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for each area it belongs to. They describe the
states of the router’s link to the area. These are
only flooded within a particular area.
Network Link advertisements. Generated by Designated
Routers. They describe the set of routers attached
to a particular network. Flooded in the area that
contains the network.
Summary Link advertisements. Generated by Area Bor-
der routers. They describe inter-area (between
areas) routes. Type 3 describes routes to networks,
also used for aggregating routes. Type 4 describes
routes to ASBR.
AS external link advertisements. Originated by ASBR.
They describe routes to destinations external to the
AS. Flooded all over except stub areas.
If you look at the OSPF database in detail, using sh ip ospf d d, you
will see different keywords such as Link-Data, Link-ID, and Link-state
ID. These terms become confusing as the value of each depends on the link
state type and the link-type. We will go over this terminology and will
provide a detailed example on the OSPF database as seen from the router.
The Link-State ID basically defines the identity of the link-state
depending on the LS type. Router Links are identified by the router ID
(RID) of the router that originated the advertisement. Network Links are
identified by the relative IP address of the DR. This makes sense because
Network Links are originated by the Designated Router. Summary Links
(type 3)are identified by the IP network numbers of the destinations they
are pointing at. ASBR Summary Links (Summary Links type 4) are identified
by the RID of the ASBR. Finally, External Links are identified by the IP
network numbers of the external destinations they are pointing at. The
following table summarizes this information:
OSPF DESIGN GUIDE-NSA group
April 25, 1996
52
Link State ID
LS type (In the high level view of the database when
referencing a router this is called Link ID)
1
2
3
4
5
The originating Router’s Router ID (RID).
The IP interface address of the network’s Designated
Router.
The destination network number.
The router ID of the described AS boundary router.
The external network number.
Next, we will describe the different links available:
Stub network links: This term has nothing to do with Stub areas. A stub
segment is a segment that has one router only attached to it. An Ethernet
or Token Ring segment that has one attached router is considered a link
to a stub network. A loopback interface is also considered a link to stub
network with a 255.255.255.255 mask (Host route).
Point-to-point links: These could be physical or logical (subinterfaces)
point-to-point serial link connections. These links could be numbered
(an IP address is configured on the link) or unnumbered.
Transit links: These are interfaces connected to networks that have more
than one router attached, hence the name transit.
Virtual links: These are logical links that connect areas that do not
have physical connections to the backbone. Virtual links are treated as
numbered point-to-point links.
The link-ID is an identification of the link itself. This is different
for each link type. A transit link is identified by the IP address of the
DR on that link. A numbered point-to-point link is identified by the RID
of the neighbor router on the point-to-point link. Virtual links are
identical to point-to-point links. Finally, links to stub networks are
identified by the IP address of the interface to the stub network. The
following table summarizes this information:
Link ID
(This applies to individual Links)
Link type
Point-to-Point
Neighbor Router ID
Link to transit network
Interface address of DR
OSPF DESIGN GUIDE-NSA group
April 25, 1996
53
Link type
Link to stub network
(In case of loopback mask is
255.255.255.255)
Virtual Link
Link ID
(This applies to individual Links)
Network/subnet number
Neighbor Router ID
The Link Data is the IP address of the link, except for stub network were
the link data is the network mask.
Link type
Link Data
Stub network
Network Mask
Other networks (applies to
Router’s associated IP inter-
router links only)
face address
Finally, an Advertising Router is the RID of the router that has sent the
LSA.
19.2 OSPF database example
203.250.15.0
255.255.255.192
RTC
Area 1
S1
15.1
15.2
RIP
16.130
E0
OSPF
E0
S0
E0
RTE
Given the above topology, and the following configs, and the IP route
tables, let us look at different ways of understanding the OSPF database.
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