RFC 2178
OSPF Version 2
8. Protocol Packet Processing This section discusses the general processing of OSPF routing protocol packets. It is very important that the router link-state databases remain synchronized. For this reason, routing protocol packets should get preferential treatment over ordinary data packets, both in sending and receiving. Routing protocol packets are sent along adjacencies only (with the exception of Hello packets, which are used to discover the adjacencies). This means that all routing protocol packets travel a single IP hop, except those sent over virtual links. All routing protocol packets begin with a standard header. The sections below provide details on how to fill in and verify this standard header. Then, for each packet type, the section giving more details on that particular packet type's processing is listed. 8.1. Sending protocol packets When a router sends a routing protocol packet, it fills in the fields of the standard OSPF packet header as follows. For more details on the header format consult Section A.3.1:
Version #
Set to 2, the version number of the protocol as documented in this
specification.
Packet type
The type of OSPF packet, such as Link state Update or Hello
Packet.
Packet length
The length of the entire OSPF packet in bytes, including the
standard OSPF packet header.
Router ID
The identity of the router itself (who is originating the packet).
Area ID
The OSPF area that the packet is being sent into.
Checksum
The standard IP 16-bit one's complement checksum of the entire
OSPF packet, excluding the 64-bit authentication field. This
checksum is calculated as part of the appropriate authentication
procedure; for some OSPF authentication types, the checksum
calculation is omitted. See Section D.4 for details.
AuType and Authentication
Each OSPF packet exchange is authenticated. Authentication types
are assigned by the protocol and are documented in Appendix D. A
different authentication procedure can be used for each IP
network/subnet. Autype indicates the type of authentication
procedure in use. The 64-bit authentication field is then for use
by the chosen authentication procedure. This procedure should be
the last called when forming the packet to be sent. See Section
D.4 for details.
The IP destination address for the packet is selected as follows. On
physical point-to-point networks, the IP destination is always set to
the address AllSPFRouters. On all other network types (including
virtual links), the majority of OSPF packets are sent as unicasts,
i.e., sent directly to the other end of the adjacency. In this case,
the IP destination is just the Neighbor IP address associated with
the other end of the adjacency (see Section 10). The only packets
not sent as unicasts are on broadcast networks; on these networks
Hello packets are sent to the multicast destination AllSPFRouters,
the Designated Router and its Backup send both Link State Update
Packets and Link State Acknowledgment Packets to the multicast
address AllSPFRouters, while all other routers send both their Link
State Update and Link State Acknowledgment Packets to the multicast
address AllDRouters.
Retransmissions of Link State Update packets are ALWAYS sent as
unicasts.
The IP source address should be set to the IP address of the sending
interface. Interfaces to unnumbered point-to-point networks have no
associated IP address. On these interfaces, the IP source should be
set to any of the other IP addresses belonging to the router. For
this reason, there must be at least one IP address assigned to the
router.[2] Note that, for most purposes, virtual links act precisely
the same as unnumbered point-to-point networks. However, each
virtual link does have an IP interface address (discovered during the
routing table build process) which is used as the IP source when
sending packets over the virtual link.
For more information on the format of specific OSPF packet types,
consult the sections listed in Table 10.
Type Packet name detailed section (transmit)
_________________________________________________________
1 Hello Section 9.5
2 Database description Section 10.8
3 Link state request Section 10.9
4 Link state update Section 13.3
5 Link state ack Section 13.5
Table 10: Sections describing OSPF protocol packet transmission.
8.2. Receiving protocol packets
Whenever a protocol packet is received by the router it is marked
with the interface it was received on. For routers that have virtual
links configured, it may not be immediately obvious which interface
to associate the packet with. For example, consider the Router RT11
depicted in Figure 6. If RT11 receives an OSPF protocol packet on
its interface to Network N8, it may want to associate the packet with
the interface to Area 2, or with the virtual link to Router RT10
(which is part of the backbone). In the following, we assume that
the packet is initially associated with the non-virtual link.[3]
In order for the packet to be accepted at the IP level, it must pass
a number of tests, even before the packet is passed to OSPF for
processing:
o The IP checksum must be correct.
o The packet's IP destination address must be the IP address
of the receiving interface, or one of the IP multicast
addresses AllSPFRouters or AllDRouters.
o The IP protocol specified must be OSPF (89).
o Locally originated packets should not be passed on to OSPF.
That is, the source IP address should be examined to make
sure this is not a multicast packet that the router itself
generated.
Next, the OSPF packet header is verified. The fields specified
in the header must match those configured for the receiving
interface. If they do not, the packet should be discarded:
o The version number field must specify protocol version 2.
o The Area ID found in the OSPF header must be verified. If
both of the following cases fail, the packet should be
discarded. The Area ID specified in the header must either:
(1) Match the Area ID of the receiving interface. In this
case, the packet has been sent over a single hop.
Therefore, the packet's IP source address is required to
be on the same network as the receiving interface. This
can be verified by comparing the packet's IP source
address to the interface's IP address, after masking
both addresses with the interface mask. This comparison
should not be performed on point-to-point networks. On
point-to-point networks, the interface addresses of each
end of the link are assigned independently, if they are
assigned at all.
(2) Indicate the backbone. In this case, the packet has
been sent over a virtual link. The receiving router
must be an area border router, and the Router ID
specified in the packet (the source router) must be the
other end of a configured virtual link. The receiving
interface must also attach to the virtual link's
configured Transit area. If all of these checks
succeed, the packet is accepted and is from now on
associated with the virtual link (and the backbone
area).
o Packets whose IP destination is AllDRouters should only be
accepted if the state of the receiving interface is DR or
Backup (see Section 9.1).
o The AuType specified in the packet must match the AuType
specified for the associated area.
o The packet must be authenticated. The authentication
procedure is indicated by the setting of AuType (see
Appendix D). The authentication procedure may use one or
more Authentication keys, which can be configured on a per-
interface basis. The authentication procedure may also
verify the checksum field in the OSPF packet header (which,
when used, is set to the standard IP 16-bit one's complement
checksum of the OSPF packet's contents after excluding the
64-bit authentication field). If the authentication
procedure fails, the packet should be discarded.
If the packet type is Hello, it should then be further processed by
the Hello Protocol (see Section 10.5). All other packet types are
sent/received only on adjacencies. This means that the packet must
have been sent by one of the router's active neighbors. If the
receiving interface connects to a broadcast network, Point-to-
MultiPoint network or NBMA network the sender is identified by the IP
source address found in the packet's IP header. If the receiving
interface connects to a point-to-point network or a virtual link, the
sender is identified by the Router ID (source router) found in the
packet's OSPF header. The data structure associated with the
receiving interface contains the list of active neighbors. Packets
not matching any active neighbor are discarded.
At this point all received protocol packets are associated with an
active neighbor. For the further input processing of specific packet
types, consult the sections listed in Table 11.
Type Packet name detailed section (receive)
________________________________________________________
1 Hello Section 10.5
2 Database description Section 10.6
3 Link state request Section 10.7
4 Link state update Section 13
5 Link state ack Section 13.7
Table 11: Sections describing OSPF protocol packet reception.
9. The Interface Data Structure
An OSPF interface is the connection between a router and a network.
We assume a single OSPF interface to each attached network/subnet,
although supporting multiple interfaces on a single network is
considered in Appendix F. Each interface structure has at most one IP
interface address.
An OSPF interface can be considered to belong to the area that
contains the attached network. All routing protocol packets
originated by the router over this interface are labelled with the
interface's Area ID. One or more router adjacencies may develop over
an interface. A router's LSAs reflect the state of its interfaces and
their associated adjacencies.
The following data items are associated with an interface. Note that
a number of these items are actually configuration for the attached
network; such items must be the same for all routers connected to the
network.
Type
The OSPF interface type is either point-to-point, broadcast, NBMA,
Point-to-MultiPoint or virtual link.
State
The functional level of an interface. State determines whether or
not full adjacencies are allowed to form over the interface.
State is also reflected in the router's LSAs.
IP interface address
The IP address associated with the interface. This appears as the
IP source address in all routing protocol packets originated over
this interface. Interfaces to unnumbered point-to-point networks
do not have an associated IP address.
IP interface mask
Also referred to as the subnet mask, this indicates the portion of
the IP interface address that identifies the attached network.
Masking the IP interface address with the IP interface mask yields
the IP network number of the attached network. On point-to-point
networks and virtual links, the IP interface mask is not defined.
On these networks, the link itself is not assigned an IP network
number, and so the addresses of each side of the link are assigned
independently, if they are assigned at all.
Area ID
The Area ID of the area to which the attached network belongs.
All routing protocol packets originating from the interface are
labelled with this Area ID.
HelloInterval
The length of time, in seconds, between the Hello packets that the
router sends on the interface. Advertised in Hello packets sent
out this interface.
RouterDeadInterval
The number of seconds before the router's neighbors will declare
it down, when they stop hearing the router's Hello Packets.
Advertised in Hello packets sent out this interface.
InfTransDelay
The estimated number of seconds it takes to transmit a Link State
Update Packet over this interface. LSAs contained in the Link
State Update packet will have their age incremented by this amount
before transmission. This value should take into account
transmission and propagation delays; it must be greater than zero.
Router Priority
An 8-bit unsigned integer. When two routers attached to a network
both attempt to become Designated Router, the one with the highest
Router Priority takes precedence. A router whose Router Priority
is set to 0 is ineligible to become Designated Router on the
attached network. Advertised in Hello packets sent out this
interface.
Hello Timer
An interval timer that causes the interface to send a Hello
packet. This timer fires every HelloInterval seconds. Note that
on non-broadcast networks a separate Hello packet is sent to each
qualified neighbor.
Wait Timer
A single shot timer that causes the interface to exit the Waiting
state, and as a consequence select a Designated Router on the
network. The length of the timer is RouterDeadInterval seconds.
List of neighboring routers
The other routers attached to this network. This list is formed
by the Hello Protocol. Adjacencies will be formed to some of
these neighbors. The set of adjacent neighbors can be determined
by an examination of all of the neighbors' states.
Designated Router
The Designated Router selected for the attached network. The
Designated Router is selected on all broadcast and NBMA networks
by the Hello Protocol. Two pieces of identification are kept for
the Designated Router: its Router ID and its IP interface address
on the network. The Designated Router advertises link state for
the network; this network-LSA is labelled with the Designated
Router's IP address. The Designated Router is initialized to
0.0.0.0, which indicates the lack of a Designated Router.
Backup Designated Router
The Backup Designated Router is also selected on all broadcast and
NBMA networks by the Hello Protocol. All routers on the attached
network become adjacent to both the Designated Router and the
Backup Designated Router. The Backup Designated Router becomes
Designated Router when the current Designated Router fails. The
Backup Designated Router is initialized to 0.0.0.0, indicating the
lack of a Backup Designated Router.
Interface output cost(s)
The cost of sending a data packet on the interface, expressed in
the link state metric. This is advertised as the link cost for
this interface in the router-LSA. The cost of an interface must be
greater than zero.
RxmtInterval
The number of seconds between LSA retransmissions, for adjacencies
belonging to this interface. Also used when retransmitting
Database Description and Link State Request Packets.
AuType
The type of authentication used on the attached network/subnet.
Authentication types are defined in Appendix D. All OSPF packet
exchanges are authenticated. Different authentication schemes may
be used on different networks/subnets.
Authentication key
This configured data allows the authentication procedure to
generate and/or verify OSPF protocol packets. The Authentication
key can be configured on a per-interface basis. For example, if
the AuType indicates simple password, the Authentication key would
be a 64-bit clear password which is inserted into the OSPF packet
header. If instead Autype indicates Cryptographic authentication,
then the Authentication key is a shared secret which enables the
generation/verification of message digests which are appended to
the OSPF protocol packets. When Cryptographic authentication is
used, multiple simultaneous keys are supported in order to achieve
smooth key transition (see Section D.3).
9.1. Interface states
The various states that router interfaces may attain is documented in
this section. The states are listed in order of progressing
functionality. For example, the inoperative state is listed first,
followed by a list of intermediate states before the final, fully
functional state is achieved. The specification makes use of this
ordering by sometimes making references such as "those interfaces in
state greater than X". Figure 11 shows the graph of interface state
changes. The arcs of the graph are labelled with the event causing
the state change. These events are documented in Section 9.2. The
interface state machine is described in more detail in Section 9.3.
Down
This is the initial interface state. In this state, the lower-
level protocols have indicated that the interface is unusable. No
protocol traffic at all will be sent or received on such a
interface. In this state, interface parameters should be set to
their initial values.
+----+ UnloopInd +--------+
|Down|<--------------|Loopback|
+----+ +--------+
|
|InterfaceUp
+-------+ | +--------------+
|Waiting|<-+-------------->|Point-to-point|
+-------+ +--------------+
|
WaitTimer|BackupSeen
|
|
| NeighborChange
+------+ +-+<---------------- +-------+
|Backup|<----------|?|----------------->|DROther|
+------+---------->+-+<-----+ +-------+
Neighbor | |
Change | |Neighbor
| |Change
| +--+
+---->|DR|
+--+
Figure 11: Interface State changes
In addition to the state transitions pictured,
Event InterfaceDown always forces Down State, and
Event LoopInd always forces Loopback State
All interface timers should be disabled, and there should be no
adjacencies associated with the interface.
Loopback
In this state, the router's interface to the network is looped
back. The interface may be looped back in hardware or software.
The interface will be unavailable for regular data traffic.
However, it may still be desirable to gain information on the
quality of this interface, either through sending ICMP pings to
the interface or through something like a bit error test. For
this reason, IP packets may still be addressed to an interface in
Loopback state. To facilitate this, such interfaces are
advertised in router-LSAs as single host routes, whose destination
is the IP interface address.[4]
Waiting
In this state, the router is trying to determine the identity of
the (Backup) Designated Router for the network. To do this, the
router monitors the Hello Packets it receives. The router is not
allowed to elect a Backup Designated Router nor a Designated
Router until it transitions out of Waiting state. This prevents
unnecessary changes of (Backup) Designated Router.
Point-to-point
In this state, the interface is operational, and connects either
to a physical point-to-point network or to a virtual link. Upon
entering this state, the router attempts to form an adjacency with
the neighboring router. Hello Packets are sent to the neighbor
every HelloInterval seconds.
DR Other
The interface is to a broadcast or NBMA network on which another
router has been selected to be the Designated Router. In this
state, the router itself has not been selected Backup Designated
Router either. The router forms adjacencies to both the
Designated Router and the Backup Designated Router (if they
exist).
Backup
In this state, the router itself is the Backup Designated Router
on the attached network. It will be promoted to Designated Router
when the present Designated Router fails. The router establishes
adjacencies to all other routers attached to the network. The
Backup Designated Router performs slightly different functions
during the Flooding Procedure, as compared to the Designated
Router (see Section 13.3). See Section 7.4 for more details on
the functions performed by the Backup Designated Router.
DR In this state, this router itself is the Designated Router
on the attached network. Adjacencies are established to all other
routers attached to the network. The router must also originate a
network-LSA for the network node. The network-LSA will contain
links to all routers (including the Designated Router itself)
attached to the network. See Section 7.3 for more details on the
functions performed by the Designated Router.
9.2. Events causing interface state changes
State changes can be effected by a number of events. These events
are pictured as the labelled arcs in Figure 11. The label
definitions are listed below. For a detailed explanation of the
effect of these events on OSPF protocol operation, consult Section
9.3.
InterfaceUp
Lower-level protocols have indicated that the network interface is
operational. This enables the interface to transition out of Down
state. On virtual links, the interface operational indication is
actually a result of the shortest path calculation (see Section
16.7).
WaitTimer
The Wait Timer has fired, indicating the end of the waiting period
that is required before electing a (Backup) Designated Router.
BackupSeen
The router has detected the existence or non-existence of a Backup
Designated Router for the network. This is done in one of two
ways. First, an Hello Packet may be received from a neighbor
claiming to be itself the Backup Designated Router.
Alternatively, an Hello Packet may be received from a neighbor
claiming to be itself the Designated Router, and indicating that
there is no Backup Designated Router. In either case there must
be bidirectional communication with the neighbor, i.e., the router
must also appear in the neighbor's Hello Packet. This event
signals an end to the Waiting state.
NeighborChange
There has been a change in the set of bidirectional neighbors
associated with the interface. The (Backup) Designated Router
needs to be recalculated. The following neighbor changes lead to
the NeighborChange event. For an explanation of neighbor states,
see Section 10.1.
o Bidirectional communication has been established to a
neighbor. In other words, the state of the neighbor has
transitioned to 2-Way or higher.
o There is no longer bidirectional communication with a
neighbor. In other words, the state of the neighbor has
transitioned to Init or lower.
o One of the bidirectional neighbors is newly declaring
itself as either Designated Router or Backup Designated
Router. This is detected through examination of that
neighbor's Hello Packets.
o One of the bidirectional neighbors is no longer
declaring itself as Designated Router, or is no longer
declaring itself as Backup Designated Router. This is
again detected through examination of that neighbor's
Hello Packets.
o The advertised Router Priority for a bidirectional
neighbor has changed. This is again detected through
examination of that neighbor's Hello Packets.
LoopInd
An indication has been received that the interface is now looped
back to itself. This indication can be received either from
network management or from the lower level protocols.
UnloopInd
An indication has been received that the interface is no longer
looped back. As with the LoopInd event, this indication can be
received either from network management or from the lower level
protocols.
InterfaceDown
Lower-level protocols indicate that this interface is no longer
functional. No matter what the current interface state is, the new
interface state will be Down.
9.3. The Interface state machine
A detailed description of the interface state changes follows. Each
state change is invoked by an event (Section 9.2). This event may
produce different effects, depending on the current state of the
interface. For this reason, the state machine below is organized by
current interface state and received event. Each entry in the state
machine describes the resulting new interface state and the required
set of additional actions.
When an interface's state changes, it may be necessary to originate a
new router-LSA. See Section 12.4 for more details.
Some of the required actions below involve generating events for the
neighbor state machine. For example, when an interface becomes
inoperative, all neighbor connections associated with the interface
must be destroyed. For more information on the neighbor state
machine, see Section 10.3.
State(s): Down
Event: InterfaceUp
New state: Depends upon action routine
Action: Start the interval Hello Timer, enabling the
periodic sending of Hello packets out the interface.
If the attached network is a physical point-to-point
network, Point-to-MultiPoint network or virtual
link, the interface state transitions to Point-to-
Point. Else, if the router is not eligible to
become Designated Router the interface state
transitions to DR Other.
Otherwise, the attached network is a broadcast or
NBMA network and the router is eligible to become
Designated Router. In this case, in an attempt to
discover the attached network's Designated Router
the interface state is set to Waiting and the single
shot Wait Timer is started. Additionally, if the
network is an NBMA network examine the configured
list of neighbors for this interface and generate
the neighbor event Start for each neighbor that is
also eligible to become Designated Router.
State(s): Waiting
Event: BackupSeen
New state: Depends upon action routine.
Action: Calculate the attached network's Backup Designated
Router and Designated Router, as shown in Section
9.4. As a result of this calculation, the new state
of the interface will be either DR Other, Backup or
DR.
State(s): Waiting
Event: WaitTimer
New state: Depends upon action routine.
Action: Calculate the attached network's Backup Designated
Router and Designated Router, as shown in Section
9.4. As a result of this calculation, the new state
of the interface will be either DR Other, Backup or
DR.
State(s): DR Other, Backup or DR
Event: NeighborChange
New state: Depends upon action routine.
Action: Recalculate the attached network's Backup Designated
Router and Designated Router, as shown in Section
9.4. As a result of this calculation, the new state
of the interface will be either DR Other, Backup or
DR.
State(s): Any State
Event: InterfaceDown
New state: Down
Action: All interface variables are reset, and interface
timers disabled. Also, all neighbor connections
associated with the interface are destroyed. This
is done by generating the event KillNbr on all
associated neighbors (see Section 10.2).
State(s): Any State
Event: LoopInd
New state: Loopback
Action: Since this interface is no longer connected to the
attached network the actions associated with the
above InterfaceDown event are executed.
State(s): Loopback
Event: UnloopInd
New state: Down
Action: No actions are necessary. For example, the
interface variables have already been reset upon
entering the Loopback state. Note that reception of
an InterfaceUp event is necessary before the
interface again becomes fully functional.
9.4. Electing the Designated Router
This section describes the algorithm used for calculating a network's
Designated Router and Backup Designated Router. This algorithm is
invoked by the Interface state machine. The initial time a router
runs the election algorithm for a network, the network's Designated
Router and Backup Designated Router are initialized to 0.0.0.0. This
indicates the lack of both a Designated Router and a Backup
Designated Router.
The Designated Router election algorithm proceeds as follows: Call
the router doing the calculation Router X. The list of neighbors
attached to the network and having established bidirectional
communication with Router X is examined. This list is precisely the
collection of Router X's neighbors (on this network) whose state is
greater than or equal to 2-Way (see Section 10.1). Router X itself
is also considered to be on the list. Discard all routers from the
list that are ineligible to become Designated Router. (Routers
having Router Priority of 0 are ineligible to become Designated
Router.) The following steps are then executed, considering only
those routers that remain on the list:
(1) Note the current values for the network's Designated Router
and Backup Designated Router. This is used later for
comparison purposes.
(2) Calculate the new Backup Designated Router for the network
as follows. Only those routers on the list that have not
declared themselves to be Designated Router are eligible to
become Backup Designated Router. If one or more of these
routers have declared themselves Backup Designated Router
(i.e., they are currently listing themselves as Backup
Designated Router, but not as Designated Router, in their
Hello Packets) the one having highest Router Priority is
declared to be Backup Designated Router. In case of a tie,
the one having the highest Router ID is chosen. If no
routers have declared themselves Backup Designated Router,
choose the router having highest Router Priority, (again
excluding those routers who have declared themselves
Designated Router), and again use the Router ID to break
ties.
(3) Calculate the new Designated Router for the network as
follows. If one or more of the routers have declared
themselves Designated Router (i.e., they are currently
listing themselves as Designated Router in their Hello
Packets) the one having highest Router Priority is declared
to be Designated Router. In case of a tie, the one having
the highest Router ID is chosen. If no routers have
declared themselves Designated Router, assign the Designated
Router to be the same as the newly elected Backup Designated
Router.
(4) If Router X is now newly the Designated Router or newly the
Backup Designated Router, or is now no longer the Designated
Router or no longer the Backup Designated Router, repeat
steps 2 and 3, and then proceed to step 5. For example, if
Router X is now the Designated Router, when step 2 is
repeated X will no longer be eligible for Backup Designated
Router election. Among other things, this will ensure that
no router will declare itself both Backup Designated Router
and Designated Router.[5]
(5) As a result of these calculations, the router itself may now
be Designated Router or Backup Designated Router. See
Sections 7.3 and 7.4 for the additional duties this would
entail. The router's interface state should be set
accordingly. If the router itself is now Designated Router,
the new interface state is DR. If the router itself is now
Backup Designated Router, the new interface state is Backup.
Otherwise, the new interface state is DR Other.
(6) If the attached network is an NBMA network, and the router
itself has just become either Designated Router or Backup
Designated Router, it must start sending Hello Packets to
those neighbors that are not eligible to become Designated
Router (see Section 9.5.1). This is done by invoking the
neighbor event Start for each neighbor having a Router
Priority of 0.
(7) If the above calculations have caused the identity of either
the Designated Router or Backup Designated Router to change,
the set of adjacencies associated with this interface will
need to be modified. Some adjacencies may need to be
formed, and others may need to be broken. To accomplish
this, invoke the event AdjOK? on all neighbors whose state
is at least 2-Way. This will cause their eligibility for
adjacency to be reexamined (see Sections 10.3 and 10.4).
The reason behind the election algorithm's complexity is the desire
for an orderly transition from Backup Designated Router to Designated
Router, when the current Designated Router fails. This orderly
transition is ensured through the introduction of hysteresis: no new
Backup Designated Router can be chosen until the old Backup accepts
its new Designated Router responsibilities.
The above procedure may elect the same router to be both Designated
Router and Backup Designated Router, although that router will never
be the calculating router (Router X) itself. The elected Designated
Router may not be the router having the highest Router Priority, nor
will the Backup Designated Router necessarily have the second highest
Router Priority. If Router X is not itself eligible to become
Designated Router, it is possible that neither a Backup Designated
Router nor a Designated Router will be selected in the above
procedure. Note also that if Router X is the only attached router
that is eligible to become Designated Router, it will select itself
as Designated Router and there will be no Backup Designated Router
for the network.
9.5. Sending Hello packets
Hello packets are sent out each functioning router interface. They
are used to discover and maintain neighbor relationships.[6] On
broadcast and NBMA networks, Hello Packets are also used to elect the
Designated Router and Backup Designated Router.
The format of an Hello packet is detailed in Section A.3.2. The
Hello Packet contains the router's Router Priority (used in choosing
the Designated Router), and the interval between Hello Packets sent
out the interface (HelloInterval). The Hello Packet also indicates
how often a neighbor must be heard from to remain active
(RouterDeadInterval). Both HelloInterval and RouterDeadInterval must
be the same for all routers attached to a common network. The Hello
packet also contains the IP address mask of the attached network
(Network Mask). On unnumbered point-to-point networks and on virtual
links this field should be set to 0.0.0.0.
The Hello packet's Options field describes the router's optional OSPF
capabilities. One optional capability is defined in this
specification (see Sections 4.5 and A.2). The E-bit of the Options
field should be set if and only if the attached area is capable of
processing AS-external-LSAs (i.e., it is not a stub area). If the E-
bit is set incorrectly the neighboring routers will refuse to accept the Hello Packet (see Section 10.5). Unrecognized bits in the Hello Packet's Options field should be set to zero. In order to ensure two-way communication between adjacent routers, the Hello packet contains the list of all routers on the network from which Hello Packets have been seen recently. The Hello packet also contains the router's current choice for Designated Router and Backup Designated Router. A value of 0.0.0.0 in these fields means that one has not yet been selected. On broadcast networks and physical point-to-point networks, Hello packets are sent every HelloInterval seconds to the IP multicast address AllSPFRouters. On virtual links, Hello packets are sent as unicasts (addressed directly to the other end of the virtual link) every HelloInterval seconds. On Point-to-MultiPoint networks, separate Hello packets are sent to each attached neighbor every HelloInterval seconds. Sending of Hello packets on NBMA networks is covered in the next section. 9.5.1. Sending Hello packets on NBMA networks Static configuration information may be necessary in order for the Hello Protocol to function on non-broadcast networks (see Sections C.5 and C.6). On NBMA networks, every attached router which is eligible to become Designated Router becomes aware of all of its neighbors on the network (either through configuration or by some unspecified mechanism). Each neighbor is labelled with the neighbor's Designated Router eligibility. The interface state must be at least Waiting for any Hello Packets to be sent out the NBMA interface. Hello Packets are then sent directly (as unicasts) to some subset of a router's neighbors. Sometimes an Hello Packet is sent periodically on a timer; at other times it is sent as a response to a received Hello Packet. A router's hello- sending behavior varies depending on whether the router itself is eligible to become Designated Router. If the router is eligible to become Designated Router, it must periodically send Hello Packets to all neighbors that are also eligible. In addition, if the router is itself the Designated Router or Backup Designated Router, it must also send periodic Hello Packets to all other neighbors. This means that any two eligible routers are always exchanging Hello Packets, which is necessary for the correct operation of the Designated Router election algorithm. To minimize the number of Hello Packets sent, the number of eligible routers on an NBMA network should be kept small.
If the router is not eligible to become Designated Router, it must periodically send Hello Packets to both the Designated Router and the Backup Designated Router (if they exist). It must also send an Hello Packet in reply to an Hello Packet received from any eligible neighbor (other than the current Designated Router and Backup Designated Router). This is needed to establish an initial bidirectional relationship with any potential Designated Router. When sending Hello packets periodically to any neighbor, the interval between Hello Packets is determined by the neighbor's state. If the neighbor is in state Down, Hello Packets are sent every PollInterval seconds. Otherwise, Hello Packets are sent every HelloInterval seconds. 10. The Neighbor Data Structure An OSPF router converses with its neighboring routers. Each separate conversation is described by a "neighbor data structure". Each conversation is bound to a particular OSPF router interface, and is identified either by the neighboring router's OSPF Router ID or by its Neighbor IP address (see below). Thus if the OSPF router and another router have multiple attached networks in common, multiple conversations ensue, each described by a unique neighbor data structure. Each separate conversation is loosely referred to in the text as being a separate "neighbor". The neighbor data structure contains all information pertinent to the forming or formed adjacency between the two neighbors. (However, remember that not all neighbors become adjacent.) An adjacency can be viewed as a highly developed conversation between two routers. State The functional level of the neighbor conversation. This is described in more detail in Section 10.1. Inactivity Timer A single shot timer whose firing indicates that no Hello Packet has been seen from this neighbor recently. The length of the timer is RouterDeadInterval seconds. Master/Slave When the two neighbors are exchanging databases, they form a master/slave relationship. The master sends the first Database Description Packet, and is the only part that is allowed to retransmit. The slave can only respond to the master's Database Description Packets. The master/slave relationship is negotiated in state ExStart.
DD Sequence Number
The DD Sequence number of the Database Description packet that is
currently being sent to the neighbor.
Last received Database Description packet
The initialize(I), more (M) and master(MS) bits, Options field,
and DD sequence number contained in the last Database Description
packet received from the neighbor. Used to determine whether the
next Database Description packet received from the neighbor is a
duplicate.
Neighbor ID
The OSPF Router ID of the neighboring router. The Neighbor ID is
learned when Hello packets are received from the neighbor, or is
configured if this is a virtual adjacency (see Section C.4).
Neighbor Priority
The Router Priority of the neighboring router. Contained in the
neighbor's Hello packets, this item is used when selecting the
Designated Router for the attached network.
Neighbor IP address
The IP address of the neighboring router's interface to the
attached network. Used as the Destination IP address when
protocol packets are sent as unicasts along this adjacency. Also
used in router-LSAs as the Link ID for the attached network if the
neighboring router is selected to be Designated Router (see
Section 12.4.1). The Neighbor IP address is learned when Hello
packets are received from the neighbor. For virtual links, the
Neighbor IP address is learned during the routing table build
process (see Section 15).
Neighbor Options
The optional OSPF capabilities supported by the neighbor. Learned
during the Database Exchange process (see Section 10.6). The
neighbor's optional OSPF capabilities are also listed in its Hello
packets. This enables received Hello Packets to be rejected (i.e.,
neighbor relationships will not even start to form) if there is a
mismatch in certain crucial OSPF capabilities (see Section 10.5).
The optional OSPF capabilities are documented in Section 4.5.
Neighbor's Designated Router
The neighbor's idea of the Designated Router. If this is the
neighbor itself, this is important in the local calculation of the
Designated Router. Defined only on broadcast and NBMA networks.
Neighbor's Backup Designated Router
The neighbor's idea of the Backup Designated Router. If this is
the neighbor itself, this is important in the local calculation of
the Backup Designated Router. Defined only on broadcast and NBMA
networks.
The next set of variables are lists of LSAs. These lists describe
subsets of the area link-state database. This memo defines five
distinct types of LSAs, all of which may be present in an area link-
state database: router-LSAs, network-LSAs, and Type 3 and 4 summary-
LSAs (all stored in the area data structure), and AS- external-LSAs
(stored in the global data structure).
Link state retransmission list
The list of LSAs that have been flooded but not acknowledged on
this adjacency. These will be retransmitted at intervals until
they are acknowledged, or until the adjacency is destroyed.
Database summary list
The complete list of LSAs that make up the area link-state
database, at the moment the neighbor goes into Database Exchange
state. This list is sent to the neighbor in Database Description
packets.
Link state request list
The list of LSAs that need to be received from this neighbor in
order to synchronize the two neighbors' link-state databases.
This list is created as Database Description packets are received,
and is then sent to the neighbor in Link State Request packets.
The list is depleted as appropriate Link State Update packets are
received.
10.1. Neighbor states
The state of a neighbor (really, the state of a conversation being
held with a neighboring router) is documented in the following
sections. The states are listed in order of progressing
functionality. For example, the inoperative state is listed first,
followed by a list of intermediate states before the final, fully
functional state is achieved. The specification makes use of this
ordering by sometimes making references such as "those
neighbors/adjacencies in state greater than X". Figures 12 and 13
show the graph of neighbor state changes. The arcs of the graphs are
labelled with the event causing the state change. The neighbor
events are documented in Section 10.2.
The graph in Figure 12 shows the state changes effected by the Hello
Protocol. The Hello Protocol is responsible for neighbor acquisition
and maintenance, and for ensuring two way communication between
neighbors.
The graph in Figure 13 shows the forming of an adjacency. Not every
two neighboring routers become adjacent (see Section 10.4). The
adjacency starts to form when the neighbor is in state ExStart.
After the two routers discover their master/slave status, the state
transitions to Exchange. At this point the neighbor starts to be
used in the flooding procedure, and the two neighboring routers begin
synchronizing their databases. When this synchronization is
finished, the neighbor is in state Full and we say that the two
routers are fully adjacent. At this point the adjacency is listed in
LSAs.
For a more detailed description of neighbor state changes, together
with the additional actions involved in each change, see Section
10.3.
Down
This is the initial state of a neighbor conversation. It
indicates that there has been no recent information received from
the neighbor. On NBMA networks, Hello packets may still be sent to
"Down" neighbors, although at a reduced frequency (see Section
9.5.1).
+----+
|Down|
+----+
|\
| \Start
| \ +-------+
Hello | +---->|Attempt|
Received | +-------+
| |
+----+<-+ |HelloReceived
|Init|<---------------+
+----+<--------+
| |
|2-Way |1-Way
|Received |Received
| |
+-------+ | +-----+
|ExStart|<--------+------->|2-Way|
+-------+ +-----+
Figure 12: Neighbor state changes (Hello Protocol)
In addition to the state transitions pictured,
Event KillNbr always forces Down State,
Event Inactivity Timer always forces Down State,
Event LLDown always forces Down State
+-------+
|ExStart|
+-------+
|
NegotiationDone|
+->+--------+
|Exchange|
+--+--------+
|
Exchange|
Done |
+----+ | +-------+
|Full|<---------+----->|Loading|
+----+<-+ +-------+
| LoadingDone |
+------------------+
Figure 13: Neighbor state changes (Database Exchange)
In addition to the state transitions pictured,
Event SeqNumberMismatch forces ExStart state,
Event BadLSReq forces ExStart state,
Event 1-Way forces Init state,
Event KillNbr always forces Down State,
Event InactivityTimer always forces Down State,
Event LLDown always forces Down State,
Event AdjOK? leads to adjacency forming/breaking
Attempt
This state is only valid for neighbors attached to NBMA networks.
It indicates that no recent information has been received from the
neighbor, but that a more concerted effort should be made to
contact the neighbor. This is done by sending the neighbor Hello
packets at intervals of HelloInterval (see Section 9.5.1).
Init
In this state, an Hello packet has recently been seen from the
neighbor. However, bidirectional communication has not yet been
established with the neighbor (i.e., the router itself did not
appear in the neighbor's Hello packet). All neighbors in this
state (or higher) are listed in the Hello packets sent from the
associated interface.
2-Way
In this state, communication between the two routers is
bidirectional. This has been assured by the operation of the
Hello Protocol. This is the most advanced state short of
beginning adjacency establishment. The (Backup) Designated Router
is selected from the set of neighbors in state 2-Way or greater.
ExStart
This is the first step in creating an adjacency between the two
neighboring routers. The goal of this step is to decide which
router is the master, and to decide upon the initial DD sequence
number. Neighbor conversations in this state or greater are
called adjacencies.
Exchange
In this state the router is describing its entire link state
database by sending Database Description packets to the neighbor.
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. In this state, Link State
Request Packets may also be sent asking for the neighbor's more
recent LSAs. All adjacencies in Exchange state or greater are
used by the flooding procedure. In fact, these adjacencies are
fully capable of transmitting and receiving all types of OSPF
routing protocol packets.
Loading
In this state, Link State Request packets are sent to the neighbor
asking for the more recent LSAs that have been discovered (but not
yet received) in the Exchange state.
Full
In this state, the neighboring routers are fully adjacent. These
adjacencies will now appear in router-LSAs and network-LSAs.
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
An Hello packet has been received from the 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 NBMA networks.
2-WayReceived
Bidirectional communication has been realized between the two
neighboring routers. This is indicated by the router seeing
itself in the neighbor'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 an LSA 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 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 the
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.
(page 76 continued on part 4)
