RFC 2328
OSPF Version 2
Pages: 244
Internet Standard: 54
→ Errata
Obsoletes: 2178
Updated by: 5709 6549 6845 6860 7474 8042 9355 9454
Internet Standard: 54
→ Errata
Obsoletes: 2178
Updated by: 5709 6549 6845 6860 7474 8042 9355 9454
Part 3 of 8 – Pages 52 to 80
7. Bringing Up Adjacencies OSPF creates adjacencies between neighboring routers for the purpose of exchanging routing information. Not every two neighboring routers will become adjacent. This section covers the generalities involved in creating adjacencies. For further details consult Section 10. 7.1. The Hello Protocol The Hello Protocol is responsible for establishing and maintaining neighbor relationships. It also ensures that communication between neighbors is bidirectional. Hello packets are sent periodically out all router interfaces. Bidirectional communication is indicated when the router sees itself listed in the neighbor's Hello Packet. On broadcast and NBMA networks, the Hello Protocol elects a Designated Router for the network. The Hello Protocol works differently on broadcast networks, NBMA networks and Point-to-MultiPoint networks. On broadcast networks, each router advertises itself by periodically multicasting Hello Packets. This allows neighbors to be discovered dynamically. These Hello Packets contain the router's view of the Designated Router's identity, and the list of routers whose Hello Packets have been seen recently. On NBMA networks some configuration information may be necessary for the operation of the Hello Protocol. Each router that may potentially become Designated Router has a list of all other
routers attached to the network. A router, having Designated
Router potential, sends Hello Packets to all other potential
Designated Routers when its interface to the NBMA network first
becomes operational. This is an attempt to find the Designated
Router for the network. If the router itself is elected
Designated Router, it begins sending Hello Packets to all other
routers attached to the network.
On Point-to-MultiPoint networks, a router sends Hello Packets to
all neighbors with which it can communicate directly. These
neighbors may be discovered dynamically through a protocol such
as Inverse ARP (see [Ref14]), or they may be configured.
After a neighbor has been discovered, bidirectional
communication ensured, and (if on a broadcast or NBMA network) a
Designated Router elected, a decision is made regarding whether
or not an adjacency should be formed with the neighbor (see
Section 10.4). If an adjacency is to be formed, the first step
is to synchronize the neighbors' link-state databases. This is
covered in the next section.
7.2. The Synchronization of Databases
In a link-state routing algorithm, it is very important for all
routers' link-state databases to stay synchronized. OSPF
simplifies this by requiring only adjacent routers to remain
synchronized. The synchronization process begins as soon as the
routers attempt to bring up the adjacency. Each router
describes its database by sending a sequence of Database
Description packets to its neighbor. Each Database Description
Packet describes a set of LSAs belonging to the router's
database. When the neighbor sees an LSA that is more recent
than its own database copy, it makes a note that this newer LSA
should be requested.
This sending and receiving of Database Description packets is
called the "Database Exchange Process". During this process,
the two routers form a master/slave relationship. Each Database
Description Packet has a sequence number. Database Description
Packets sent by the master (polls) are acknowledged by the slave
through echoing of the sequence number. Both polls and their
responses contain summaries of link state data. The master is
the only one allowed to retransmit Database Description Packets.
It does so only at fixed intervals, the length of which is the
configured per-interface constant RxmtInterval.
Each Database Description contains an indication that there are
more packets to follow --- the M-bit. The Database Exchange
Process is over when a router has received and sent Database
Description Packets with the M-bit off.
During and after the Database Exchange Process, each router has
a list of those LSAs for which the neighbor has more up-to-date
instances. These LSAs are requested in Link State Request
Packets. Link State Request packets that are not satisfied are
retransmitted at fixed intervals of time RxmtInterval. When the
Database Description Process has completed and all Link State
Requests have been satisfied, the databases are deemed
synchronized and the routers are marked fully adjacent. At this
time the adjacency is fully functional and is advertised in the
two routers' router-LSAs.
The adjacency is used by the flooding procedure as soon as the
Database Exchange Process begins. This simplifies database
synchronization, and guarantees that it finishes in a
predictable period of time.
7.3. The Designated Router
Every broadcast and NBMA network has a Designated Router. The
Designated Router performs two main functions for the routing
protocol:
o The Designated Router originates a network-LSA on behalf of
the network. This LSA lists the set of routers (including
the Designated Router itself) currently attached to the
network. The Link State ID for this LSA (see Section
12.1.4) is the IP interface address of the Designated
Router. The IP network number can then be obtained by using
the network's subnet/network mask.
o The Designated Router becomes adjacent to all other routers
on the network. Since the link state databases are
synchronized across adjacencies (through adjacency bring-up
and then the flooding procedure), the Designated Router
plays a central part in the synchronization process.
The Designated Router is elected by the Hello Protocol. A
router's Hello Packet contains its Router Priority, which is
configurable on a per-interface basis. In general, when a
router's interface to a network first becomes functional, it
checks to see whether there is currently a Designated Router for
the network. If there is, it accepts that Designated Router,
regardless of its Router Priority. (This makes it harder to
predict the identity of the Designated Router, but ensures that
the Designated Router changes less often. See below.)
Otherwise, the router itself becomes Designated Router if it has
the highest Router Priority on the network. A more detailed
(and more accurate) description of Designated Router election is
presented in Section 9.4.
The Designated Router is the endpoint of many adjacencies. In
order to optimize the flooding procedure on broadcast networks,
the Designated Router multicasts its Link State Update Packets
to the address AllSPFRouters, rather than sending separate
packets over each adjacency.
Section 2 of this document discusses the directed graph
representation of an area. Router nodes are labelled with their
Router ID. Transit network nodes are actually labelled with the
IP address of their Designated Router. It follows that when the
Designated Router changes, it appears as if the network node on
the graph is replaced by an entirely new node. This will cause
the network and all its attached routers to originate new LSAs.
Until the link-state databases again converge, some temporary
loss of connectivity may result. This may result in ICMP
unreachable messages being sent in response to data traffic.
For that reason, the Designated Router should change only
infrequently. Router Priorities should be configured so that
the most dependable router on a network eventually becomes
Designated Router.
7.4. The Backup Designated Router In order to make the transition to a new Designated Router smoother, there is a Backup Designated Router for each broadcast and NBMA network. The Backup Designated Router is also adjacent to all routers on the network, and becomes Designated Router when the previous Designated Router fails. If there were no Backup Designated Router, when a new Designated Router became necessary, new adjacencies would have to be formed between the new Designated Router and all other routers attached to the network. Part of the adjacency forming process is the synchronizing of link-state databases, which can potentially take quite a long time. During this time, the network would not be available for transit data traffic. The Backup Designated obviates the need to form these adjacencies, since they already exist. This means the period of disruption in transit traffic lasts only as long as it takes to flood the new LSAs (which announce the new Designated Router). The Backup Designated Router does not generate a network-LSA for the network. (If it did, the transition to a new Designated Router would be even faster. However, this is a tradeoff between database size and speed of convergence when the Designated Router disappears.) The Backup Designated Router is also elected by the Hello Protocol. Each Hello Packet has a field that specifies the Backup Designated Router for the network. In some steps of the flooding procedure, the Backup Designated Router plays a passive role, letting the Designated Router do more of the work. This cuts down on the amount of local routing traffic. See Section 13.3 for more information. 7.5. The graph of adjacencies An adjacency is bound to the network that the two routers have in common. If two routers have multiple networks in common, they may have multiple adjacencies between them.
One can picture the collection of adjacencies on a network as
forming an undirected graph. The vertices consist of routers,
with an edge joining two routers if they are adjacent. The
graph of adjacencies describes the flow of routing protocol
packets, and in particular Link State Update Packets, through
the Autonomous System.
Two graphs are possible, depending on whether a Designated
Router is elected for the network. On physical point-to-point
networks, Point-to-MultiPoint networks and virtual links,
neighboring routers become adjacent whenever they can
communicate directly. In contrast, on broadcast and NBMA
networks only the Designated Router and the Backup Designated
Router become adjacent to all other routers attached to the
network.
+---+ +---+
|RT1|------------|RT2| o---------------o
+---+ N1 +---+ RT1 RT2
RT7
o---------+
+---+ +---+ +---+ /|\ |
|RT7| |RT3| |RT4| / | \ |
+---+ +---+ +---+ / | \ |
| | | / | \ |
+-----------------------+ RT5o RT6o oRT4 |
| | N2 * * * |
+---+ +---+ * * * |
|RT5| |RT6| * * * |
+---+ +---+ *** |
o---------+
RT3
Figure 10: The graph of adjacencies
These graphs are shown in Figure 10. It is assumed that Router
RT7 has become the Designated Router, and Router RT3 the Backup
Designated Router, for the Network N2. The Backup Designated
Router performs a lesser function during the flooding procedure
than the Designated Router (see Section 13.3). This is the
reason for the dashed lines connecting the Backup Designated
Router RT3.
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
directly to the neighbor. On multi-access networks, this means
that retransmissions should be sent to the neighbor's IP
address.
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. 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
+----+ 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 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.
(page 80 continued on part 4)