rfc2328.hastabs.txt

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Network	Working	Group						  J. Moy
Request	for Comments: 2328		     Ascend Communications, Inc.
STD: 54							      April 1998
Obsoletes: 2178
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.

Copyright Notice

    Copyright (C) The Internet Society (1998).	All Rights Reserved.

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 2178 are explained in
    Appendix G.	All differences	are backward-compatible	in nature.




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    Implementations of this memo and of	RFCs 2178, 1583, and 1247 will
    interoperate.

    Please send	comments to [e-mail].

Table of Contents

    1	     Introduction ........................................... 6
    1.1	     Protocol Overview ...................................... 6
    1.2	     Definitions of commonly used terms	..................... 8
    1.3	     Brief history of link-state routing technology ........ 11
    1.4	     Organization of this document ......................... 12
    1.5	     Acknowledgments ....................................... 12
    2	     The link-state database: organization and calculations  13
    2.1	     Representation of routers and networks ................ 13
    2.1.1    Representation of non-broadcast networks .............. 15
    2.1.2    An	example	link-state database ........................ 18
    2.2	     The shortest-path tree ................................ 21
    2.3	     Use of external routing information ................... 23
    2.4	     Equal-cost	multipath .................................. 26
    3	     Splitting the AS into Areas ........................... 26
    3.1	     The backbone of the Autonomous System ................. 27
    3.2	     Inter-area	routing	.................................... 27
    3.3	     Classification of routers ............................. 28
    3.4	     A sample area configuration ........................... 29
    3.5	     IP	subnetting support ................................. 35
    3.6	     Supporting	stub areas ................................. 37
    3.7	     Partitions	of areas ................................... 38
    4	     Functional	Summary	.................................... 40
    4.1	     Inter-area	routing	.................................... 41
    4.2	     AS	external routes	.................................... 41
    4.3	     Routing protocol packets .............................. 42
    4.4	     Basic implementation requirements ..................... 43
    4.5	     Optional OSPF capabilities	............................ 46
    5	     Protocol data structures .............................. 47
    6	     The Area Data Structure ............................... 49
    7	     Bringing Up Adjacencies ............................... 52
    7.1	     The Hello Protocol	.................................... 52
    7.2	     The Synchronization of Databases ...................... 53
    7.3	     The Designated Router ................................. 54
    7.4	     The Backup	Designated Router .......................... 56
    7.5	     The graph of adjacencies .............................. 56



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    8	     Protocol Packet Processing	............................ 58
    8.1	     Sending protocol packets .............................. 58
    8.2	     Receiving protocol	packets	............................ 61
    9	     The Interface Data	Structure .......................... 63
    9.1	     Interface states ...................................... 67
    9.2	     Events causing interface state changes ................ 70
    9.3	     The Interface state machine ........................... 72
    9.4	     Electing the Designated Router ........................ 75
    9.5	     Sending Hello packets ................................. 77
    9.5.1    Sending Hello packets on NBMA networks ................ 79
    10	     The Neighbor Data Structure ........................... 80
    10.1     Neighbor states ....................................... 83
    10.2     Events causing neighbor state changes ................. 87
    10.3     The Neighbor state	machine	............................ 89
    10.4     Whether to	become adjacent	............................ 95
    10.5     Receiving Hello Packets ............................... 96
    10.6     Receiving Database	Description Packets ................ 99
    10.7     Receiving Link State Request Packets ................. 102
    10.8     Sending Database Description Packets ................. 103
    10.9     Sending Link State	Request	Packets	................... 104
    10.10    An	Example	........................................... 105
    11	     The Routing Table Structure .......................... 107
    11.1     Routing table lookup ................................. 111
    11.2     Sample routing table, without areas .................. 111
    11.3     Sample routing table, with	areas ..................... 112
    12	     Link State	Advertisements (LSAs) ..................... 115
    12.1     The LSA Header ....................................... 116
    12.1.1   LS	age ............................................... 116
    12.1.2   Options .............................................. 117
    12.1.3   LS	type .............................................. 117
    12.1.4   Link State	ID ........................................ 117
    12.1.5   Advertising Router	................................... 119
    12.1.6   LS	sequence number	................................... 120
    12.1.7   LS	checksum .......................................... 121
    12.2     The link state database .............................. 121
    12.3     Representation of TOS ................................ 122
    12.4     Originating LSAs ..................................... 123
    12.4.1   Router-LSAs .......................................... 126
    12.4.1.1 Describing	point-to-point interfaces ................. 130
    12.4.1.2 Describing	broadcast and NBMA interfaces ............. 130
    12.4.1.3 Describing	virtual	links ............................. 131
    12.4.1.4 Describing	Point-to-MultiPoint interfaces ............ 131



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    12.4.1.5 Examples of router-LSAs .............................. 132
    12.4.2   Network-LSAs ......................................... 133
    12.4.2.1 Examples of network-LSAs ............................. 134
    12.4.3   Summary-LSAs ......................................... 135
    12.4.3.1 Originating summary-LSAs into stub	areas ............. 137
    12.4.3.2 Examples of summary-LSAs ............................. 138
    12.4.4   AS-external-LSAs ..................................... 139
    12.4.4.1 Examples of AS-external-LSAs ......................... 140
    13	     The Flooding Procedure ............................... 143
    13.1     Determining which LSA is newer ....................... 146
    13.2     Installing	LSAs in	the database ...................... 147
    13.3     Next step in the flooding procedure .................. 148
    13.4     Receiving self-originated LSAs ....................... 151
    13.5     Sending Link State	Acknowledgment packets ............ 152
    13.6     Retransmitting LSAs .................................. 154
    13.7     Receiving link state acknowledgments ................. 155
    14	     Aging The Link State Database ........................ 156
    14.1     Premature aging of	LSAs .............................. 157
    15	     Virtual Links ........................................ 158
    16	     Calculation of the	routing	table ..................... 160
    16.1     Calculating the shortest-path tree	for an area ....... 161
    16.1.1   The next hop calculation ............................. 167
    16.2     Calculating the inter-area	routes .................... 178
    16.3     Examining transit areas' summary-LSAs ................ 170
    16.4     Calculating AS external routes ....................... 173
    16.4.1   External path preferences ............................ 175
    16.5     Incremental updates -- summary-LSAs .................. 175
    16.6     Incremental updates -- AS-external-LSAs .............. 177
    16.7     Events generated as a result of routing table changes  177
    16.8     Equal-cost	multipath ................................. 178
	     Footnotes ............................................ 179
	     References	........................................... 183
    A	     OSPF data formats .................................... 185
    A.1	     Encapsulation of OSPF packets ........................ 185
    A.2	     The Options field .................................... 187
    A.3	     OSPF Packet Formats .................................. 189
    A.3.1    The OSPF packet header ............................... 190
    A.3.2    The Hello packet ..................................... 193
    A.3.3    The Database Description packet ...................... 195
    A.3.4    The Link State Request packet ........................ 197
    A.3.5    The Link State Update packet ......................... 199
    A.3.6    The Link State Acknowledgment packet ................. 201



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    A.4	     LSA formats .......................................... 203
    A.4.1    The LSA header ....................................... 204
    A.4.2    Router-LSAs .......................................... 206
    A.4.3    Network-LSAs ......................................... 210
    A.4.4    Summary-LSAs ......................................... 212
    A.4.5    AS-external-LSAs ..................................... 214
    B	     Architectural Constants .............................. 217
    C	     Configurable Constants ............................... 219
    C.1	     Global parameters .................................... 219
    C.2	     Area parameters ...................................... 220
    C.3	     Router interface parameters .......................... 221
    C.4	     Virtual link parameters .............................. 224
    C.5	     NBMA network parameters .............................. 224
    C.6	     Point-to-MultiPoint network parameters ............... 225
    C.7	     Host route	parameters ................................ 226
    D	     Authentication ....................................... 227
    D.1	     Null authentication .................................. 227
    D.2	     Simple password authentication ....................... 228
    D.3	     Cryptographic authentication ......................... 228
    D.4	     Message generation	................................... 231
    D.4.1    Generating	Null authentication ....................... 231
    D.4.2    Generating	Simple password	authentication ............ 232
    D.4.3    Generating	Cryptographic authentication .............. 232
    D.5	     Message verification ................................. 234
    D.5.1    Verifying Null authentication ........................ 234
    D.5.2    Verifying Simple password authentication ............. 234
    D.5.3    Verifying Cryptographic authentication ............... 235
    E	     An	algorithm for assigning	Link State IDs ............ 236
    F	     Multiple interfaces to the	same network/subnet ....... 239
    G	     Differences from RFC 2178 ............................ 240
    G.1	     Flooding modifications ............................... 240
    G.2	     Changes to	external path preferences ................. 241
    G.3	     Incomplete	resolution of virtual next hops	........... 241
    G.4	     Routing table lookup ................................. 241
	     Security Considerations .............................. 243
	     Author's Address ..................................... 243
	     Full Copyright Statement ............................. 244








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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.





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	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