Chapter 7 661
4
What are link-state advertisements? Name four types of link-state advertisements.
Link State Advertisements (LSAs) advertise the state of the link to other OSPF routers.
Router LSA--Type 1 LSAs contain information about router links, interfaces, state of
links, and cost.
Network LSA--Type 2 LSAs contain lists of routers connected to a multiaccess network
segment.
Summary LSA--Type 3 LSAs originate at area border routers and are sent into an area to
advertise networks that have been summarized into a single route.
Summary LSA (ASBR)--Type 4 LSAs originate as area system border routers and are
sent to networks that have been summarized into a single route to the OSPF network to
advertise the ASBR.
AS-external LSA--Type 5 LSAs contain information that describes destinations external
to the OSPF AS.
5
True or false: All OSPF areas must be physically adjacent to the backbone.
False. Virtual links can connect areas that are not physically attached to the backbone.
6
The following are OSPF routes. Perform summarization to one route. What configuration
commands would be needed if the routes were external? What commands would be
needed if the routes were internal?
Route 1 ________________________________________________________________
172.26.30.0
Route 2 ________________________________________________________________
172.26.31.0
Route 3 ________________________________________________________________
172.26.32.0
External routes: summary address 172.26.0.0 255.255.192.0
Internal routes: area 1 range 172.26.0.0 255.255.192.0
7
Compare and contrast stub, totally stubby, and not-so-stubby areas.
In a stub area, a default route summarizes all external routes. Stub areas are similar to
regular areas, except that the routers do not enter external routes in the area's databases.
Type 3 LSAs are still flooded into the area.
A totally stubby area only allows the default summary link to be propagated into the area
by the ARB. Type 3 LSAs are not flooded into the area.
Not-so-stubby areas are similar to stub areas, except that they allow limited importing of
external routes.
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