RFC 1583
OSPF Version 2
10.2. Events causing neighbor state changes State changes can be effected by a number of events. These events are shown in the labels of the arcs in Figures 12 and 13. The label definitions are as follows: HelloReceived A Hello packet has been received from a neighbor. Start This is an indication that Hello Packets should now be sent to the neighbor at intervals of HelloInterval seconds. This event is generated only for neighbors associated with non- broadcast networks. 2-WayReceived Bidirectional communication has been realized between the two neighboring routers. This is indicated by this router seeing itself in the other's Hello packet. NegotiationDone The Master/Slave relationship has been negotiated, and DD sequence numbers have been exchanged. This signals the start of the sending/receiving of Database Description packets. For more information on the generation of this event, consult Section 10.8. ExchangeDone Both routers have successfully transmitted a full sequence of Database Description packets. Each router now knows what parts of its link state database are out of date. For more information on the generation of this event, consult Section
10.8.
BadLSReq
A Link State Request has been received for a link state
advertisement not contained in the database. This indicates
an error in the Database Exchange process.
Loading Done
Link State Updates have been received for all out-of-date
portions of the database. This is indicated by the Link
state request list becoming empty after the Database
Exchange process has completed.
AdjOK?
A decision must be made (again) as to whether an adjacency
should be established/maintained with the neighbor. This
event will start some adjacencies forming, and destroy
others.
The following events cause well developed neighbors to revert to
lesser states. Unlike the above events, these events may occur
when the neighbor conversation is in any of a number of states.
SeqNumberMismatch
A Database Description packet has been received that either
a) has an unexpected DD sequence number, b) unexpectedly has
the Init bit set or c) has an Options field differing from
the last Options field received in a Database Description
packet. Any of these conditions indicate that some error
has occurred during adjacency establishment.
1-Way
An Hello packet has been received from the neighbor, in
which this router is not mentioned. This indicates that
communication with the neighbor is not bidirectional.
KillNbr
This is an indication that all communication with the
neighbor is now impossible, forcing the neighbor to
revert to Down state.
InactivityTimer
The inactivity Timer has fired. This means that no Hello
packets have been seen recently from the neighbor. The
neighbor reverts to Down state.
LLDown
This is an indication from the lower level protocols that
the neighbor is now unreachable. For example, on an X.25
network this could be indicated by an X.25 clear indication
with appropriate cause and diagnostic fields. This event
forces the neighbor into Down state.
10.3. The Neighbor state machine
A detailed description of the neighbor state changes follows.
Each state change is invoked by an event (Section 10.2). This
event may produce different effects, depending on the current
state of the neighbor. For this reason, the state machine below
is organized by current neighbor state and received event. Each
entry in the state machine describes the resulting new neighbor
state and the required set of additional actions.
When a neighbor's state changes, it may be necessary to rerun
the Designated Router election algorithm. This is determined by
whether the interface NeighborChange event is generated (see
Section 9.2). Also, if the Interface is in DR state (the router
is itself Designated Router), changes in neighbor state may
cause a new network links advertisement to be originated (see
Section 12.4).
When the neighbor state machine needs to invoke the interface
state machine, it should be done as a scheduled task (see
Section 4.4). This simplifies things, by ensuring that neither
state machine will be executed recursively.
State(s): Down
Event: Start
New state: Attempt
Action: Send an Hello Packet to the neighbor (this neighbor
is always associated with a non-broadcast network)
and start the Inactivity Timer for the neighbor.
The timer's later firing would indicate that
communication with the neighbor was not attained.
State(s): Attempt
Event: HelloReceived
New state: Init
Action: Restart the Inactivity Timer for the neighbor, since
the neighbor has now been heard from.
State(s): Down
Event: HelloReceived
New state: Init
Action: Start the Inactivity Timer for the neighbor. The
timer's later firing would indicate that the
neighbor is dead.
State(s): Init or greater
Event: HelloReceived
New state: No state change.
Action: Restart the Inactivity Timer for the neighbor, since
the neighbor has again been heard from.
State(s): Init
Event: 2-WayReceived
New state: Depends upon action routine.
Action: Determine whether an adjacency should be established
with the neighbor (see Section 10.4). If not, the
new neighbor state is 2-Way.
Otherwise (an adjacency should be established) the
neighbor state transitions to ExStart. Upon
entering this state, the router increments the DD
sequence number for this neighbor. If this is the
first time that an adjacency has been attempted, the
DD sequence number should be assigned some unique
value (like the time of day clock). It then
declares itself master (sets the master/slave bit to
master), and starts sending Database Description
Packets, with the initialize (I), more (M) and
master (MS) bits set. This Database Description
Packet should be otherwise empty. This Database
Description Packet should be retransmitted at
intervals of RxmtInterval until the next state is
entered (see Section 10.8).
State(s): ExStart
Event: NegotiationDone
New state: Exchange
Action: The router must list the contents of its entire area
link state database in the neighbor Database summary
list. The area link state database consists of the
router links, network links and summary links
contained in the area structure, along with the AS
external links contained in the global structure.
AS external link advertisements are omitted from a
virtual neighbor's Database summary list. AS
external advertisements are omitted from the
Database summary list if the area has been
configured as a stub (see Section 3.6).
Advertisements whose age is equal to MaxAge are
instead added to the neighbor's Link state
retransmission list. A summary of the Database
summary list will be sent to the neighbor in
Database Description packets. Each Database
Description Packet has a DD sequence number, and is
explicitly acknowledged. Only one Database
Description Packet is allowed outstanding at any one
time. For more detail on the sending and receiving
of Database Description packets, see Sections 10.8
and 10.6.
State(s): Exchange
Event: ExchangeDone
New state: Depends upon action routine.
Action: If the neighbor Link state request list is empty,
the new neighbor state is Full. No other action is
required. This is an adjacency's final state.
Otherwise, the new neighbor state is Loading. Start
(or continue) sending Link State Request packets to
the neighbor (see Section 10.9). These are requests
for the neighbor's more recent advertisements (which
were discovered but not yet received in the Exchange
state). These advertisements are listed in the Link
state request list associated with the neighbor.
State(s): Loading
Event: Loading Done
New state: Full
Action: No action required. This is an adjacency's final
state.
State(s): 2-Way
Event: AdjOK?
New state: Depends upon action routine.
Action: Determine whether an adjacency should be formed with
the neighboring router (see Section 10.4). If not,
the neighbor state remains at 2-Way. Otherwise,
transition the neighbor state to ExStart and perform
the actions associated with the above state machine
entry for state Init and event 2-WayReceived.
State(s): ExStart or greater
Event: AdjOK?
New state: Depends upon action routine.
Action: Determine whether the neighboring router should
still be adjacent. If yes, there is no state change
and no further action is necessary.
Otherwise, the (possibly partially formed) adjacency
must be destroyed. The neighbor state transitions
to 2-Way. The Link state retransmission list,
Database summary list and Link state request list
are cleared of link state advertisements.
State(s): Exchange or greater
Event: SeqNumberMismatch
New state: ExStart
Action: The (possibly partially formed) adjacency is torn
down, and then an attempt is made at
reestablishment. The neighbor state first
transitions to ExStart. The Link state
retransmission list, Database summary list and Link
state request list are cleared of link state
advertisements. Then the router increments the DD
sequence number for this neighbor, declares itself
master (sets the master/slave bit to master), and
starts sending Database Description Packets, with
the initialize (I), more (M) and master (MS) bits
set. This Database Description Packet should be
otherwise empty (see Section 10.8).
State(s): Exchange or greater
Event: BadLSReq
New state: ExStart
Action: The action for event BadLSReq is exactly the same as
for the neighbor event SeqNumberMismatch. The
(possibly partially formed) adjacency is torn down,
and then an attempt is made at reestablishment. For
more information, see the neighbor state machine
entry that is invoked when event SeqNumberMismatch
is generated in state Exchange or greater.
State(s): Any state
Event: KillNbr
New state: Down
Action: The Link state retransmission list, Database summary
list and Link state request list are cleared of link
state advertisements. Also, the Inactivity Timer is
disabled.
State(s): Any state
Event: LLDown
New state: Down
Action: The Link state retransmission list, Database summary
list and Link state request list are cleared of link
state advertisements. Also, the Inactivity Timer is
disabled.
State(s): Any state
Event: InactivityTimer
New state: Down
Action: The Link state retransmission list, Database summary
list and Link state request list are cleared of link
state advertisements.
State(s): 2-Way or greater
Event: 1-WayReceived
New state: Init
Action: The Link state retransmission list, Database summary
list and Link state request list are cleared of link
state advertisements.
State(s): 2-Way or greater
Event: 2-WayReceived
New state: No state change.
Action: No action required.
State(s): Init
Event: 1-WayReceived
New state: No state change.
Action: No action required.
10.4. Whether to become adjacent
Adjacencies are established with some subset of the router's
neighbors. Routers connected by point-to-point networks and
virtual links always become adjacent. On multi-access networks,
all routers become adjacent to both the Designated Router and
the Backup Designated Router.
The adjacency-forming decision occurs in two places in the
neighbor state machine. First, when bidirectional communication
is initially established with the neighbor, and secondly, when
the identity of the attached network's (Backup) Designated
Router changes. If the decision is made to not attempt an
adjacency, the state of the neighbor communication stops at 2-
Way.
An adjacency should be established with a bidirectional neighbor
when at least one of the following conditions holds:
o The underlying network type is point-to-point
o The underlying network type is virtual link
o The router itself is the Designated Router
o The router itself is the Backup Designated Router
o The neighboring router is the Designated Router
o The neighboring router is the Backup Designated Router
10.5. Receiving Hello Packets
This section explains the detailed processing of a received
Hello Packet. (See Section A.3.2 for the format of Hello
packets.) The generic input processing of OSPF packets will
have checked the validity of the IP header and the OSPF packet
header. Next, the values of the Network Mask, HelloInterval,
and RouterDeadInterval fields in the received Hello packet must
be checked against the values configured for the receiving
interface. Any mismatch causes processing to stop and the
packet to be dropped. In other words, the above fields are
really describing the attached network's configuration. However,
there is one exception to the above rule: on point-to-point
networks and on virtual links, the Network Mask in the received
Hello Packet should be ignored.
The receiving interface attaches to a single OSPF area (this
could be the backbone). The setting of the E-bit found in the
Hello Packet's Options field must match this area's
ExternalRoutingCapability. If AS external advertisements are
not flooded into/throughout the area (i.e, the area is a "stub")
the E-bit must be clear in received Hello Packets, otherwise the
E-bit must be set. A mismatch causes processing to stop and the
packet to be dropped. The setting of the rest of the bits in
the Hello Packet's Options field should be ignored.
At this point, an attempt is made to match the source of the
Hello Packet to one of the receiving interface's neighbors. If
the receiving interface is a multi-access network (either
broadcast or non-broadcast) the source is identified by the IP
source address found in the Hello's IP header. If the receiving
interface is a point-to-point link or a virtual link, the source
is identified by the Router ID found in the Hello's OSPF packet
header. The interface's current list of neighbors is contained
in the interface's data structure. If a matching neighbor
structure cannot be found, (i.e., this is the first time the
neighbor has been detected), one is created. The initial state
of a newly created neighbor is set to Down.
When receiving an Hello Packet from a neighbor on a multi-access
network (broadcast or non-broadcast), set the neighbor
structure's Neighbor ID equal to the Router ID found in the
packet's OSPF header. When receiving an Hello on a point-to-
point network (but not on a virtual link) set the neighbor
structure's Neighbor IP address to the packet's IP source
address.
Now the rest of the Hello Packet is examined, generating events
to be given to the neighbor and interface state machines. These
state machines are specified either to be executed or scheduled
(see Section 4.4). For example, by specifying below that the
neighbor state machine be executed in line, several neighbor
state transitions may be effected by a single received Hello:
o Each Hello Packet causes the neighbor state machine to be
executed with the event HelloReceived.
o Then the list of neighbors contained in the Hello Packet is
examined. If the router itself appears in this list, the
neighbor state machine should be executed with the event 2-
WayReceived. Otherwise, the neighbor state machine should
be executed with the event 1-WayReceived, and the processing
of the packet stops.
o Next, the Hello Packet's Router Priority field is examined.
If this field is different than the one previously received
from the neighbor, the receiving interface's state machine
is scheduled with the event NeighborChange. In any case,
the Router Priority field in the neighbor data structure
should be updated accordingly.
o Next the Designated Router field in the Hello Packet is
examined. If the neighbor is both declaring itself to be
Designated Router (Designated Router field = Neighbor IP
address) and the Backup Designated Router field in the
packet is equal to 0.0.0.0 and the receiving interface is in
state Waiting, the receiving interface's state machine is
scheduled with the event BackupSeen. Otherwise, if the
neighbor is declaring itself to be Designated Router and it
had not previously, or the neighbor is not declaring itself
Designated Router where it had previously, the receiving
interface's state machine is scheduled with the event
NeighborChange. In any case, the Neighbors' Designated
Router item in the neighbor structure is updated
accordingly.
o Finally, the Backup Designated Router field in the Hello
Packet is examined. If the neighbor is declaring itself to
be Backup Designated Router (Backup Designated Router field
= Neighbor IP address) and the receiving interface is in
state Waiting, the receiving interface's state machine is
scheduled with the event BackupSeen. Otherwise, if the
neighbor is declaring itself to be Backup Designated Router
and it had not previously, or the neighbor is not declaring
itself Backup Designated Router where it had previously, the
receiving interface's state machine is scheduled with the
event NeighborChange. In any case, the Neighbor's Backup
Designated Router item in the neighbor structure is updated
accordingly.
On non-broadcast multi-access networks, receipt of an Hello
Packet may also cause an Hello Packet to be sent back to the
neighbor in response. See Section 9.5.1 for more details.
10.6. Receiving Database Description Packets This section explains the detailed processing of a received Database Description Packet. The incoming Database Description Packet has already been associated with a neighbor and receiving interface by the generic input packet processing (Section 8.2). The further processing of the Database Description Packet depends on the neighbor state. If the neighbor's state is Down or Attempt the packet should be ignored. Otherwise, if the state is: Init The neighbor state machine should be executed with the event 2-WayReceived. This causes an immediate state change to either state 2-Way or state ExStart. If the new state is ExStart, the processing of the current packet should then continue in this new state by falling through to case ExStart below. 2-Way The packet should be ignored. Database Description Packets are used only for the purpose of bringing up adjacencies.[7] ExStart If the received packet matches one of the following cases, then the neighbor state machine should be executed with the event NegotiationDone (causing the state to transition to Exchange), the packet's Options field should be recorded in the neighbor structure's Neighbor Options field and the packet should be accepted as next in sequence and processed further (see below). Otherwise, the packet should be ignored. o The initialize(I), more (M) and master(MS) bits are set, the contents of the packet are empty, and the neighbor's Router ID is larger than the router's own. In this case the router is now Slave. Set the master/slave bit to slave, and set the DD sequence number to that specified by the master. o The initialize(I) and master(MS) bits are off, the packet's DD sequence number equals the router's own DD sequence number (indicating acknowledgment) and the neighbor's Router ID is smaller than the router's own. In this case the router is Master.
Exchange
If the state of the MS-bit is inconsistent with the
master/slave state of the connection, generate the neighbor
event SeqNumberMismatch and stop processing the packet.
Otherwise:
o If the initialize(I) bit is set, generate the neighbor
event SeqNumberMismatch and stop processing the packet.
o If the packet's Options field indicates a different set
of optional OSPF capabilities than were previously
received from the neighbor (recorded in the Neighbor
Options field of the neighbor structure), generate the
neighbor event SeqNumberMismatch and stop processing the
packet.
o If the router is master, and the packet's DD sequence
number equals the router's own DD sequence number (this
packet is the next in sequence) the packet should be
accepted and its contents processed (below).
o If the router is master, and the packet's DD sequence
number is one less than the router's DD sequence number,
the packet is a duplicate. Duplicates should be
discarded by the master.
o If the router is slave, and the packet's DD sequence
number is one more than the router's own DD sequence
number (this packet is the next in sequence) the packet
should be accepted and its contents processed (below).
o If the router is slave, and the packet's DD sequence
number is equal to the router's DD sequence number, the
packet is a duplicate. The slave must respond to
duplicates by repeating the last Database Description
packet that it had sent.
o Else, generate the neighbor event SeqNumberMismatch and
stop processing the packet.
Loading or Full
In this state, the router has sent and received an entire
sequence of Database Description Packets. The only packets
received should be duplicates (see above). In particular,
the packet's Options field should match the set of optional
OSPF capabilities previously indicated by the neighbor
(stored in the neighbor structure's Neighbor Options field).
Any other packets received, including the reception of a
packet with the Initialize(I) bit set, should generate the
neighbor event SeqNumberMismatch.[8] Duplicates should be
discarded by the master. The slave must respond to
duplicates by repeating the last Database Description packet
that it had sent.
When the router accepts a received Database Description Packet
as the next in sequence the packet contents are processed as
follows. For each link state advertisement listed, the
advertisement's LS type is checked for validity. If the LS type
is unknown (e.g., not one of the LS types 1-5 defined by this
specification), or if this is a AS external advertisement (LS
type = 5) and the neighbor is associated with a stub area,
generate the neighbor event SeqNumberMismatch and stop
processing the packet. Otherwise, the router looks up the
advertisement in its database to see whether it also has an
instance of the link state advertisement. If it does not, or if
the database copy is less recent (see Section 13.1), the link
state advertisement is put on the Link state request list so
that it can be requested (immediately or at some later time) in
Link State Request Packets.
When the router accepts a received Database Description Packet
as the next in sequence, it also performs the following actions,
depending on whether it is master or slave:
Master
Increments the DD sequence number. If the router has
already sent its entire sequence of Database Description
Packets, and the just accepted packet has the more bit (M)
set to 0, the neighbor event ExchangeDone is generated.
Otherwise, it should send a new Database Description to the
slave.
Slave
Sets the DD sequence number to the DD sequence number
appearing in the received packet. The slave must send a
Database Description Packet in reply. If the received
packet has the more bit (M) set to 0, and the packet to be
sent by the slave will also have the M-bit set to 0, the
neighbor event ExchangeDone is generated. Note that the
slave always generates this event before the master.
10.7. Receiving Link State Request Packets This section explains the detailed processing of received Link State Request packets. Received Link State Request Packets specify a list of link state advertisements that the neighbor wishes to receive. Link State Request Packets should be accepted when the neighbor is in states Exchange, Loading, or Full. In all other states Link State Request Packets should be ignored. Each link state advertisement specified in the Link State Request packet should be located in the router's database, and copied into Link State Update packets for transmission to the neighbor. These link state advertisements should NOT be placed on the Link state retransmission list for the neighbor. If a link state advertisement cannot be found in the database, something has gone wrong with the Database Exchange process, and neighbor event BadLSReq should be generated. 10.8. Sending Database Description Packets This section describes how Database Description Packets are sent to a neighbor. The router's optional OSPF capabilities (see Section 4.5) are transmitted to the neighbor in the Options field of the Database Description packet. The router should maintain the same set of optional capabilities throughout the Database Exchange and flooding procedures. If for some reason the router's optional capabilities change, the Database Exchange procedure should be restarted by reverting to neighbor state ExStart. There are currently two optional capabilities defined. The T-bit should be set if and only if the router is capable of calculating separate routes for each IP TOS. The E-bit should be set if and only if the attached network belongs to a non-stub area. The rest of the Options field should be set to zero. The sending of Database Description packets depends on the neighbor's state. In state ExStart the router sends empty Database Description packets, with the initialize (I), more (M) and master (MS) bits set. These packets are retransmitted every RxmtInterval seconds. In state Exchange the Database Description Packets actually contain summaries of the link state information contained in the router's database. Each link state advertisement in the area's topological database (at the time the neighbor transitions into Exchange state) is listed in the neighbor Database summary list. When a new Database Description Packet is to be sent, the
packet's DD sequence number is incremented, and the (new) top of
the Database summary list is described by the packet. Items are
removed from the Database summary list when the previous packet
is acknowledged.
In state Exchange, the determination of when to send a Database
Description packet depends on whether the router is master or
slave:
Master
Database Description packets are sent when either a) the
slave acknowledges the previous Database Description packet
by echoing the DD sequence number or b) RxmtInterval seconds
elapse without an acknowledgment, in which case the previous
Database Description packet is retransmitted.
Slave
Database Description packets are sent only in response to
Database Description packets received from the master. If
the Database Description packet received from the master is
new, a new Database Description packet is sent, otherwise
the previous Database Description packet is resent.
In states Loading and Full the slave must resend its last
Database Description packet in response to duplicate Database
Description packets received from the master. For this reason
the slave must wait RouterDeadInterval seconds before freeing
the last Database Description packet. Reception of a Database
Description packet from the master after this interval will
generate a SeqNumberMismatch neighbor event.
10.9. Sending Link State Request Packets
In neighbor states Exchange or Loading, the Link state request
list contains a list of those link state advertisements that
need to be obtained from the neighbor. To request these
advertisements, a router sends the neighbor the beginning of the
Link state request list, packaged in a Link State Request
packet.
When the neighbor responds to these requests with the proper
Link State Update packet(s), the Link state request list is
truncated and a new Link State Request packet is sent. This
process continues until the Link state request list becomes
empty. Unsatisfied Link State Request packets are retransmitted
at intervals of RxmtInterval. There should be at most one Link
State Request packet outstanding at any one time.
When the Link state request list becomes empty, and the neighbor
state is Loading (i.e., a complete sequence of Database
Description packets has been sent to and received from the
neighbor), the Loading Done neighbor event is generated.
10.10. An Example
Figure 14 shows an example of an adjacency forming. Routers RT1
and RT2 are both connected to a broadcast network. It is
assumed that RT2 is the Designated Router for the network, and
that RT2 has a higher Router ID than Router RT1.
The neighbor state changes realized by each router are listed on
the sides of the figure.
At the beginning of Figure 14, Router RT1's interface to the
network becomes operational. It begins sending Hello Packets,
although it doesn't know the identity of the Designated Router
or of any other neighboring routers. Router RT2 hears this
hello (moving the neighbor to Init state), and in its next Hello
Packet indicates that it is itself the Designated Router and
that it has heard Hello Packets from RT1. This in turn causes
RT1 to go to state ExStart, as it starts to bring up the
adjacency.
RT1 begins by asserting itself as the master. When it sees that
RT2 is indeed the master (because of RT2's higher Router ID),
RT1 transitions to slave state and adopts its neighbor's DD
sequence number. Database Description packets are then
exchanged, with polls coming from the master (RT2) and responses
from the slave (RT1). This sequence of Database Description
Packets ends when both the poll and associated response has the
M-bit off.
In this example, it is assumed that RT2 has a completely up to
date database. In that case, RT2 goes immediately into Full
state. RT1 will go into Full state after updating the necessary
parts of its database. This is done by sending Link State
Request Packets, and receiving Link State Update Packets in
response. Note that, while RT1 has waited until a complete set
of Database Description Packets has been received (from RT2)
before sending any Link State Request Packets, this need not be
the case. RT1 could have interleaved the sending of Link State
Request Packets with the reception of Database Description
+---+ +---+
|RT1| |RT2|
+---+ +---+
Down Down
Hello(DR=0,seen=0)
------------------------------>
Hello (DR=RT2,seen=RT1,...) Init
<------------------------------
ExStart D-D (Seq=x,I,M,Master)
------------------------------>
D-D (Seq=y,I,M,Master) ExStart
<------------------------------
Exchange D-D (Seq=y,M,Slave)
------------------------------>
D-D (Seq=y+1,M,Master) Exchange
<------------------------------
D-D (Seq=y+1,M,Slave)
------------------------------>
...
...
...
D-D (Seq=y+n, Master)
<------------------------------
D-D (Seq=y+n, Slave)
Loading ------------------------------>
LS Request Full
------------------------------>
LS Update
<------------------------------
LS Request
------------------------------>
LS Update
<------------------------------
Full
Figure 14: An adjacency bring-up example
Packets.
11. The Routing Table Structure
The routing table data structure contains all the information
necessary to forward an IP data packet toward its destination. Each
routing table entry describes the collection of best paths to a
particular destination. When forwarding an IP data packet, the
routing table entry providing the best match for the packet's IP
destination is located. The matching routing table entry then
provides the next hop towards the packet's destination. OSPF also
provides for the existence of a default route (Destination ID =
DefaultDestination, Address Mask = 0x00000000). When the default
route exists, it matches all IP destinations (although any other
matching entry is a better match). Finding the routing table entry
that best matches an IP destination is further described in Section
11.1.
There is a single routing table in each router. Two sample routing
tables are described in Sections 11.2 and 11.3. The building of the
routing table is discussed in Section 16.
The rest of this section defines the fields found in a routing table
entry. The first set of fields describes the routing table entry's
destination.
Destination Type
The destination can be one of three types. Only the first type,
Network, is actually used when forwarding IP data traffic. The
other destinations are used solely as intermediate steps in the
routing table build process.
Network
A range of IP addresses, to which IP data traffic may be
forwarded. This includes IP networks (class A, B, or C), IP
subnets, IP supernets and single IP hosts. The default
route also falls in this category.
Area border router
Routers that are connected to multiple OSPF areas. Such
routers originate summary link advertisements. These
routing table entries are used when calculating the inter-
area routes (see Section 16.2). These routing table entries
may also be associated with configured virtual links.
AS boundary router
Routers that originate AS external link advertisements.
These routing table entries are used when calculating the AS
external routes (see Section 16.4).
Destination ID
The destination's identifier or name. This depends on the
Destination Type. For networks, the identifier is their
associated IP address. For all other types, the identifier is
the OSPF Router ID.[9]
Address Mask
Only defined for networks. The network's IP address together
with its address mask defines a range of IP addresses. For IP
subnets, the address mask is referred to as the subnet mask.
For host routes, the mask is "all ones" (0xffffffff).
Optional Capabilities
When the destination is a router (either an area border router
or an AS boundary router) this field indicates the optional OSPF
capabilities supported by the destination router. The two
optional capabilities currently defined by this specification
are the ability to route based on IP TOS and the ability to
process AS external link advertisements. For a further
discussion of OSPF's optional capabilities, see Section 4.5.
The set of paths to use for a destination may vary based on IP Type
of Service and the OSPF area to which the paths belong. This means
that there may be multiple routing table entries for the same
destination, depending on the values of the next two fields.
Type of Service
There can be a separate set of routes for each IP Type of
Service. The encoding of TOS in OSPF link state advertisements
is described in Section 12.3.
Area
This field indicates the area whose link state information has
led to the routing table entry's collection of paths. This is
called the entry's associated area. For sets of AS external
paths, this field is not defined. For destinations of type
"area border router", there may be separate sets of paths (and
therefore separate routing table entries) associated with each
of several areas. This will happen when two area border routers
share multiple areas in common. For all other destination
types, only the set of paths associated with the best area (the
one providing the shortest route) is kept.
The rest of the routing table entry describes the set of paths to
the destination. The following fields pertain to the set of paths
as a whole. In other words, each one of the paths contained in a
routing table entry is of the same path-type and cost (see below).
Path-type
There are four possible types of paths used to route traffic to
the destination, listed here in order of preference: intra-area,
inter-area, type 1 external or type 2 external. Intra-area
paths indicate destinations belonging to one of the router's
attached areas. Inter-area paths are paths to destinations in
other OSPF areas. These are discovered through the examination
of received summary link advertisements. AS external paths are
paths to destinations external to the AS. These are detected
through the examination of received AS external link
advertisements.
Cost
The link state cost of the path to the destination. For all
paths except type 2 external paths this describes the entire
path's cost. For Type 2 external paths, this field describes
the cost of the portion of the path internal to the AS. This
cost is calculated as the sum of the costs of the path's
constituent links.
Type 2 cost
Only valid for type 2 external paths. For these paths, this
field indicates the cost of the path's external portion. This
cost has been advertised by an AS boundary router, and is the
most significant part of the total path cost. For example, a
type 2 external path with type 2 cost of 5 is always preferred
over a path with type 2 cost of 10, regardless of the cost of
the two paths' internal components.
Link State Origin
Valid only for intra-area paths, this field indicates the link
state advertisement (router links or network links) that
directly references the destination. For example, if the
destination is a transit network, this is the transit network's
network links advertisement. If the destination is a stub
network, this is the router links advertisement for the attached
router. The advertisement is discovered during the shortest-
path tree calculation (see Section 16.1). Multiple
advertisements may reference the destination, however a tie-
breaking scheme always reduces the choice to a single
advertisement. The Link State Origin field is not used by the
OSPF protocol, but it is used by the routing table calculation
in OSPF's Multicast routing extensions (MOSPF).
When multiple paths of equal path-type and cost exist to a
destination (called elsewhere "equal-cost" paths), they are stored
in a single routing table entry. Each one of the "equal-cost" paths
is distinguished by the following fields:
Next hop
The outgoing router interface to use when forwarding traffic to
the destination. On multi-access networks, the next hop also
includes the IP address of the next router (if any) in the path
towards the destination. This next router will always be one of
the adjacent neighbors.
Advertising router
Valid only for inter-area and AS external paths. This field
indicates the Router ID of the router advertising the summary
link or AS external link that led to this path.
11.1. Routing table lookup
When an IP data packet is received, an OSPF router finds the
routing table entry that best matches the packet's destination.
This routing table entry then provides the outgoing interface
and next hop router to use in forwarding the packet. This
section describes the process of finding the best matching
routing table entry. The process consists of a number of steps,
wherein the collection of routing table entries is progressively
pruned. In the end, the single routing table entry remaining is
the called best match.
Note that the steps described below may fail to produce a best
match routing table entry (i.e., all existing routing table
entries are pruned for some reason or another). In this case,
the packet's IP destination is considered unreachable. Instead
of being forwarded, the packet should be dropped and an ICMP
destination unreachable message should be returned to the
packet's source.
(1) Select the complete set of "matching" routing table entries
from the routing table. Each routing table entry describes
a (set of) path(s) to a range of IP addresses. If the data
packet's IP destination falls into an entry's range of IP
addresses, the routing table entry is called a match. (It is
quite likely that multiple entries will match the data
packet. For example, a default route will match all
packets.)
(2) Suppose that the packet's IP destination falls into one of
the router's configured area address ranges (see Section
3.5), and that the particular area address range is active.
This means that there are one or more reachable (by intra-
area paths) networks contained in the area address range.
The packet's IP destination is then required to belong to
one of these constituent networks. For this reason, only
matching routing table entries with path-type of intra-area
are considered (all others are pruned). If no such matching
entries exist, the destination is unreachable (see above).
Otherwise, skip to step 4.
(3) Reduce the set of matching entries to those having the most
preferential path-type (see Section 11). OSPF has a four
level hierarchy of paths. Intra-area paths are the most
preferred, followed in order by inter-area, type 1 external
and type 2 external paths.
(4) Select the remaining routing table entry that provides the
longest (most specific) match. Another way of saying this is
to choose the remaining entry that specifies the narrowest
range of IP addresses.[10] For example, the entry for the
address/mask pair of (128.185.1.0, 0xffffff00) is more
specific than an entry for the pair (128.185.0.0,
0xffff0000). The default route is the least specific match,
since it matches all destinations.
(5) At this point, there may still be multiple routing table
entries remaining. Each routing entry will specify the same
range of IP addresses, but a different IP Type of Service.
Select the routing table entry whose TOS value matches the
TOS found in the packet header. If there is no routing table
entry for this TOS, select the routing table entry for TOS
0. In other words, packets requesting TOS X are routed along
the TOS 0 path if a TOS X path does not exist.
11.2. Sample routing table, without areas
Consider the Autonomous System pictured in Figure 2. No OSPF
areas have been configured. A single metric is shown per
outbound interface, indicating that routes will not vary based
on TOS. The calculation of Router RT6's routing table proceeds
as described in Section 2.1. The resulting routing table is
shown in Table 12. Destination types are abbreviated: Network
as "N", area border router as "BR" and AS boundary router as
"ASBR".
There are no instances of multiple equal-cost shortest paths in
this example. Also, since there are no areas, there are no
inter-area paths.
Routers RT5 and RT7 are AS boundary routers. Intra-area routes
have been calculated to Routers RT5 and RT7. This allows
external routes to be calculated to the destinations advertised
by RT5 and RT7 (i.e., Networks N12, N13, N14 and N15). It is
assumed all AS external advertisements originated by RT5 and RT7
are advertising type 1 external metrics. This results in type 1
external paths being calculated to destinations N12-N15.
11.3. Sample routing table, with areas
Consider the previous example, this time split into OSPF areas.
An OSPF area configuration is pictured in Figure 6. Router
RT4's routing table will be described for this area
configuration. Router RT4 has a connection to Area 1 and a
backbone connection. This causes Router RT4 to view the AS as
the concatenation of the two graphs shown in Figures 7 and 8.
The resulting routing table is displayed in Table 13.
Again, Routers RT5 and RT7 are AS boundary routers. Routers
RT3, RT4, RT7, RT10 and RT11 are area border routers. Note that
there are two routing table entries (in this case having
identical paths) for Router RT7, in its dual capacities as an
area border router and an AS boundary router. Note also that
there are two routing entries for the area border router RT3,
since it has two areas in common with RT4 (Area 1 and the
backbone).
Backbone paths have been calculated to all area border routers
(BR). These are used when determining the inter-area routes.
Note that all of the inter-area routes are associated with the
backbone; this is always the case when the calculating router is
itself an area border router. Routing information is condensed
at area boundaries. In this example, we assume that Area 3 has
been defined so that networks N9-N11 and the host route to H1
are all condensed to a single route when advertised into the
backbone (by Router RT11). Note that the cost of this route is
Type Dest Area Path Type Cost Next Adv.
Hop(s) Router(s)
____________________________________________________________
N N1 0 intra-area 10 RT3 *
N N2 0 intra-area 10 RT3 *
N N3 0 intra-area 7 RT3 *
N N4 0 intra-area 8 RT3 *
N Ib 0 intra-area 7 * *
N Ia 0 intra-area 12 RT10 *
N N6 0 intra-area 8 RT10 *
N N7 0 intra-area 12 RT10 *
N N8 0 intra-area 10 RT10 *
N N9 0 intra-area 11 RT10 *
N N10 0 intra-area 13 RT10 *
N N11 0 intra-area 14 RT10 *
N H1 0 intra-area 21 RT10 *
ASBR RT5 0 intra-area 6 RT5 *
ASBR RT7 0 intra-area 8 RT10 *
____________________________________________________________
N N12 * type 1 ext. 10 RT10 RT7
N N13 * type 1 ext. 14 RT5 RT5
N N14 * type 1 ext. 14 RT5 RT5
N N15 * type 1 ext. 17 RT10 RT7
Table 12: The routing table for Router RT6
(no configured areas).
the minimum of the set of costs to its individual components.
There is a virtual link configured between Routers RT10 and
RT11. Without this configured virtual link, RT11 would be
unable to advertise a route for networks N9-N11 and Host H1 into
the backbone, and there would not be an entry for these networks
in Router RT4's routing table.
In this example there are two equal-cost paths to Network N12.
However, they both use the same next hop (Router RT5).
Router RT4's routing table would improve (i.e., some of the
paths in the routing table would become shorter) if an
additional virtual link were configured between Router RT4 and
Router RT3. The new virtual link would itself be associated
with the first entry for area border router RT3 in Table 13 (an
Type Dest Area Path Type Cost Next Adv.
Hops(s) Router(s)
__________________________________________________________________
N N1 1 intra-area 4 RT1 *
N N2 1 intra-area 4 RT2 *
N N3 1 intra-area 1 * *
N N4 1 intra-area 3 RT3 *
BR RT3 1 intra-area 1 * *
__________________________________________________________________
N Ib 0 intra-area 22 RT5 *
N Ia 0 intra-area 27 RT5 *
BR RT3 0 intra-area 21 RT5 *
BR RT7 0 intra-area 14 RT5 *
BR RT10 0 intra-area 22 RT5 *
BR RT11 0 intra-area 25 RT5 *
ASBR RT5 0 intra-area 8 * *
ASBR RT7 0 intra-area 14 RT5 *
__________________________________________________________________
N N6 0 inter-area 15 RT5 RT7
N N7 0 inter-area 19 RT5 RT7
N N8 0 inter-area 18 RT5 RT7
N N9-N11,H1 0 inter-area 26 RT5 RT11
__________________________________________________________________
N N12 * type 1 ext. 16 RT5 RT5,RT7
N N13 * type 1 ext. 16 RT5 RT5
N N14 * type 1 ext. 16 RT5 RT5
N N15 * type 1 ext. 23 RT5 RT7
Table 13: Router RT4's routing table
in the presence of areas.
intra-area path through Area 1). This would yield a cost of 1
for the virtual link. The routing table entries changes that
would be caused by the addition of this virtual link are shown
in Table 14.
(page 100 continued on part 5)
