2. The Link State Database
-
The Link-state Database: organization and calculations
The following subsections describe the organization of OSPF's link- state database, and the routing calculations that are performed on the database in order to produce a router's routing table.
2.1. Representation of routers and networks
The Autonomous System's link-state database describes a directed
graph. The vertices of the graph consist of routers and
networks. A graph edge connects two routers when they are
attached via a physical point-to-point network. An edge
connecting a router to a network indicates that the router has
an interface on the network. Networks can be either transit or
stub networks. Transit networks are those capable of carrying
data traffic that is neither locally originated nor locally
destined. A transit network is represented by a graph vertex
having both incoming and outgoing edges. A stub network's vertex
has only incoming edges.
The neighborhood of each network node in the graph depends on
the network's type (point-to-point, broadcast, NBMA or Point-
to-MultiPoint) and the number of routers having an interface to
the network. Three cases are depicted in Figure 1a. Rectangles
indicate routers. Circles and oblongs indicate networks.
Router names are prefixed with the letters RT and network names
with the letter N. Router interface names are prefixed by the
letter I. Lines between routers indicate point-to-point
networks. The left side of the figure shows networks with their
connected routers, with the resulting graphs shown on the right.
**FROM**
* |RT1|RT2|
+---+Ia +---+ * ------------
|RT1|------|RT2| T RT1| | X |
+---+ Ib+---+ O RT2| X | |
* Ia| | X |
* Ib| X | |
Physical point-to-point networks
**FROM**
+---+ *
|RT7| * |RT7| N3|
+---+ T ------------
| O RT7| | |
+----------------------+ * N3| X | |
N3 *
Stub networks
**FROM**
+---+ +---+
|RT3| |RT4| |RT3|RT4|RT5|RT6|N2 |
+---+ +---+ * ------------------------
| N2 | * RT3| | | | | X |
+----------------------+ T RT4| | | | | X |
| | O RT5| | | | | X |
+---+ +---+ * RT6| | | | | X |
|RT5| |RT6| * N2| X | X | X | X | |
+---+ +---+
Broadcast or NBMA networks
Figure 1a: Network map components
Networks and routers are represented by vertices.
An edge connects Vertex A to Vertex B iff the
intersection of Column A and Row B is marked with
an X.
The top of Figure 1a shows two routers connected by a point-to-
point link. In the resulting link-state database graph, the two
router vertices are directly connected by a pair of edges, one
in each direction. Interfaces to point-to-point networks need
not be assigned IP addresses. When interface addresses are
assigned, they are modelled as stub links, with each router
advertising a stub connection to the other router's interface
address. Optionally, an IP subnet can be assigned to the point-
to-point network. In this case, both routers advertise a stub
link to the IP subnet, instead of advertising each others' IP
interface addresses.
The middle of Figure 1a shows a network with only one attached
router (i.e., a stub network). In this case, the network appears
on the end of a stub connection in the link-state database's
graph.
When multiple routers are attached to a broadcast network, the
link-state database graph shows all routers bidirectionally
connected to the network vertex. This is pictured at the bottom
of Figure 1a.
Each network (stub or transit) in the graph has an IP address
and associated network mask. The mask indicates the number of
nodes on the network. Hosts attached directly to routers
(referred to as host routes) appear on the graph as stub
networks. The network mask for a host route is always
0xffffffff, which indicates the presence of a single node.
2.1.1. Representation of non-broadcast networks
As mentioned previously, OSPF can run over non-broadcast
networks in one of two modes: NBMA or Point-to-MultiPoint.
The choice of mode determines the way that the Hello
protocol and flooding work over the non-broadcast network,
and the way that the network is represented in the link-
state database.
In NBMA mode, OSPF emulates operation over a broadcast
network: a Designated Router is elected for the NBMA
network, and the Designated Router originates an LSA for the
network. The graph representation for broadcast networks and
NBMA networks is identical. This representation is pictured
in the middle of Figure 1a.
NBMA mode is the most efficient way to run OSPF over non-
broadcast networks, both in terms of link-state database
size and in terms of the amount of routing protocol traffic.
However, it has one significant restriction: it requires all
routers attached to the NBMA network to be able to
communicate directly. This restriction may be met on some
non-broadcast networks, such as an ATM subnet utilizing
SVCs. But it is often not met on other non-broadcast
networks, such as PVC-only Frame Relay networks. On non-
broadcast networks where not all routers can communicate
directly you can break the non-broadcast network into
logical subnets, with the routers on each subnet being able
to communicate directly, and then run each separate subnet
as an NBMA network (see [Ref15]). This however requires
quite a bit of administrative overhead, and is prone to
misconfiguration. It is probably better to run such a non-
broadcast network in Point-to-Multipoint mode.
In Point-to-MultiPoint mode, OSPF treats all router-to-
router connections over the non-broadcast network as if they
were point-to-point links. No Designated Router is elected
for the network, nor is there an LSA generated for the
network. In fact, a vertex for the Point-to-MultiPoint
network does not appear in the graph of the link-state
database.
Figure 1b illustrates the link-state database representation
of a Point-to-MultiPoint network. On the left side of the
figure, a Point-to-MultiPoint network is pictured. It is
assumed that all routers can communicate directly, except
for routers RT4 and RT5. I3 though I6 indicate the routers'
IP interface addresses on the Point-to-MultiPoint network.
In the graphical representation of the link-state database,
routers that can communicate directly over the Point-to-
MultiPoint network are joined by bidirectional edges, and
each router also has a stub connection to its own IP
interface address (which is in contrast to the
representation of real point-to-point links; see Figure 1a).
On some non-broadcast networks, use of Point-to-MultiPoint
mode and data-link protocols such as Inverse ARP (see
[Ref14]) will allow autodiscovery of OSPF neighbors even
though broadcast support is not available.
**FROM**
+---+ +---+
|RT3| |RT4| |RT3|RT4|RT5|RT6|
+---+ +---+ * --------------------
I3| N2 |I4 * RT3| | X | X | X |
+----------------------+ T RT4| X | | | X |
I5| |I6 O RT5| X | | | X |
+---+ +---+ * RT6| X | X | X | |
|RT5| |RT6| * I3| X | | | |
+---+ +---+ I4| | X | | |
I5| | | X | |
I6| | | | X |
Figure 1b: Network map components
Point-to-MultiPoint networks
All routers can communicate directly over N2, except
routers RT4 and RT5. I3 through I6 indicate IP
interface addresses
2.1.2. An example link-state database
Figure 2 shows a sample map of an Autonomous System. The
rectangle labelled H1 indicates a host, which has a SLIP
connection to Router RT12. Router RT12 is therefore
advertising a host route. Lines between routers indicate
physical point-to-point networks. The only point-to-point
network that has been assigned interface addresses is the
one joining Routers RT6 and RT10. Routers RT5 and RT7 have
BGP connections to other Autonomous Systems. A set of BGP-
learned routes have been displayed for both of these
routers.
A cost is associated with the output side of each router
interface. This cost is configurable by the system
administrator. The lower the cost, the more likely the
interface is to be used to forward data traffic. Costs are
also associated with the externally derived routing data
(e.g., the BGP-learned routes).
The directed graph resulting from the map in Figure 2 is
depicted in Figure 3. Arcs are labelled with the cost of
the corresponding router output interface. Arcs having no
labelled cost have a cost of 0. Note that arcs leading from
networks to routers always have cost 0; they are significant
nonetheless. Note also that the externally derived routing
data appears on the graph as stubs.
The link-state database is pieced together from LSAs
generated by the routers. In the associated graphical
representation, the neighborhood of each router or transit
network is represented in a single, separate LSA. Figure 4
shows these LSAs graphically. Router RT12 has an interface
to two broadcast networks and a SLIP line to a host.
Network N6 is a broadcast network with three attached
routers. The cost of all links from Network N6 to its
attached routers is 0. Note that the LSA for Network N6 is
actually generated by one of the network's attached routers:
the router that has been elected Designated Router for the
network.
+
| 3+---+ N12 N14
N1|--|RT1|\ 1 \ N13 /
| +---+ \ 8\ |8/8
+ \ ____ \|/
/ \ 1+---+8 8+---+6
* N3 *---|RT4|------|RT5|--------+
\____/ +---+ +---+ |
+ / | |7 |
| 3+---+ / | | |
N2|--|RT2|/1 |1 |6 |
| +---+ +---+8 6+---+ |
+ |RT3|--------------|RT6| |
+---+ +---+ |
|2 Ia|7 |
| | |
+---------+ | |
N4 | |
| |
| |
N11 | |
+---------+ | |
| | | N12
|3 | |6 2/
+---+ | +---+/
|RT9| | |RT7|---N15
+---+ | +---+ 9
|1 + | |1
_|__ | Ib|5 __|_
/ \ 1+----+2 | 3+----+1 / \
* N9 *------|RT11|----|---|RT10|---* N6 *
\____/ +----+ | +----+ \____/
| | |
|1 + |1
+--+ 10+----+ N8 +---+
|H1|-----|RT12| |RT8|
+--+SLIP +----+ +---+
|2 |4
| |
+---------+ +--------+
N10 N7
Figure 2: A sample Autonomous System
**FROM**
|RT|RT|RT|RT|RT|RT|RT|RT|RT|RT|RT|RT|
|1 |2 |3 |4 |5 |6 |7 |8 |9 |10|11|12|N3|N6|N8|N9|
----- ---------------------------------------------
RT1| | | | | | | | | | | | |0 | | | |
RT2| | | | | | | | | | | | |0 | | | |
RT3| | | | | |6 | | | | | | |0 | | | |
RT4| | | | |8 | | | | | | | |0 | | | |
RT5| | | |8 | |6 |6 | | | | | | | | | |
RT6| | |8 | |7 | | | | |5 | | | | | | |
RT7| | | | |6 | | | | | | | | |0 | | |
* RT8| | | | | | | | | | | | | |0 | | |
* RT9| | | | | | | | | | | | | | | |0 |
T RT10| | | | | |7 | | | | | | | |0 |0 | |
O RT11| | | | | | | | | | | | | | |0 |0 |
* RT12| | | | | | | | | | | | | | | |0 |
* N1|3 | | | | | | | | | | | | | | | |
N2| |3 | | | | | | | | | | | | | | |
N3|1 |1 |1 |1 | | | | | | | | | | | | |
N4| | |2 | | | | | | | | | | | | | |
N6| | | | | | |1 |1 | |1 | | | | | | |
N7| | | | | | | |4 | | | | | | | | |
N8| | | | | | | | | |3 |2 | | | | | |
N9| | | | | | | | |1 | |1 |1 | | | | |
N10| | | | | | | | | | | |2 | | | | |
N11| | | | | | | | |3 | | | | | | | |
N12| | | | |8 | |2 | | | | | | | | | |
N13| | | | |8 | | | | | | | | | | | |
N14| | | | |8 | | | | | | | | | | | |
N15| | | | | | |9 | | | | | | | | | |
H1| | | | | | | | | | | |10| | | | |
Figure 3: The resulting directed graph
Networks and routers are represented by vertices.
An edge of cost X connects Vertex A to Vertex B iff
the intersection of Column A and Row B is marked
with an X.
**FROM** **FROM**
|RT12|N9|N10|H1| |RT9|RT11|RT12|N9|
* -------------------- * ----------------------
* RT12| | | | | * RT9| | | |0 |
T N9|1 | | | | T RT11| | | |0 |
O N10|2 | | | | O RT12| | | |0 |
* H1|10 | | | | * N9| | | | |
* *
RT12's router-LSA N9's network-LSA
Figure 4: Individual link state components
Networks and routers are represented by vertices.
An edge of cost X connects Vertex A to Vertex B iff
the intersection of Column A and Row B is marked
with an X.2.2. The shortest-path tree
When no OSPF areas are configured, each router in the Autonomous
System has an identical link-state database, leading to an
identical graphical representation. A router generates its
routing table from this graph by calculating a tree of shortest
paths with the router itself as root. Obviously, the shortest-
path tree depends on the router doing the calculation. The
shortest-path tree for Router RT6 in our example is depicted in
Figure 5.
The tree gives the entire path to any destination network or
host. However, only the next hop to the destination is used in
the forwarding process. Note also that the best route to any
router has also been calculated. For the processing of external
data, we note the next hop and distance to any router
advertising external routes. The resulting routing table for
Router RT6 is pictured in Table 2. Note that there is a
separate route for each end of a numbered point-to-point network
(in this case, the serial line between Routers RT6 and RT10).
Routes to networks belonging to other AS'es (such as N12) appear
as dashed lines on the shortest path tree in Figure 5. Use of
RT6(origin)
RT5 o------------o-----------o Ib
/|\ 6 |\ 7
8/8|8\ | \
/ | \ 6| \
o | o | \7
N12 o N14 | \
N13 2 | \
N4 o-----o RT3 \
/ \ 5
1/ RT10 o-------o Ia
/ |\
RT4 o-----o N3 3| \1
/| | \ N6 RT7
/ | N8 o o---------o
/ | | | /|
RT2 o o RT1 | | 2/ |9
/ | | |RT8 / |
/3 |3 RT11 o o o o
/ | | | N12 N15
N2 o o N1 1| |4
| |
N9 o o N7
/|
/ |
N11 RT9 / |RT12
o--------o-------o o--------o H1
3 | 10
|2
|
o N10
Figure 5: The SPF tree for Router RT6
Edges that are not marked with a cost have a cost of
of zero (these are network-to-router links). Routes
to networks N12-N15 are external information that is
considered in Section 2.3
Destination Next Hop Distance
__________________________________
N1 RT3 10
N2 RT3 10
N3 RT3 7
N4 RT3 8
Ib * 7
Ia RT10 12
N6 RT10 8
N7 RT10 12
N8 RT10 10
N9 RT10 11
N10 RT10 13
N11 RT10 14
H1 RT10 21
__________________________________
RT5 RT5 6
RT7 RT10 8Table 2: The portion of Router RT6's routing table listing local destinations.
this externally derived routing information is considered in the
next section.2.3. Use of external routing information
After the tree is created the external routing information is
examined. This external routing information may originate from
another routing protocol such as BGP, or be statically
configured (static routes). Default routes can also be included
as part of the Autonomous System's external routing information.
External routing information is flooded unaltered throughout the
AS. In our example, all the routers in the Autonomous System
know that Router RT7 has two external routes, with metrics 2 and
9.
OSPF supports two types of external metrics. Type 1 external
metrics are expressed in the same units as OSPF interface cost
(i.e., in terms of the link state metric). Type 2 external
metrics are an order of magnitude larger; any Type 2 metric is
considered greater than the cost of any path internal to the AS.
Use of Type 2 external metrics assumes that routing between
AS'es is the major cost of routing a packet, and eliminates the
need for conversion of external costs to internal link state
metrics.
As an example of Type 1 external metric processing, suppose that
the Routers RT7 and RT5 in Figure 2 are advertising Type 1
external metrics. For each advertised external route, the total
cost from Router RT6 is calculated as the sum of the external
route's advertised cost and the distance from Router RT6 to the
advertising router. When two routers are advertising the same
external destination, RT6 picks the advertising router providing
the minimum total cost. RT6 then sets the next hop to the
external destination equal to the next hop that would be used
when routing packets to the chosen advertising router.
In Figure 2, both Router RT5 and RT7 are advertising an external
route to destination Network N12. Router RT7 is preferred since
it is advertising N12 at a distance of 10 (8+2) to Router RT6,
which is better than Router RT5's 14 (6+8). Table 3 shows the
entries that are added to the routing table when external routes
are examined:
Destination Next Hop Distance
__________________________________
N12 RT10 10
N13 RT5 14
N14 RT5 14
N15 RT10 17
Table 3: The portion of Router RT6's routing table
listing external destinations.
Processing of Type 2 external metrics is simpler. The AS
boundary router advertising the smallest external metric is
chosen, regardless of the internal distance to the AS boundary
router. Suppose in our example both Router RT5 and Router RT7
were advertising Type 2 external routes. Then all traffic
destined for Network N12 would be forwarded to Router RT7, since
2 < 8. When several equal-cost Type 2 routes exist, the
internal distance to the advertising routers is used to break
the tie.
Both Type 1 and Type 2 external metrics can be present in the AS
at the same time. In that event, Type 1 external metrics always
take precedence.
This section has assumed that packets destined for external
destinations are always routed through the advertising AS
boundary router. This is not always desirable. For example,
suppose in Figure 2 there is an additional router attached to
Network N6, called Router RTX. Suppose further that RTX does
not participate in OSPF routing, but does exchange BGP
information with the AS boundary router RT7. Then, Router RT7
would end up advertising OSPF external routes for all
destinations that should be routed to RTX. An extra hop will
sometimes be introduced if packets for these destinations need
always be routed first to Router RT7 (the advertising router).
To deal with this situation, the OSPF protocol allows an AS
boundary router to specify a "forwarding address" in its AS-
external-LSAs. In the above example, Router RT7 would specify
RTX's IP address as the "forwarding address" for all those
destinations whose packets should be routed directly to RTX.
The "forwarding address" has one other application. It enables
routers in the Autonomous System's interior to function as
"route servers". For example, in Figure 2 the router RT6 could
become a route server, gaining external routing information
through a combination of static configuration and external
routing protocols. RT6 would then start advertising itself as
an AS boundary router, and would originate a collection of OSPF
AS-external-LSAs. In each AS-external-LSA, Router RT6 would
specify the correct Autonomous System exit point to use for the
destination through appropriate setting of the LSA's "forwarding
address" field.2.4. Equal-cost multipath
The above discussion has been simplified by considering only a
single route to any destination. In reality, if multiple
equal-cost routes to a destination exist, they are all
discovered and used. This requires no conceptual changes to the
algorithm, and its discussion is postponed until we consider the
tree-building process in more detail.
With equal cost multipath, a router potentially has several
available next hops towards any given destination.