RFC2178 - OSPF Version 2

Network Working Group J. Moy
Request for Comments: 2178 Cascade Communications Corp.
Obsoletes: 1583 July 1997
Category: Standards Track
OSPF Version 2
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
This memo documents version 2 of the OSPF protocol. OSPF is a link-
state routing protocol. It is designed to be run internal to a
single Autonomous System. Each OSPF router maintains an identical
database describing the Autonomous System"s topology. From this
database, a routing table is calculated by constrUCting a shortest-
path tree.
OSPF recalculates routes quickly in the face of topological changes,
utilizing a minimum of routing protocol traffic. OSPF provides
support for equal-cost multipath. An area routing capability is
provided, enabling an additional level of routing protection and a
reduction in routing protocol traffic. In addition, all OSPF routing
protocol exchanges are authenticated.
The differences between this memo and RFC1583 are eXPlained in
Appendix G. All differences are backward-compatible in nature.
Implementations of this memo and of RFC1583 will interoperate.
Please send comments to ospf@gated.cornell.edu.
Table of Contents
1 Introduction ........................................... 5
1.1 Protocol Overview ...................................... 5
1.2 Definitions of commonly used terms ..................... 6
1.3 Brief history of link-state routing technology ........ 9
1.4 Organization of this document ......................... 10
1.5 Acknowledgments ....................................... 11
2 The link-state database: organization and calculations 11
2.1 Representation of routers and networks ................ 11
2.1.1 Representation of non-broadcast networks .............. 13
2.1.2 An example link-state database ........................ 14
2.2 The shortest-path tree ................................ 18
2.3 Use of external routing information ................... 20
2.4 Equal-cost multipath .................................. 22
3 Splitting the AS into Areas ........................... 22
3.1 The backbone of the Autonomous System ................. 23
3.2 Inter-area routing .................................... 23
3.3 Classification of routers ............................. 24
3.4 A sample area configuration ........................... 25
3.5 IP subnetting support ................................. 31
3.6 Supporting stub areas ................................. 32
3.7 Partitions of areas ................................... 33
4 Functional Summary .................................... 34
4.1 Inter-area routing .................................... 35
4.2 AS external routes .................................... 35
4.3 Routing protocol packets .............................. 35
4.4 Basic implementation requirements ..................... 38
4.5 Optional OSPF capabilities ............................ 39
5 Protocol data structures .............................. 40
6 The Area Data Structure ............................... 42
7 Bringing Up Adjacencies ............................... 44
7.1 The Hello Protocol .................................... 44
7.2 The Synchronization of Databases ...................... 45
7.3 The Designated Router ................................. 46
7.4 The Backup Designated Router .......................... 47
7.5 The graph of adjacencies .............................. 48
8 Protocol Packet Processing ............................ 49
8.1 Sending protocol packets .............................. 49
8.2 Receiving protocol packets ............................ 51
9 The Interface Data Structure .......................... 54
9.1 Interface states ...................................... 57
9.2 Events causing interface state changes ................ 59
9.3 The Interface state machine ........................... 61
9.4 Electing the Designated Router ........................ 64
9.5 Sending Hello packets ................................. 66
9.5.1 Sending Hello packets on NBMA networks ................ 67
10 The Neighbor Data Structure ........................... 68
10.1 Neighbor states ....................................... 70
10.2 Events causing neighbor state changes ................. 75
10.3 The Neighbor state machine ............................ 76
10.4 Whether tocome adjacent ............................ 82
10.5 Receiving Hello Packets ............................... 83
10.6 Receiving Database Description Packets ................ 85
10.7 Receiving Link State Request Packets .................. 88
10.8 Sending Database Description Packets .................. 89
10.9 Sending Link State Request Packets .................... 90
10.10 An Example ............................................ 91
11 The Routing Table Structure ........................... 93
11.1 Routing table lookup .................................. 96
11.2 Sample routing table, without areas ................... 97
11.3 Sample routing table, with areas ...................... 97
12 Link State Advertisements (LSAs) ......................100
12.1 The LSA Header ........................................100
12.1.1 LS age ............................................... 101
12.1.2 Options .............................................. 101
12.1.3 LS type .............................................. 102
12.1.4 Link State ID ........................................ 102
12.1.5 Advertising Router ................................... 104
12.1.6 LS sequence number ................................... 104
12.1.7 LS checksum .......................................... 105
12.2 The link state database .............................. 105
12.3 Representation of TOS ................................ 106
12.4 Originating LSAs ..................................... 107
12.4.1 Router-LSAs .......................................... 110
12.4.1.1 Describing point-to-point interfaces ................. 112
12.4.1.2 Describing broadcast and NBMA interfaces ............. 113
12.4.1.3 Describing virtual links ............................. 113
12.4.1.4 Describing Point-to-MultiPoint interfaces ............ 114
12.4.1.5 Examples of router-LSAs .............................. 114
12.4.2 Network-LSAs ......................................... 116
12.4.2.1 Examples of network-LSAs ............................. 116
12.4.3 Summary-LSAs ......................................... 117
12.4.3.1 Originating summary-LSAs into stub areas ............. 119
12.4.3.2 Examples of summary-LSAs ............................. 119
12.4.4 AS-external-LSAs ..................................... 120
12.4.4.1 Examples of AS-external-LSAs ......................... 121
13 The Flooding Procedure ............................... 122
13.1 Determining which LSA is newer ....................... 126
13.2 Installing LSAs in the database ...................... 127
13.3 Next step in the flooding procedure .................. 128
13.4 Receiving self-originated LSAs ....................... 130
13.5 Sending Link State Acknowledgment packets ............ 131
13.6 Retransmitting LSAs .................................. 133
13.7 Receiving link state acknowledgments ................. 134
14 Aging The Link State Database ........................ 134
14.1 Premature aging of LSAs .............................. 135
15 Virtual Links ........................................ 135
16 Calculation of the routing table ..................... 137
16.1 Calculating the shortest-path tree for an area ....... 138
16.1.1 The next hop calculation ............................. 144
16.2 Calculating the inter-area routes .................... 145
16.3 Examining transit areas" summary-LSAs ................ 146
16.4 Calculating AS external routes ....................... 149
16.4.1 External path preferences ............................ 151
16.5 Incremental updates -- summary-LSAs .................. 151
16.6 Incremental updates -- AS-external-LSAs .............. 152
16.7 Events generated as a result of routing table changes 153
16.8 Equal-cost multipath ................................. 154
Footnotes ............................................ 155
References ........................................... 158
A OSPF data formats .................................... 160
A.1 Encapsulation of OSPF packets ........................ 160
A.2 The Options field .................................... 162
A.3 OSPF Packet Formats .................................. 163
A.3.1 The OSPF packet header ............................... 164
A.3.2 The Hello packet ..................................... 166
A.3.3 The Database Description packet ...................... 168
A.3.4 The Link State Request packet ........................ 170
A.3.5 The Link State Update packet ......................... 171
A.3.6 The Link State Acknowledgment packet ................. 172
A.4 LSA formats .......................................... 173
A.4.1 The LSA header ....................................... 174
A.4.2 Router-LSAs .......................................... 176
A.4.3 Network-LSAs ......................................... 179
A.4.4 Summary-LSAs ......................................... 180
A.4.5 AS-external-LSAs ..................................... 182
B Architectural Constants .............................. 184
C Configurable Constants ............................... 186
C.1 Global parameters .................................... 186
C.2 Area parameters ...................................... 187
C.3 Router interface parameters .......................... 188
C.4 Virtual link parameters .............................. 190
C.5 NBMA network parameters .............................. 191
C.6 Point-to-MultiPoint network parameters ............... 191
C.7 Host route parameters ................................ 192
D Authentication ....................................... 193
D.1 Null authentication .................................. 193
D.2 Simple passWord authentication ....................... 193
D.3 Cryptographic authentication ......................... 194
D.4 Message generation ................................... 196
D.4.1 Generating Null authentication ....................... 196
D.4.2 Generating Simple password authentication ............ 197
D.4.3 Generating Cryptographic authentication .............. 197
D.5 Message verification ................................. 198
D.5.1 Verifying Null authentication ........................ 199
D.5.2 Verifying Simple password authentication ............. 199
D.5.3 Verifying Cryptographic authentication ............... 199
E An algorithm for assigning Link State IDs ............ 201
F Multiple interfaces to the same network/subnet ....... 203
G Differences from RFC1583 ............................ 204
G.1 Enhancements to OSPF authentication .................. 204
G.2 Addition of Point-to-MultiPoint interface ............ 204
G.3 Support for overlapping area ranges .................. 205
G.4 A modification to the flooding algorithm ............. 206
G.5 Introduction of the MinLSArrival constant ............ 206
G.6 Optionally advertising point-to-point links as subnets 207
G.7 Advertising same external route from multiple areas .. 207
G.8 Retransmission of initial Database Description packets 209
G.9 Detecting interface MTU mismatches ................... 209
G.10 Deleting the TOS routing option ...................... 209
Security Considerations .............................. 210
Author"s Address ..................................... 211
1. Introduction
This document is a specification of the Open Shortest Path First
(OSPF) TCP/IP internet routing protocol. OSPF is classified as an
Interior Gateway Protocol (IGP). This means that it distributes
routing information between routers belonging to a single Autonomous
System. The OSPF protocol is based on link-state or SPF technology.
This is a departure from the Bellman-Ford base used by traditional
TCP/IP internet routing protocols.
The OSPF protocol was developed by the OSPF working group of the
Internet Engineering Task Force. It has been designed expressly for
the TCP/IP internet environment, including explicit support for CIDR
and the tagging of externally-derived routing information. OSPF also
provides for the authentication of routing updates, and utilizes IP
multicast when sending/receiving the updates. In addition, much work
has been done to produce a protocol that responds quickly to topology
changes, yet involves small amounts of routing protocol traffic.
1.1. Protocol overview
OSPF routes IP packets based solely on the destination IP address
found in the IP packet header. IP packets are routed "as is" -- they
are not encapsulated in any further protocol headers as they transit
the Autonomous System. OSPF is a dynamic routing protocol. It
quickly detects topological changes in the AS (such as router
interface failures) and calculates new loop-free routes after a
period of convergence. This period of convergence is short and
involves a minimum of routing traffic.
In a link-state routing protocol, each router maintains a database
describing the Autonomous System"s topology. This database is
referred to as the link-state database. Each participating router has
an identical database. Each individual piece of this database is a
particular router"s local state (e.g., the router"s usable interfaces
and reachable neighbors). The router distributes its local state
throughout the Autonomous System by flooding.
All routers run the exact same algorithm, in parallel. From the
link-state database, each router constructs a tree of shortest paths
with itself as root. This shortest-path tree gives the route to each
destination in the Autonomous System. Externally derived routing
information appears on the tree as leaves.
When several equal-cost routes to a destination exist, traffic is
distributed equally among them. The cost of a route is described by
a single dimensionless metric.
OSPF allows sets of networks to be grouped together. Such a grouping
is called an area. The topology of an area is hidden from the rest
of the Autonomous System. This information hiding enables a
significant reduction in routing traffic. Also, routing within the
area is determined only by the area"s own topology, lending the area
protection from bad routing data. An area is a generalization of an
IP subnetted network.
OSPF enables the flexible configuration of IP subnets. Each route
distributed by OSPF has a destination and mask. Two different
subnets of the same IP network number may have different sizes (i.e.,
different masks). This is commonly referred to as variable length
subnetting. A packet is routed to the best (i.e., longest or most
specific) match. Host routes are considered to be subnets whose
masks are "all ones" (0xffffffff).
All OSPF protocol exchanges are authenticated. This means that only
trusted routers can participate in the Autonomous System"s routing.
A variety of authentication schemes can be used; in fact, separate
authentication schemes can be configured for each IP subnet.
Externally derived routing data (e.g., routes learned from an
Exterior Gateway Protocol such as BGP; see [Ref23]) is advertised
throughout the Autonomous System. This externally derived data is
kept separate from the OSPF protocol"s link state data. Each
external route can also be tagged by the advertising router, enabling
the passing of additional information between routers on the boundary
of the Autonomous System.
1.2. Definitions of commonly used terms
This section provides definitions for terms that have a specific
meaning to the OSPF protocol and that are used throughout the text.
The reader unfamiliar with the Internet Protocol Suite is referred to
[Ref13] for an introduction to IP.
Router
A level three Internet Protocol packet switch. Formerly called a
gateway in much of the IP literature.
Autonomous System
A group of routers exchanging routing information via a common
routing protocol. Abbreviated as AS.
Interior Gateway Protocol
The routing protocol spoken by the routers belonging to an
Autonomous system. Abbreviated as IGP. Each Autonomous System has
a single IGP. Separate Autonomous Systems may be running
different IGPs.
Router ID
A 32-bit number assigned to each router running the OSPF protocol.
This number uniquely identifies the router within an Autonomous
System.
Network
In this memo, an IP network/subnet/supernet. It is possible for
one physical network to be assigned multiple IP network/subnet
numbers. We consider these to be separate networks. Point-to-
point physical networks are an exception - they are considered a
single network no matter how many (if any at all) IP
network/subnet numbers are assigned to them.
Network mask
A 32-bit number indicating the range of IP addresses residing on a
single IP network/subnet/supernet. This specification displays
network masks as hexadecimal numbers. For example, the network
mask for a class C IP network is displayed as 0xffffff00. Such a
mask is often displayed elsewhere in the literature as
255.255.255.0.
Point-to-point networks
A network that joins a single pair of routers. A 56Kb serial line
is an example of a point-to-point network.
Broadcast networks
Networks supporting many (more than two) attached routers,
together with the capability to address a single physical message
to all of the attached routers (broadcast). Neighboring routers
are discovered dynamically on these nets using OSPF"s Hello
Protocol. The Hello Protocol itself takes advantage of the
broadcast capability. The OSPF protocol makes further use of
multicast capabilities, if they exist. Each pair of routers on a
broadcast network is assumed to be able to communicate directly.
An ethernet is an example of a broadcast network.
Non-broadcast networks
Networks supporting many (more than two) routers, but having no
broadcast capability. Neighboring routers are maintained on these
nets using OSPF"s Hello Protocol. However, due to the lack of
broadcast capability, some configuration information may be
necessary to aid in the discovery of neighbors. On non-broadcast
networks, OSPF protocol packets that are normally multicast need
to be sent to each neighboring router, in turn. An X.25 Public
Data Network (PDN) is an example of a non-broadcast network.
OSPF runs in one of two modes over non-broadcast networks. The
first mode, called non-broadcast multi-Access or NBMA, simulates
the operation of OSPF on a broadcast network. The second mode,
called Point-to-MultiPoint, treats the non-broadcast network as a
collection of point-to-point links. Non-broadcast networks are
referred to as NBMA networks or Point-to-MultiPoint networks,
depending on OSPF"s mode of operation over the network.
Interface
The connection between a router and one of its attached networks.
An interface has state information associated with it, which is
oBTained from the underlying lower level protocols and the routing
protocol itself. An interface to a network has associated with it
a single IP address and mask (unless the network is an unnumbered
point-to-point network). An interface is sometimes also referred
to as a link.
Neighboring routers
Two routers that have interfaces to a common network. Neighbor
relationships are maintained by, and usually dynamically
discovered by, OSPF"s Hello Protocol.
Adjacency
A relationship formed between selected neighboring routers for the
purpose of exchanging routing information. Not every pair of
neighboring routers become adjacent.
Link state advertisement
Unit of data describing the local state of a router or network.
For a router, this includes the state of the router"s interfaces
and adjacencies. Each link state advertisement is flooded
throughout the routing domain. The collected link state
advertisements of all routers and networks forms the protocol"s
link state database. Throughout this memo, link state
advertisement is abbreviated as LSA.
Hello Protocol
The part of the OSPF protocol used to establish and maintain
neighbor relationships. On broadcast networks the Hello Protocol
can also dynamically discover neighboring routers.
Flooding
The part of the OSPF protocol that distributes and synchronizes
the link-state database between OSPF routers.
Designated Router
Each broadcast and NBMA network that has at least two attached
routers has a Designated Router. The Designated Router generates
an LSA for the network and has other special responsibilities in
the running of the protocol. The Designated Router is elected by
the Hello Protocol.
The Designated Router concept enables a reduction in the number of
adjacencies required on a broadcast or NBMA network. This in turn
reduces the amount of routing protocol traffic and the size of the
link-state database.
Lower-level protocols
The underlying network access protocols that provide services to
the Internet Protocol and in turn the OSPF protocol. Examples of
these are the X.25 packet and frame levels for X.25 PDNs, and the
ethernet data link layer for ethernets.
1.3. Brief history of link-state routing technology
OSPF is a link state routing protocol. Such protocols are also
referred to in the literature as SPF-based or distributed-database
protocols. This section gives a brief description of the
developments in link-state technology that have influenced the OSPF
protocol.
The first link-state routing protocol was developed for use in the
ARPANET packet switching network. This protocol is described in
[Ref3]. It has formed the starting point for all other link-state
protocols. The homogeneous ARPANET environment, i.e., single-vendor
packet switches connected by synchronous serial lines, simplified the
design and implementation of the original protocol.
Modifications to this protocol were proposed in [Ref4]. These
modifications dealt with increasing the fault tolerance of the
routing protocol through, among other things, adding a checksum to
the LSAs (thereby detecting database corruption). The paper also
included means for reducing the routing traffic overhead in a link-
state protocol. This was accomplished by introducing mechanisms
which enabled the interval between LSA originations to be increased
by an order of magnitude.
A link-state algorithm has also been proposed for use as an ISO IS-IS
routing protocol. This protocol is described in [Ref2]. The
protocol includes methods for data and routing traffic reduction when
operating over broadcast networks. This is accomplished by election
of a Designated Router for each broadcast network, which then
originates an LSA for the network.
The OSPF Working Group of the IETF has extended this work in
developing the OSPF protocol. The Designated Router concept has been
greatly enhanced to further reduce the amount of routing traffic
required. Multicast capabilities are utilized for additional routing
bandwidth reduction. An area routing scheme has been developed
enabling information hiding/protection/reduction. Finally, the
algorithms have been tailored for efficient operation in TCP/IP
internets.
1.4. Organization of this document
The first three sections of this specification give a general
overview of the protocol"s capabilities and functions. Sections 4-16
explain the protocol"s mechanisms in detail. Packet formats,
protocol constants and configuration items are specified in the
appendices.
Labels such as HelloInterval encountered in the text refer to
protocol constants. They may or may not be configurable.
Architectural constants are summarized in Appendix B. Configurable
constants are summarized in Appendix C.
The detailed specification of the protocol is presented in terms of
data structures. This is done in order to make the explanation more
precise. Implementations of the protocol are required to support the
functionality described, but need not use the precise data structures
that appear in this memo.
1.5. Acknowledgments
The author would like to thank Ran Atkinson, Fred Baker, Jeffrey
Burgan, Rob Coltun, Dino Farinacci, Vince Fuller, Phanindra
JujJavarapu, Milo Medin, Tom Pusateri, Kannan Varadhan, Zhaohui Zhang
and the rest of the OSPF Working Group for the ideas and support they
have given to this project.
The OSPF Point-to-MultiPoint interface is based on work done by Fred
Baker.
The OSPF Cryptographic Authentication option was developed by Fred
Baker and Ran Atkinson.
2. The Link-state Database: organization and calculations
The following subsections describe the organization of OSPF"s link-
state database, and the routing calculations that are performed on
the database in order to produce a router"s routing table.
2.1. Representation of routers and networks
The Autonomous System"s link-state database describes a directed
graph. The vertices of the graph consist of routers and networks. A
graph edge connects two routers when they are attached via a physical
point-to-point network. An edge connecting a router to a network
indicates that the router has an interface on the network. Networks
can be either transit or stub networks. Transit networks are those
capable of carrying data traffic that is neither locally originated
nor locally destined. A transit network is represented by a graph
vertex having both incoming and outgoing edges. A stub network"s
vertex has only incoming edges.
The neighborhood of each network node in the graph depends on the
network"s type (point-to-point, broadcast, NBMA or Point-to-
MultiPoint) and the number of routers having an interface to the
network. Three cases are depicted in Figure 1a. Rectangles indicate
routers. Circles and oblongs indicate networks. Router names are
prefixed with the letters RT and network names with the letter N.
Router interface names are prefixed by the letter I. Lines between
routers indicate point-to-point networks. The left side of the
figure shows networks with their connected routers, with the
resulting graphs shown on the right.
**FROM**
* RT1RT2
+---+Ia +---+ * ------------
RT1------RT2 T RT1 X
+---+ Ib+---+ O RT2 X
* Ia X
* Ib X
Physical point-to-point networks
**FROM**
+---+ *
RT7 * RT7 N3
+---+ T ------------
O RT7
+----------------------+ * N3 X
N3 *
Stub networks
+---+ +---+
RT3 RT4 RT3RT4RT5RT6N2
+---+ +---+ * ------------------------
N2 * RT3 X
+----------------------+ T RT4 X
O RT5 X
+---+ +---+ * RT6 X
RT5 RT6 * N2 X X X X
+---+ +---+
Broadcast or NBMA networks
Figure 1a: Network map components
Networks and routers are represented by vertices. An edge connects
Vertex A to Vertex B iff the intersection of Column A and Row B is
marked with an X.
The top of Figure 1a shows two routers connected by a point-to-point
link. In the resulting link-state database graph, the two router
vertices are directly connected by a pair of edges, one in each
direction. Interfaces to point-to-point networks need not be assigned
IP addresses. When interface addresses are assigned, they are
modelled as stub links, with each router advertising a stub
connection to the other router"s interface address. Optionally, an IP
subnet can be assigned to the point-to-point network. In this case,
both routers advertise a stub link to the IP subnet, instead of
advertising each others" IP interface addresses.
The middle of Figure 1a shows a network with only one attached router
(i.e., a stub network). In this case, the network appears on the end
of a stub connection in the link-state database"s graph.
When multiple routers are attached to a broadcast network, the link-
state database graph shows all routers bidirectionally connected to
the network vertex. This is pictured at the bottom of Figure 1a.
Each network (stub or transit) in the graph has an IP address and
associated network mask. The mask indicates the number of nodes on
the network. Hosts attached directly to routers (referred to as host
routes) appear on the graph as stub networks. The network mask for a
host route is always 0xffffffff, which indicates the presence of a
single node.
2.1.1. Representation of non-broadcast networks
As mentioned previously, OSPF can run over non-broadcast networks in
one of two modes: NBMA or Point-to-MultiPoint. The choice of mode
determines the way that the Hello protocol and flooding work over the
non-broadcast network, and the way that the network is represented in
the link-state database.
In NBMA mode, OSPF emulates operation over a broadcast network: a
Designated Router is elected for the NBMA network, and the Designated
Router originates an LSA for the network. The graph representation
for broadcast networks and NBMA networks is identical. This
representation is pictured in the middle of Figure 1a.
NBMA mode is the most efficient way to run OSPF over non-broadcast
networks, both in terms of link-state database size and in terms of
the amount of routing protocol traffic. However, it has one
significant restriction: it requires all routers attached to the NBMA
network to be able to communicate directly. This restriction may be
met on some non-broadcast networks, such as an ATM subnet utilizing
SVCs. But it is often not met on other non-broadcast networks, such
as PVC-only Frame Relay networks. On non-broadcast networks where not
all routers can communicate directly you can break the non-broadcast
network into logical subnets, with the routers on each subnet being
able to communicate directly, and then run each separate subnet as an
NBMA network (see [Ref15]). This however requires quite a bit of
administrative overhead, and is prone to misconfiguration. It is
probably better to run such a non-broadcast network in Point-to-
Multipoint mode.
In Point-to-MultiPoint mode, OSPF treats all router-to-router
connections over the non-broadcast network as if they were point-to-
point links. No Designated Router is elected for the network, nor is
there an LSA generated for the network. In fact, a vertex for the
Point-to-MultiPoint network does not appear in the graph of the
link-state database.
Figure 1b illustrates the link-state database representation of a
Point-to-MultiPoint network. On the left side of the figure, a
Point-to-MultiPoint network is pictured. It is assumed that all
routers can communicate directly, except for routers RT4 and RT5. I3
though I6 indicate the routers" IP interface addresses on the Point-
to-MultiPoint network. In the graphical representation of the link-
state database, routers that can communicate directly over the
Point-to-MultiPoint network are joined by bidirectional edges, and
each router also has a stub connection to its own IP interface
address (which is in contrast to the representation of real point-
to-point links; see Figure 1a).
On some non-broadcast networks, use of Point-to-MultiPoint mode and
data-link protocols such as Inverse ARP (see [Ref14]) will allow
autodiscovery of OSPF neighbors even though broadcast support is not
available.
2.1.2. An example link-state database
Figure 2 shows a sample map of an Autonomous System. The rectangle
labelled H1 indicates a host, which has a SLIP connection to Router
RT12. Router RT12 is therefore advertising a host route. Lines
between routers indicate physical point-to-point networks. The only
point-to-point network that has been assigned interface addresses is
the one joining Routers RT6 and RT10. Routers RT5 and RT7 have BGP
connections to other Autonomous Systems. A set of BGP-learned routes
have been displayed for both of these routers.
A cost is associated with the output side of each router interface.
This cost is configurable by the system administrator. The lower the
cost,the more likely the interface is to be used to forward data
traffic. Costs are also associated with the externally derived
routing data (e.g., the BGP-learned routes).
The directed graph resulting from the map in Figure 2 is depicted in
Figure 3. Arcs are labelled with the cost of the corresponding
router output interface. Arcs having no labelled cost have a cost of
0. Note that arcs leading from networks to routers always have cost
0; they are significant nonetheless. Note also that the externally
derived routing data appears on the graph as stubs.
**FROM**
+---+ +---+
RT3 RT4 RT3RT4RT5RT6
+---+ +---+ * --------------------
I3 N2 I4 * RT3 X X X
+----------------------+ T RT4 X X
I5 I6 O RT5 X X
+---+ +---+ * RT6 X X X
RT5 RT6 * I3 X
+---+ +---+ I4 X
I5 X
I6 X
Figure 1b: Network map components
Point-to-MultiPoint networks
All routers can communicate directly over N2, except
routers RT4 and RT5. I3 through I6 indicate IP
interface addresses
+
3+---+ N12 N14
N1--RT1 1 N13 /
+---+ 8 8/8
+ ____ /
/ 1+---+8 8+---+6
* N3 *---RT4------RT5--------+
____/ +---+ +---+
+ / 7
3+---+ /
N2--RT2/1 1 6
+---+ +---+8 6+---+
+ RT3--------------RT6
+---+ +---+
2 Ia7

+---------+
N4


N11
+---------+
N12
3 6 2/
+---+ +---+/
RT9 RT7---N15
+---+ +---+ 9
1 + 1
___ Ib5 ___
/ 1+----+2 3+----+1 /
* N9 *------RT11-------RT10---* N6 *
____/ +----+ +----+ ____/

1 + 1
+--+ 10+----+ N8 +---+
H1-----RT12 RT8
+--+SLIP +----+ +---+
2 4

+---------+ +--------+
N10 N7
Figure 2: A sample Autonomous System
**FROM**
RTRTRTRTRTRTRTRTRTRTRTRT
1 2 3 4 5 6 7 8 9 101112N3N6N8N9
----- ---------------------------------------------
RT1 0
RT2 0
RT3 6 0
RT4 80
RT5 8 6 6
RT6 8 7 5
RT7 6 0
* RT8 0
* RT9 0
T RT10 70 0
O RT11 0 0
* RT12 0
* N13
N2 3
N31 1 1 1
N4 2
N6 1 1 1
N7 4
N8 3 2
N91 1 1
N10 2
N113
N12 8 2
N13 8
N14 8
N15 9
H1 10
Figure 3: The resulting directed graph
Networks and routers are represented by vertices.
An edge of cost X connects Vertex A to Vertex B iff
the intersection of Column A and Row B is marked
with an X.
The link-state database is pieced together from LSAs generated by the
routers. In the associated graphical representation, the
neighborhood of each router or transit network is represented in a
single, separate LSA. Figure 4 shows these LSAs graphically. Router
RT12 has an interface to two broadcast networks and a SLIP line to a
host. Network N6 is a broadcast network with three attached routers.
The cost of all links from Network N6 to its attached routers is 0.
Note that the LSA for Network N6 is actually generated by one of the
network"s attached routers: the router that has been elected
Designated Router for the network.
2.2. The shortest-path tree
When no OSPF areas are configured, each router in the Autonomous
System has an identical link-state database, leading to an identical
graphical representation. A router generates its routing table from
this graph by calculating a tree of shortest paths with the router
itself as root. Obviously, the shortest- path tree depends on the
router doing the calculation. The shortest-path tree for Router RT6
in our example is depicted in Figure 5.
The tree gives the entire path to any destination network or host.
However, only the next hop to the destination is used in the
forwarding process. Note also that the best route to any router has
also been calculated. For the processing of external data, we note
the next hop and distance to any router advertising external routes.
The resulting routing table for Router RT6 is pictured in Table 2.
Note that there is a separate route for each end of a numbered
point-to-point network (in this case, the serial line between Routers
RT6 and RT10).
**FROM** **FROM**
RT12N9N10H1 RT9RT11RT12N9
* -------------------- * ----------------------
* RT12 * RT9 0
T N91 T RT11 0
O N102 O RT12 0
* H110 * N9
* *
RT12"s router-LSA N9"s network-LSA
Figure 4: Individual link state components
Networks and routers are represented by vertices.
An edge of cost X connects Vertex A to Vertex B iff
the intersection of Column A and Row B is marked
with an X.
RT6(origin)
RT5 o------------o-----------o Ib
/ 6 7
8/88
/ 6
o o 7
N12 o N14
N13 2
N4 o-----o RT3
/ 5
1/ RT10 o-------o Ia
/
RT4 o-----o N3 3 1
/ N6 RT7
/ N8 o o---------o
/ /
RT2 o o RT1 2/ 9
/ RT8 /
/3 3 RT11 o o o o
/ N12 N15
N2 o o N1 1 4

N9 o o N7
/
/
N11 RT9 / RT12
o--------o-------o o--------o H1
3 10
2

o N10
Figure 5: The SPF tree for Router RT6
Edges that are not marked with a cost have a cost of of zero (these
are network-to-router links). Routes to networks N12-N15 are external
information that is considered in Section 2.3
Destination Next Hop Distance
__________________________________
N1 RT3 10
N2 RT3 10
N3 RT3 7
N4 RT3 8
Ib * 7
Ia RT10 12
N6 RT10 8
N7 RT10 12
N8 RT10 10
N9 RT10 11
N10 RT10 13
N11 RT10 14
H1 RT10 21
__________________________________
RT5 RT5 6
RT7 RT10 8
Table 2: The portion of Router RT6"s routing table listing local
destinations.
Routes to networks belonging to other AS"es (such as N12) appear as
dashed lines on the shortest path tree in Figure 5. Use of this
externally derived routing information is considered in the next
section.
2.3. Use of external routing information
After the tree is created the external routing information is
examined. This external routing information may originate from
another routing protocol such as BGP, or be statically configured
(static routes). Default routes can also be included as part of the
Autonomous System"s external routing information.
External routing information is flooded unaltered throughout the AS.
In our example, all the routers in the Autonomous System know that
Router RT7 has two external routes, with metrics 2 and 9.
OSPF supports two types of external metrics. Type 1 external metrics
are expressed in the same units as OSPF interface cost (i.e., in
terms of the link state metric). Type 2 external metrics are an
order of magnitude larger; any Type 2 metric is considered greater
than the cost of any path internal to the AS. Use of Type 2 external
metrics assumes that routing between AS"es is the major cost of
routing a packet, and eliminates the need for conversion of external
costs to internal link state metrics.
As an example of Type 1 external metric processing, suppose that the
Routers RT7 and RT5 in Figure 2 are advertising Type 1 external
metrics. For each advertised external route, the total cost from
Router RT6 is calculated as the sum of the external route"s
advertised cost and the distance from Router RT6 to the advertising
router. When two routers are advertising the same external
destination, RT6 picks the advertising router providing the minimum
total cost. RT6 then sets the next hop to the external destination
equal to the next hop that would be used when routing packets to the
chosen advertising router.
In Figure 2, both Router RT5 and RT7 are advertising an external
route to destination Network N12. Router RT7 is preferred since it
is advertising N12 at a distance of 10 (8+2) to Router RT6, which is
better than Router RT5"s 14 (6+8). Table 3 shows the entries that
are added to the routing table when external routes are examined:
Destination Next Hop Distance
__________________________________
N12 RT10 10
N13 RT5 14
N14 RT5 14
N15 RT10 17
Table 3: The portion of Router RT6"s routing table
listing external destinations.
Processing of Type 2 external metrics is simpler. The AS boundary
router advertising the smallest external metric is chosen, regardless
of the internal distance to the AS boundary router. Suppose in our
example both Router RT5 and Router RT7 were advertising Type 2
external routes. Then all traffic destined for Network N12 would be
forwarded to Router RT7, since 2 < 8. When several equal-cost Type 2
routes exist, the internal distance to the advertising routers is
used to break the tie.
Both Type 1 and Type 2 external metrics can be present in the AS at
the same time. In that event, Type 1 external metrics always take
precedence.
This section has assumed that packets destined for external
destinations are always routed through the advertising AS boundary
router. This is not always desirable. For example, suppose in
Figure 2 there is an additional router attached to Network N6, called
Router RTX. Suppose further that RTX does not participate in OSPF
routing, but does exchange BGP information with the AS boundary
router RT7. Then, Router RT7 would end up advertising OSPF external
routes for all destinations that should be routed to RTX. An extra
hop will sometimes be introduced if packets for these destinations
need always be routed first to Router RT7 (the advertising router).
To deal with this situation, the OSPF protocol allows an AS boundary
router to specify a "forwarding address" in its AS- external-LSAs. In
the above example, Router RT7 would specify RTX"s IP address as the
"forwarding address" for all those destinations whose packets should
be routed directly to RTX.
The "forwarding address" has one other application. It enables
routers in the Autonomous System"s interior to function as "route
servers". For example, in Figure 2 the router RT6 could become a
route server, gaining external routing information through a
combination of static configuration and external routing protocols.
RT6 would then start advertising itself as an AS boundary router, and
would originate a collection of OSPF AS-external-LSAs. In each AS-
external-LSA, Router RT6 would specify the correct Autonomous System
exit point to use for the destination through appropriate setting of
the LSA"s "forwarding address" field.
2.4. Equal-cost multipath
The above discussion has been simplified by considering only a single
route to any destination. In reality, if multiple equal-cost routes
to a destination exist, they are all discovered and used. This
requires no conceptual changes to the algorithm, and its discussion
is postponed until we consider the tree-building process in more
detail.
With equal cost multipath, a router potentially has several available
next hops towards any given destination.
3. Splitting the AS into Areas
OSPF allows collections of contiguous networks and hosts to be
grouped together. Such a group, together with the routers having
interfaces to any one of the included networks, is called an area.
Each area runs a separate copy of the basic link-state routing
algorithm. This means that each area has its own link-state database
and corresponding graph, as explained in the previous section.
The topology of an area is invisible from the outside of the area.
Conversely, routers internal to a given area know nothing of the
detailed topology external to the area. This isolation of knowledge
enables the protocol to effect a marked reduction in routing traffic
as compared to treating the entire Autonomous System as a single
link-state domain.
With the introduction of areas, it is no longer true that all routers
in the AS have an identical link-state database. A router actually
has a separate link-state database for each area it is connected to.
(Routers connected to multiple areas are called area border routers).
Two routers belonging to the same area have, for that area, identical
area link-state databases.
Routing in the Autonomous System takes place on two levels, depending
on whether the source and destination of a packet reside in the same
area (intra-area routing is used) or different areas (inter-area
routing is used). In intra-area routing, the packet is routed solely
on information obtained within the area; no routing information
obtained from outside the area can be used. This protects intra-area
routing from the injection of bad routing information. We discuss
inter-area routing in Section 3.2.
3.1. The backbone of the Autonomous System
The OSPF backbone is the special OSPF Area 0 (often written as Area
0.0.0.0, since OSPF Area ID"s are typically formatted as IP
addresses). The OSPF backbone always contains all area border
routers. The backbone is responsible for distributing routing
information between non-backbone areas. The backbone must be
contiguous. However, it need not be physically contiguous; backbone
connectivity can be established/maintained through the configuration
of virtual links.
Virtual links can be configured between any two backbone routers that
have an interface to a common non-backbone area. Virtual links
belong to the backbone. The protocol treats two routers joined by a
virtual link as if they were connected by an unnumbered point-to-
point backbone network. On the graph of the backbone, two such
routers are joined by arcs whose costs are the intra-area distances
between the two routers. The routing protocol traffic that flows
along the virtual link uses intra-area routing only.
3.2. Inter-area routing
When routing a packet between two non-backbone areas the backbone is
used. The path that the packet will travel can be broken up into
three contiguous pieces: an intra-area path from the source to an
area border router, a backbone path between the source and
destination areas, and then another intra-area path to the
destination. The algorithm finds the set of such paths that have the
smallest cost.
Looking at this another way, inter-area routing can be pictured as
forcing a star configuration on the Autonomous System, with the
backbone as hub and each of the non-backbone areas as spokes.
The topology of the backbone dictates the backbone paths used between
areas. The topology of the backbone can be enhanced by adding
virtual links. This gives the system administrator some control over
the routes taken by inter-area traffic.
The correct area border router to use as the packet exits the source
area is chosen in exactly the same way routers advertising external
routes are chosen. Each area border router in an area summarizes for
the area its cost to all networks external to the area. After the
SPF tree is calculated for the area, routes to all inter-area
destinations are calculated by examining the summaries of the area
border routers.
3.3. Classification of routers
Before the introduction of areas, the only OSPF routers having a
specialized function were those advertising external routing
information, such as Router RT5 in Figure 2. When the AS is split
into OSPF areas, the routers are further divided according to
function into the following four overlapping categories:
Internal routers
A router with all directly connected networks belonging to the
same area. These routers run a single copy of the basic routing
algorithm.
Area border routers
A router that attaches to multiple areas. Area border routers run
multiple copies of the basic algorithm, one copy for each attached
area. Area border routers condense the topological information of
their attached areas for distribution to the backbone. The
backbone in turn distributes the information to the other areas.
Backbone routers
A router that has an interface to the backbone area. This
includes all routers that interface to more than one area (i.e.,
area border routers). However, backbone routers do not have to be
area border routers. Routers with all interfaces connecting to
the backbone area are supported.
AS boundary routers
A router that exchanges routing information with routers belonging
to other Autonomous Systems. Such a router advertises AS external
routing information throughout the Autonomous System. The paths
to each AS boundary router are known by every router in the AS.
This classification is completely independent of the previous
classifications: AS boundary routers may be internal or area
border routers, and may or may not participate in the backbone.
3.4. A sample area configuration
Figure 6 shows a sample area configuration. The first area consists
of networks N1-N4, along with their attached routers RT1-RT4. The
second area consists of networks N6-N8, along with their attached
routers RT7, RT8, RT10 and RT11. The third area consists of networks
N9-N11 and Host H1, along with their attached routers RT9, RT11 and
RT12. The third area has been configured so that networks N9-N11 and
Host H1 will all be grouped into a single route, when advertised
external to the area (see Section 3.5 for more details).
In Figure 6, Routers RT1, RT2, RT5, RT6, RT8, RT9 and RT12 are
internal routers. Routers RT3, RT4, RT7, RT10 and RT11 are area
border routers. Finally, as before, Routers RT5 and RT7 are AS
boundary routers.
Figure 7 shows the resulting link-state database for the Area 1. The
figure completely describes that area"s intra-area routing.
...........................
. + .
. 3+---+ . N12 N14
. N1--RT1 1 . N13 /
. +---+ . 8 8/8
. + ____ . /
. / 1+---+8 8+---+6
. * N3 *---RT4------RT5--------+
. ____/ +---+ +---+
. + / . 7
. 3+---+ / .
. N2--RT2/1 1 . 6
. +---+ +---+8 6+---+
. + RT3------RT6
. +---+ +---+
. 2/ . Ia7
. / .
. +---------+ .
.Area 1 N4 .
...........................
..........................
. N11 .
. +---------+ .
. . N12
. 3 . Ib5 6 2/
. +---+ . +----+ +---+/
. RT9 . .........RT10.....RT7---N15.
. +---+ . . +----+ +---+ 9 .
. 1 . . + /3 1 1 .
. ___ . . / ___ .
. / 1+----+2 / / .
. * N9 *------RT11---- * N6 * .
. ____/ +----+ ____/ .
. . . .
. 1 . . + 1 .
. +--+ 10+----+ . . N8 +---+ .
. H1-----RT12 . . RT8 .
. +--+SLIP +----+ . . +---+ .
. 2 . . 4 .
. . . .
. +---------+ . . +--------+ .
. N10 . . N7 .
. . .Area 2 .
.Area 3 . ................................
..........................
Figure 6: A sample OSPF area configuration
It also shows the complete view of the internet for the two internal
routers RT1 and RT2. It is the job of the area border routers, RT3
and RT4, to advertise into Area 1 the distances to all destinations
external to the area. These are indicated in Figure 7 by the dashed
stub routes. Also, RT3 and RT4 must advertise into Area 1 the
location of the AS boundary routers RT5 and RT7. Finally, AS-
external-LSAs from RT5 and RT7 are flooded throughout the entire AS,
and in particular throughout Area 1. These LSAs are included in Area
1"s database, and yield routes to Networks N12-N15.
Routers RT3 and RT4 must also summarize Area 1"s topology for
distribution to the backbone. Their backbone LSAs are shown in Table
4. These summaries show which networks are contained in Area 1
(i.e., Networks N1-N4), and the distance to these networks from the
routers RT3 and RT4 respectively.
The link-state database for the backbone is shown in Figure 8. The
set of routers pictured are the backbone routers. Router RT11 is a
backbone router because it belongs to two areas. In order to make
the backbone connected, a virtual link has been configured between
Routers R10 and R11.
The area border routers RT3, RT4, RT7, RT10 and RT11 condense the
routing information of their attached non-backbone areas for
distribution via the backbone; these are the dashed stubs that appear
in Figure 8. Remember that the third area has been configured to
condense Networks N9-N11 and Host H1 into a single route. This
yields a single dashed line for networks N9-N11 and Host H1 in Figure
8. Routers RT5 and RT7 are AS boundary routers; their externally
derived information also appears on the graph in Figure 8 as stubs.
Network RT3 adv. RT4 adv.
_____________________________
N1 4 4
N2 4 4
N3 1 1
N4 2 3
Table 4: Networks advertised to the backbone
by Routers RT3 and RT4.
RTRTRTRTRTRT
1 2 3 4 5 7 N3
----- -------------------
RT1 0
RT2 0
RT3 0
* RT4 0
* RT5 148
T RT7 2014
O N13
* N2 3
* N31 1 1 1
N4 2
Ia,Ib 2027
N6 1615
N7 2019
N8 1818
N9-N11,H1 2936
N12 8 2
N13 8
N14 8
N15 9
Figure 7: Area 1"s Database.
Networks and routers are represented by vertices.
An edge of cost X connects Vertex A to Vertex B iff
the intersection of Column A and Row B is marked
with an X.
**FROM**
RTRTRTRTRTRTRT
3 4 5 6 7 1011
------------------------
RT3 6
RT4 8
RT5 8 6 6
RT68 7 5
RT7 6
* RT10 7 2
* RT11 3
T N14 4
O N24 4
* N31 1
* N42 3
Ia 5
Ib 7
N6 1 1 3
N7 5 5 7
N8 4 3 2
N9-N11,H1 11
N12 8 2
N13 8
N14 8
N15 9
Figure 8: The backbone"s database.
Networks and routers are represented by vertices.
An edge of cost X connects Vertex A to Vertex B iff
the intersection of Column A and Row B is marked
with an X.
The backbone enables the exchange of summary information between area
border routers. Every area border router hears the area summaries
from all other area border routers. It then forms a picture of the
distance to all networks outside of its area by examining the
collected LSAs, and adding in the backbone distance to each
advertising router.
Again using Routers RT3 and RT4 as an example, the procedure goes as
follows: They first calculate the SPF tree for the backbone. This
gives the distances to all other area border routers. Also noted are
the distances to networks (Ia and Ib) and AS boundary routers (RT5
and RT7) that belong to the backbone. This calculation is shown in
Table 5.
Next, by looking at the area summaries from these area border
routers, RT3 and RT4 can determine the distance to all networks
outside their area. These distances are then advertised internally
to the area by RT3 and RT4. The advertisements that Router RT3 and
RT4 will make into Area 1 are shown in Table 6. Note that Table 6
assumes that an area range has been configured for the backbone which
groups Ia and Ib into a single LSA.
The information imported into Area 1 by Routers RT3 and RT4 enables
an internal router, such as RT1, to choose an area border router
intelligently. Router RT1 would use RT4 for traffic to Network N6,
RT3 for traffic to Network N10, and would load share between the two
for traffic to Network N8.
dist from dist from
RT3 RT4
__________________________________
to RT3 * 21
to RT4 22 *
to RT7 20 14
to RT10 15 22
to RT11 18 25
__________________________________
to Ia 20 27
to Ib 15 22
__________________________________
to RT5 14 8
to RT7 20 14
Table 5: Backbone distances calculated
by Routers RT3 and RT4.
Destination RT3 adv. RT4 adv.
_________________________________
Ia,Ib 20 27
N6 16 15
N7 20 19
N8 18 18
N9-N11,H1 29 36
_________________________________
RT5 14 8
RT7 20 14
Table 6: Destinations advertised into Area 1
by Routers RT3 and RT4.
Router RT1 can also determine in this manner the shortest path to the
AS boundary routers RT5 and RT7. Then, by looking at RT5 and RT7"s
AS-external-LSAs, Router RT1 can decide between RT5 or RT7 when
sending to a destination in another Autonomous System (one of the
networks N12-N15).
Note that a failure of the line between Routers RT6 and RT10 will
cause the backbone to become disconnected. Configuring a virtual
link between Routers RT7 and RT10 will give the backbone more
connectivity and more resistance to such failures.
3.5. IP subnetting support
OSPF attaches an IP address mask to each advertised route. The mask
indicates the range of addresses being described by the particular
route. For example, a summary-LSA for the destination 128.185.0.0
with a mask of 0xffff0000 actually is describing a single route to
the collection of destinations 128.185.0.0 - 128.185.255.255.
Similarly, host routes are always advertised with a mask of
0xffffffff, indicating the presence of only a single destination.
Including the mask with each advertised destination enables the
implementation of what is commonly referred to as variable-length
subnetting. This means that a single IP class A, B, or C network
number can be broken up into many subnets of various sizes. For
example, the network 128.185.0.0 could be broken up into 62
variable-sized subnets: 15 subnets of size 4K, 15 subnets of size
256, and 32 subnets of size 8. Table 7 shows some of the resulting
network addresses together with their masks.
Network address IP address mask Subnet size
_______________________________________________
128.185.16.0 0xfffff000 4K
128.185.1.0 0xffffff00 256
128.185.0.8 0xfffffff8 8
Table 7: Some sample subnet sizes.
There are many possible ways of dividing up a class A, B, and C
network into variable sized subnets. The precise procedure for doing
so is beyond the scope of this specification. This specification
however establishes the following guideline: When an IP packet is
forwarded, it is always forwarded to the network that is the best
match for the packet"s destination. Here best match is synonymous
with the longest or most specific match. For example, the default
route with destination of 0.0.0.0 and mask 0x00000000 is always a
match for every IP destination. Yet it is always less specific than
any other match. Subnet masks must be assigned so that the best
match for any IP destination is unambiguous.
Attaching an address mask to each route also enables the support of
IP supernetting. For example, a single physical network segment could
be assigned the [address,mask] pair [192.9.4.0,0xfffffc00]. The
segment would then be single IP network, containing addresses from
the four consecutive class C network numbers 192.9.4.0 through
192.9.7.0. Such addressing is now becoming commonplace with the
advent of CIDR (see [Ref10]).
In order to get better aggregation at area boundaries, area address
ranges can be employed (see Section C.2 for more details). Each
address range is defined as an [address,mask] pair. Many separate
networks may then be contained in a single address range, just as a
subnetted network is composed of many separate subnets. Area border
routers then summarize the area contents (for distribution to the
backbone) by advertising a single route for each address range. The
cost of the route is the maximum cost to any of the networks falling
in the specified range.
For example, an IP subnetted network might be configured as a single
OSPF area. In that case, a single address range could be configured:
a class A, B, or C network number along with its natural IP mask.
Inside the area, any number of variable sized subnets could be
defined. However, external to the area a single route for the entire
subnetted network would be distributed, hiding even the fact that the
network is subnetted at all. The cost of this route is the maximum
of the set of