Answer / jm

LSA = link state
Following are the types of LSAs you would usually find in an
OSPF network:
Type 1: router LSA -- A router sends this to describe
neighbors and its own interfaces.
Type 2: network LSA -- For broadcast networks only; this LSA
is flooded by the DR and lists OSPF-speaking routers on the
network.
Type 3: network summary LSA -- Sent by an ASBR to advertise
networks reachable through it. A stub area router will also
use this for the default route.
Type 4: ASBR-summary LSA -- Sent by ASBR, but only
internally. This describes to the others how to get to the
ASBR itself, and uses only internal metrics.
Type 5: AS-external LSA -- Used to describe external routes
to internal areas. Can be used to advertise “this is the way
to the Internet” (or some subset of).
Type 6: Group summary -- Used in multicast (MOSPF). Ignore
this.
Type 7: NSSA area import.
Type 8: External Attributes LSA –- This is very rarely
deployed and is used when BGP information is carried across
OSPF AS.
Type 9, 10, 11 are Opaque LSAs -- They are designated for
future specifically for application specific purpos

Answer / jitendera sinha

lSA types
OSPF routers originate LSAs to describe the local
environment. All LSAs, taken together, form the link state
database.
Type 1: Router-LSA

Reports router's active interfaces, IP addresses, and neighbors.
One such LSA per router.
Distributed using reliable flooding: if a new, different LSA
is received, the router acks it, then sends it out all of
its other interfaces.

Type 2: Network-LSA

On broadcast subnets, for all routers to form adjacencies
with all other routers would require n*(n-1) links.
Instead, each router has a link to the subnet's Network-LSA
(and vice versa).
The designated router (DR) originates the network-LSA on
behalf of the subnet.
This LSA has a list of all of the attached routers.
On NBMA networks, all routers need to know (by
configuration?) the DR and BDR, not the others. They must be
able to reach the others (full mesh).
(On point-to-multipoint networks, a full mesh is not
required. There is no DR, and hence no Type 2 LSAs; only
Type 1 LSAs are used.)

Type 3: Summary-LSA

* Generated by ABR to leak one area's summary address
info into another area
* ABR will generate more than one summary-LSA for an
area if that area's addresses cannot be properly aggregated
by only one prefix.
* Reduces the size of the link state database
* Constrains flooding to an area
* Makes areas somewhat insensitive to link or router
failures in another area.
* Since OSPF prefers intra-area paths, duplicate routes
in another area won't affect this one.

Type 5: AS-external-LSA

* Unlike all other LSAs, these have "AS flooding scope",
not"area flooding scope". In other works, they are flooded
unmodified across area boundaries.
* Originated by ASBRs
* Advertises a single prefix, learned from another
routing protocol.
* Coming from another internal protocol, usually use
Type 1 metrics.
* Coming from an external source, usually use Type 2
metrics.
* Flooded across area boundaries, instead of being
re-originated by ABRs.

Type 4: ASBR-summary-LSA

* Originated by ABRs when an ASBR is present to let
other areas know where the ASBR is.
* Handled just like summary-LSAs.

Type 7: Type-7-LSA

* Used only by NSSAs, which are stub areas that contain
one or more ASBRs.
* Type 7s are originated by ASBRs (only if area is
configured to be a NSSA).
* ABRs hear the Type 7s, then change them into Type 5s
before forwarding them.
* Identical in format to type 5s, except that they have
only area flooding scope.
* Can have a N/P (do not propagate) bit set.

Type 8: external-attributes-LSA

* Never been implemented or deployed.
* Used when BGP info is carried across OSPF AS.
* Only needed when the BGP AS-path does not fit into the
available 4 bytes in the AS-external-LSA "Tag" field.
* If used, the associated Type 5's tag field is set
equal to the link state ID of the Type 8.
* Allows OSPF to be used instead of IBGP, and still
preserves the AS-path attribute. Advantages of OSPF over IBGP:
o IBGP requires n2 mesh
o With OSPF, each BGP route traverses a subnet
only once, using the OSPF flooding mechanism
o Some OSPF routers would be required to store BGP
info in the form of Type 8s, requiring more memory.

Special area types
Stub areas

* Design goal: allow the use of routers with limited
memory and processor.
* No AS-external-LSAs (5).
* No summary LSAs (3,4).

NSSA (not-so-stubby-area)

* Allows stub area to provide transit for small sub
routing domains that are not OSPF (such as a small RIP network.
* Still protects internal routers from other area's
external-LSAs.
* All routers in an area must agree that it is an NSSA,
so that there are no AS-external-LSAs. Therefore, N/P bit is
also part of the Hello packet.

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2 Answers  

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