooded to the entire AS.
What command is used to configure OSPF LSDB overload protection?
max-lsa max-number [threshold-percentage] [warning-only] [ignore-time minutes] [ignore-count number] [reset-time minutes]
What command is used to change the formula that is used to calculate the OSPF cost of an interface?
auto-cost reference-bandwidth reference-bandwidth
By default, how are summary routes summarized in summary LSAs (type3)?
Summary LSAs (type 3) and external LSAs (type 5) by default do not contain summarized routes. By default, summary LSAs are not summarized.
Describe the two different types of summarizations used in OSPF.

– Interarea route summarization – Interarea route summarization occurs on ABRs and applies to routes from within each area.

 

– External route summarization – External route summarization is specific to external routes that are injected into OSPF via route redistribution. 

What command is used to create summarized routes on OSPF ABRs?
area area-id range address mask [advertise | not-advertise] [cost cost]
What command is used to create summarized routes on OSPF ASBRs?
summary-address ip-address mask [not-advertise] [tag tag]
What command is used to generate a default route into an OSPF routing domain?
default-information originate [always] [metric metric-value] [metric-type type-value] [route-map map-name]
When using the default-information originate command, what is the default metric when none is specified?
Cisco documentation says that the default metric is 10, but actual tests show thatthe default metric is 1.
Describe a stub area.
Stub areas do not accept information about routes external to the AS. If they need to route to networks outside the AS, they use a default route.
Describe a totally subby area.
A totally stubby area does not accept external AS routes or summary routes from other areas internal to the AS. If a router needs to send a packet to a network external to the area, it sends the packet using a default route.
Describe a not-so-stubby area.
A not-so-stubby area offers benefits that are similar to those of a stub or totally stub area, however, they allow ASBRs, using a special LSA type 7
What characteristics are required for an area to qualify as a stub or totally stub area?

– There is a single exit form that area

 

– All OSPF routers inside the stub area, including the ABR and internal routers, are configured as stub routers

 

– The area is not needed as a transit area for virtual links

 

– No ASBR is inside the stub area

 

– The area is not the backbone area 

What command is used to define an area as a stub area?
area area-id stub
What command is used to change the default cost that is advertised to a stub or totally stubby area?
area area-id default-cost cost
What command is used to prevent an ABR from advertising summary LSAs into a stub area, making it a totally stubby area?
area area-id stub no-summary
What command is used to configure a NSSA in OSPF?
area area-id nssa [no-redistribution] [default-information-originate] [metric value] [metric-type type] [no-summary]
What command is used to display details of LSAs?
show ip ospf database
In what types of areas are default routes generated?
For stub and totally stubby areas, the ABR generates a summary LSA with the link-state ID 0.0.0.0. This is true even is the ABR does not have a default route.
What command is used to build and configure OSPF virtual links?
area area-id virtual-link neighbor-id
What command is used to verify OSPF virtual link operation?
show ip ospf virtual-links
What command is used to assign a password to be used when using OSPF simple password authentication with neighboring routers?
ip ospf authentication-key password
What command is used to specify the authentication type used with OSPF?
ip ospf authentication [message-digest | null]
What command is used to enable OSPF authentication for an entire area?
area area-id authentication [message-digest]
What command is used to create a key ID and key to be used for OSPF authentication?
ip ospf message-digest-key key-id md5 key
What happens when more that one key is configured for OSPF authentication?
If an interface is configured with a new key, the router will send multiple copies of the same packet, each authenticated by different keys. The router will stop sending duplicate packets when it detects that all of its neighbors have adopted the new key.