RFC 2178
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 RFC 1583 are explained in Appendix G. All differences are backward-compatible in nature. Implementations of this memo and of RFC 1583 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 RFC 1583 ............................ 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**
* |RT1|RT2|
+---+Ia +---+ * ------------
|RT1|------|RT2| T RT1| | X |
+---+ Ib+---+ O RT2| X | |
* Ia| | X |
* Ib| X | |
Physical point-to-point networks
**FROM**
+---+ *
|RT7| * |RT7| N3|
+---+ T ------------
| O RT7| | |
+----------------------+ * N3| X | |
N3 *
Stub networks
+---+ +---+
|RT3| |RT4| |RT3|RT4|RT5|RT6|N2 |
+---+ +---+ * ------------------------
| N2 | * RT3| | | | | X |
+----------------------+ T RT4| | | | | X |
| | O RT5| | | | | X |
+---+ +---+ * RT6| | | | | X |
|RT5| |RT6| * N2| X | X | X | X | |
+---+ +---+
Broadcast or NBMA networks
Figure 1a: Network map components
Networks and routers are represented by vertices. An edge connects
Vertex A to Vertex B iff the intersection of Column A and Row B is
marked with an X.
The top of Figure 1a shows two routers connected by a point-to-point
link. In the resulting link-state database graph, the two router
vertices are directly connected by a pair of edges, one in each
direction. Interfaces to point-to-point networks need not be assigned
IP addresses. When interface addresses are assigned, they are
modelled as stub links, with each router advertising a stub
connection to the other router's interface address. Optionally, an IP
subnet can be assigned to the point-to-point network. In this case, both routers advertise a stub link to the IP subnet, instead of advertising each others' IP interface addresses. The middle of Figure 1a shows a network with only one attached router (i.e., a stub network). In this case, the network appears on the end of a stub connection in the link-state database's graph. When multiple routers are attached to a broadcast network, the link- state database graph shows all routers bidirectionally connected to the network vertex. This is pictured at the bottom of Figure 1a. Each network (stub or transit) in the graph has an IP address and associated network mask. The mask indicates the number of nodes on the network. Hosts attached directly to routers (referred to as host routes) appear on the graph as stub networks. The network mask for a host route is always 0xffffffff, which indicates the presence of a single node. 2.1.1. Representation of non-broadcast networks As mentioned previously, OSPF can run over non-broadcast networks in one of two modes: NBMA or Point-to-MultiPoint. The choice of mode determines the way that the Hello protocol and flooding work over the non-broadcast network, and the way that the network is represented in the link-state database. In NBMA mode, OSPF emulates operation over a broadcast network: a Designated Router is elected for the NBMA network, and the Designated Router originates an LSA for the network. The graph representation for broadcast networks and NBMA networks is identical. This representation is pictured in the middle of Figure 1a. NBMA mode is the most efficient way to run OSPF over non-broadcast networks, both in terms of link-state database size and in terms of the amount of routing protocol traffic. However, it has one significant restriction: it requires all routers attached to the NBMA network to be able to communicate directly. This restriction may be met on some non-broadcast networks, such as an ATM subnet utilizing SVCs. But it is often not met on other non-broadcast networks, such as PVC-only Frame Relay networks. On non-broadcast networks where not all routers can communicate directly you can break the non-broadcast network into logical subnets, with the routers on each subnet being able to communicate directly, and then run each separate subnet as an NBMA network (see [Ref15]). This however requires quite a bit of administrative overhead, and is prone to misconfiguration. It is probably better to run such a non-broadcast network in Point-to- Multipoint mode.
In Point-to-MultiPoint mode, OSPF treats all router-to-router connections over the non-broadcast network as if they were point-to- point links. No Designated Router is elected for the network, nor is there an LSA generated for the network. In fact, a vertex for the Point-to-MultiPoint network does not appear in the graph of the link-state database. Figure 1b illustrates the link-state database representation of a Point-to-MultiPoint network. On the left side of the figure, a Point-to-MultiPoint network is pictured. It is assumed that all routers can communicate directly, except for routers RT4 and RT5. I3 though I6 indicate the routers' IP interface addresses on the Point- to-MultiPoint network. In the graphical representation of the link- state database, routers that can communicate directly over the Point-to-MultiPoint network are joined by bidirectional edges, and each router also has a stub connection to its own IP interface address (which is in contrast to the representation of real point- to-point links; see Figure 1a). On some non-broadcast networks, use of Point-to-MultiPoint mode and data-link protocols such as Inverse ARP (see [Ref14]) will allow autodiscovery of OSPF neighbors even though broadcast support is not available. 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| |RT3|RT4|RT5|RT6|
+---+ +---+ * --------------------
I3| N2 |I4 * RT3| | X | X | X |
+----------------------+ T RT4| X | | | X |
I5| |I6 O RT5| X | | | X |
+---+ +---+ * RT6| X | X | X | |
|RT5| |RT6| * I3| X | | | |
+---+ +---+ I4| | X | | |
I5| | | X | |
I6| | | | X |
Figure 1b: Network map components
Point-to-MultiPoint networks
All routers can communicate directly over N2, except
routers RT4 and RT5. I3 through I6 indicate IP
interface addresses
+
| 3+---+ N12 N14
N1|--|RT1|\ 1 \ N13 /
| +---+ \ 8\ |8/8
+ \ ____ \|/
/ \ 1+---+8 8+---+6
* N3 *---|RT4|------|RT5|--------+
\____/ +---+ +---+ |
+ / | |7 |
| 3+---+ / | | |
N2|--|RT2|/1 |1 |6 |
| +---+ +---+8 6+---+ |
+ |RT3|--------------|RT6| |
+---+ +---+ |
|2 Ia|7 |
| | |
+---------+ | |
N4 | |
| |
| |
N11 | |
+---------+ | |
| | | N12
|3 | |6 2/
+---+ | +---+/
|RT9| | |RT7|---N15
+---+ | +---+ 9
|1 + | |1
_|__ | Ib|5 __|_
/ \ 1+----+2 | 3+----+1 / \
* N9 *------|RT11|----|---|RT10|---* N6 *
\____/ +----+ | +----+ \____/
| | |
|1 + |1
+--+ 10+----+ N8 +---+
|H1|-----|RT12| |RT8|
+--+SLIP +----+ +---+
|2 |4
| |
+---------+ +--------+
N10 N7
Figure 2: A sample Autonomous System
**FROM**
|RT|RT|RT|RT|RT|RT|RT|RT|RT|RT|RT|RT|
|1 |2 |3 |4 |5 |6 |7 |8 |9 |10|11|12|N3|N6|N8|N9|
----- ---------------------------------------------
RT1| | | | | | | | | | | | |0 | | | |
RT2| | | | | | | | | | | | |0 | | | |
RT3| | | | | |6 | | | | | | |0 | | | |
RT4| | | | |8 | | | | | | | |0 | | | |
RT5| | | |8 | |6 |6 | | | | | | | | | |
RT6| | |8 | |7 | | | | |5 | | | | | | |
RT7| | | | |6 | | | | | | | | |0 | | |
* RT8| | | | | | | | | | | | | |0 | | |
* RT9| | | | | | | | | | | | | | | |0 |
T RT10| | | | | |7 | | | | | | | |0 |0 | |
O RT11| | | | | | | | | | | | | | |0 |0 |
* RT12| | | | | | | | | | | | | | | |0 |
* N1|3 | | | | | | | | | | | | | | | |
N2| |3 | | | | | | | | | | | | | | |
N3|1 |1 |1 |1 | | | | | | | | | | | | |
N4| | |2 | | | | | | | | | | | | | |
N6| | | | | | |1 |1 | |1 | | | | | | |
N7| | | | | | | |4 | | | | | | | | |
N8| | | | | | | | | |3 |2 | | | | | |
N9| | | | | | | | |1 | |1 |1 | | | | |
N10| | | | | | | | | | | |2 | | | | |
N11| | | | | | | | |3 | | | | | | | |
N12| | | | |8 | |2 | | | | | | | | | |
N13| | | | |8 | | | | | | | | | | | |
N14| | | | |8 | | | | | | | | | | | |
N15| | | | | | |9 | | | | | | | | | |
H1| | | | | | | | | | | |10| | | | |
Figure 3: The resulting directed graph
Networks and routers are represented by vertices.
An edge of cost X connects Vertex A to Vertex B iff
the intersection of Column A and Row B is marked
with an X.
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** |RT12|N9|N10|H1| |RT9|RT11|RT12|N9| * -------------------- * ---------------------- * RT12| | | | | * RT9| | | |0 | T N9|1 | | | | T RT11| | | |0 | O N10|2 | | | | O RT12| | | |0 | * H1|10 | | | | * N9| | | | | * * RT12's router-LSA N9's network-LSA Figure 4: Individual link state components Networks and routers are represented by vertices. An edge of cost X connects Vertex A to Vertex B iff the intersection of Column A and Row B is marked with an X.
RT6(origin)
RT5 o------------o-----------o Ib
/|\ 6 |\ 7
8/8|8\ | \
/ | \ 6| \
o | o | \7
N12 o N14 | \
N13 2 | \
N4 o-----o RT3 \
/ \ 5
1/ RT10 o-------o Ia
/ |\
RT4 o-----o N3 3| \1
/| | \ N6 RT7
/ | N8 o o---------o
/ | | | /|
RT2 o o RT1 | | 2/ |9
/ | | |RT8 / |
/3 |3 RT11 o o o o
/ | | | N12 N15
N2 o o N1 1| |4
| |
N9 o o N7
/|
/ |
N11 RT9 / |RT12
o--------o-------o o--------o H1
3 | 10
|2
|
o N10
Figure 5: The SPF tree for Router RT6
Edges that are not marked with a cost have a cost of of zero (these
are network-to-router links). Routes to networks N12-N15 are external
information that is considered in Section 2.3
Destination Next Hop Distance
__________________________________
N1 RT3 10
N2 RT3 10
N3 RT3 7
N4 RT3 8
Ib * 7
Ia RT10 12
N6 RT10 8
N7 RT10 12
N8 RT10 10
N9 RT10 11
N10 RT10 13
N11 RT10 14
H1 RT10 21
__________________________________
RT5 RT5 6
RT7 RT10 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.
(page 22 continued on part 2)
