draft-ietf-ospf-version2-09.txt   draft-ietf-ospf-version2-10.txt 
Network Working Group J. Moy Network Working Group J. Moy
Internet Draft Cascade Communications Corp. Internet Draft Cascade Communications Corp.
Expiration Date: July 1997 January 1997 Expiration Date: August 1997 February 1997
File name: draft-ietf-ospf-version2-09.txt File name: draft-ietf-ospf-version2-10.txt
OSPF Version 2 OSPF Version 2
Status of this Memo Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas, documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts. working documents as Internet-Drafts.
skipping to change at page 3, line 20 skipping to change at page 3, line 20
1.3 Brief history of link-state routing technology ........ 11 1.3 Brief history of link-state routing technology ........ 11
1.4 Organization of this document ......................... 12 1.4 Organization of this document ......................... 12
1.5 Acknowledgments ....................................... 13 1.5 Acknowledgments ....................................... 13
2 The link-state database: organization and calculations 13 2 The link-state database: organization and calculations 13
2.1 Representation of routers and networks ................ 13 2.1 Representation of routers and networks ................ 13
2.1.1 Representation of non-broadcast networks .............. 15 2.1.1 Representation of non-broadcast networks .............. 15
2.1.2 An example link-state database ........................ 16 2.1.2 An example link-state database ........................ 16
2.2 The shortest-path tree ................................ 20 2.2 The shortest-path tree ................................ 20
2.3 Use of external routing information ................... 22 2.3 Use of external routing information ................... 22
2.4 Equal-cost multipath .................................. 24 2.4 Equal-cost multipath .................................. 24
2.5 TOS-based routing ..................................... 24 2.5 TOS-based routing ..................................... 25
3 Splitting the AS into Areas ........................... 25 3 Splitting the AS into Areas ........................... 25
3.1 The backbone of the Autonomous System ................. 26 3.1 The backbone of the Autonomous System ................. 26
3.2 Inter-area routing .................................... 26 3.2 Inter-area routing .................................... 27
3.3 Classification of routers ............................. 27 3.3 Classification of routers ............................. 27
3.4 A sample area configuration ........................... 28 3.4 A sample area configuration ........................... 28
3.5 IP subnetting support ................................. 34 3.5 IP subnetting support ................................. 34
3.6 Supporting stub areas ................................. 35 3.6 Supporting stub areas ................................. 35
3.7 Partitions of areas ................................... 36 3.7 Partitions of areas ................................... 36
4 Functional Summary .................................... 38 4 Functional Summary .................................... 38
4.1 Inter-area routing .................................... 38 4.1 Inter-area routing .................................... 38
4.2 AS external routes .................................... 39 4.2 AS external routes .................................... 39
4.3 Routing protocol packets .............................. 39 4.3 Routing protocol packets .............................. 39
4.4 Basic implementation requirements ..................... 41 4.4 Basic implementation requirements ..................... 42
4.5 Optional OSPF capabilities ............................ 43 4.5 Optional OSPF capabilities ............................ 43
5 Protocol data structures .............................. 44 5 Protocol data structures .............................. 44
6 The Area Data Structure ............................... 46 6 The Area Data Structure ............................... 46
7 Bringing Up Adjacencies ............................... 49 7 Bringing Up Adjacencies ............................... 49
7.1 The Hello Protocol .................................... 49 7.1 The Hello Protocol .................................... 49
7.2 The Synchronization of Databases ...................... 50 7.2 The Synchronization of Databases ...................... 50
7.3 The Designated Router ................................. 51 7.3 The Designated Router ................................. 51
7.4 The Backup Designated Router .......................... 52 7.4 The Backup Designated Router .......................... 52
7.5 The graph of adjacencies .............................. 53 7.5 The graph of adjacencies .............................. 53
8 Protocol Packet Processing ............................ 54 8 Protocol Packet Processing ............................ 53
8.1 Sending protocol packets .............................. 54 8.1 Sending protocol packets .............................. 54
8.2 Receiving protocol packets ............................ 56 8.2 Receiving protocol packets ............................ 56
9 The Interface Data Structure .......................... 59 9 The Interface Data Structure .......................... 59
9.1 Interface states ...................................... 62 9.1 Interface states ...................................... 62
9.2 Events causing interface state changes ................ 64 9.2 Events causing interface state changes ................ 64
9.3 The Interface state machine ........................... 66 9.3 The Interface state machine ........................... 66
9.4 Electing the Designated Router ........................ 68 9.4 Electing the Designated Router ........................ 68
9.5 Sending Hello packets ................................. 71 9.5 Sending Hello packets ................................. 71
9.5.1 Sending Hello packets on NBMA networks ................ 72 9.5.1 Sending Hello packets on NBMA networks ................ 72
10 The Neighbor Data Structure ........................... 73 10 The Neighbor Data Structure ........................... 73
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10.9 Sending Link State Request Packets .................... 94 10.9 Sending Link State Request Packets .................... 94
10.10 An Example ............................................ 95 10.10 An Example ............................................ 95
11 The Routing Table Structure ........................... 95 11 The Routing Table Structure ........................... 95
11.1 Routing table lookup ................................. 100 11.1 Routing table lookup ................................. 100
11.2 Sample routing table, without areas .................. 101 11.2 Sample routing table, without areas .................. 101
11.3 Sample routing table, with areas ..................... 102 11.3 Sample routing table, with areas ..................... 102
12 Link State Advertisements (LSAs) ..................... 103 12 Link State Advertisements (LSAs) ..................... 103
12.1 The LSA Header ....................................... 105 12.1 The LSA Header ....................................... 105
12.1.1 LS age ............................................... 105 12.1.1 LS age ............................................... 105
12.1.2 Options .............................................. 106 12.1.2 Options .............................................. 106
12.1.3 LS type .............................................. 106 12.1.3 LS type .............................................. 107
12.1.4 Link State ID ........................................ 106 12.1.4 Link State ID ........................................ 108
12.1.5 Advertising Router ................................... 108 12.1.5 Advertising Router ................................... 109
12.1.6 LS sequence number ................................... 108 12.1.6 LS sequence number ................................... 109
12.1.7 LS checksum .......................................... 109 12.1.7 LS checksum .......................................... 110
12.2 The link state database .............................. 110 12.2 The link state database .............................. 110
12.3 Representation of TOS ................................ 111 12.3 Representation of TOS ................................ 111
12.4 Originating LSAs ..................................... 112 12.4 Originating LSAs ..................................... 112
12.4.1 Router-LSAs .......................................... 115 12.4.1 Router-LSAs .......................................... 115
12.4.1.1 Describing point-to-point interfaces ................. 117 12.4.1.1 Describing point-to-point interfaces ................. 118
12.4.1.2 Describing broadcast and NBMA interfaces ............. 118 12.4.1.2 Describing broadcast and NBMA interfaces ............. 119
12.4.1.3 Describing virtual links ............................. 119 12.4.1.3 Describing virtual links ............................. 119
12.4.1.4 Describing Point-to-MultiPoint interfaces ............ 119 12.4.1.4 Describing Point-to-MultiPoint interfaces ............ 120
12.4.1.5 Examples of router-LSAs .............................. 119 12.4.1.5 Examples of router-LSAs .............................. 120
12.4.2 Network-LSAs ......................................... 122 12.4.2 Network-LSAs ......................................... 122
12.4.2.1 Examples of network-LSAs ............................. 122 12.4.2.1 Examples of network-LSAs ............................. 123
12.4.3 Summary-LSAs ......................................... 123 12.4.3 Summary-LSAs ......................................... 123
12.4.3.1 Originating summary-LSAs into stub areas ............. 126 12.4.3.1 Originating summary-LSAs into stub areas ............. 126
12.4.3.2 Examples of summary-LSAs ............................. 126 12.4.3.2 Examples of summary-LSAs ............................. 127
12.4.4 AS-external-LSAs ..................................... 127 12.4.4 AS-external-LSAs ..................................... 128
12.4.4.1 Examples of AS-external-LSAs ......................... 128 12.4.4.1 Examples of AS-external-LSAs ......................... 128
13 The Flooding Procedure ............................... 130 13 The Flooding Procedure ............................... 130
13.1 Determining which LSA is newer ....................... 133 13.1 Determining which LSA is newer ....................... 134
13.2 Installing LSAs in the database ...................... 134 13.2 Installing LSAs in the database ...................... 134
13.3 Next step in the flooding procedure .................. 135 13.3 Next step in the flooding procedure .................. 135
13.4 Receiving self-originated LSAs ....................... 138 13.4 Receiving self-originated LSAs ....................... 138
13.5 Sending Link State Acknowledgment packets ............ 138 13.5 Sending Link State Acknowledgment packets ............ 139
13.6 Retransmitting LSAs .................................. 141 13.6 Retransmitting LSAs .................................. 141
13.7 Receiving link state acknowledgments ................. 141 13.7 Receiving link state acknowledgments ................. 142
14 Aging The Link State Database ........................ 142 14 Aging The Link State Database ........................ 142
14.1 Premature aging of LSAs .............................. 142 14.1 Premature aging of LSAs .............................. 143
15 Virtual Links ........................................ 143 15 Virtual Links ........................................ 144
16 Calculation of the routing table ..................... 145 16 Calculation of the routing table ..................... 146
16.1 Calculating the shortest-path tree for an area ....... 146 16.1 Calculating the shortest-path tree for an area ....... 147
16.1.1 The next hop calculation ............................. 152 16.1.1 The next hop calculation ............................. 152
16.2 Calculating the inter-area routes .................... 153 16.2 Calculating the inter-area routes .................... 153
16.3 Examining transit areas' summary-LSAs ................ 154 16.3 Examining transit areas' summary-LSAs ................ 155
16.4 Calculating AS external routes ....................... 157 16.4 Calculating AS external routes ....................... 157
16.4.1 External path preferences ............................ 159 16.4.1 External path preferences ............................ 159
16.5 Incremental updates -- summary-LSAs .................. 159 16.5 Incremental updates -- summary-LSAs .................. 160
16.6 Incremental updates -- AS-external-LSAs .............. 160 16.6 Incremental updates -- AS-external-LSAs .............. 161
16.7 Events generated as a result of routing table changes 161 16.7 Events generated as a result of routing table changes 161
16.8 Equal-cost multipath ................................. 161 16.8 Equal-cost multipath ................................. 162
16.9 Building the non-zero-TOS portion of the routing table 162 16.9 Building the non-zero-TOS portion of the routing table 162
Footnotes ............................................ 164 Footnotes ............................................ 165
References ........................................... 168 References ........................................... 169
A OSPF data formats .................................... 170 A OSPF data formats .................................... 171
A.1 Encapsulation of OSPF packets ........................ 170 A.1 Encapsulation of OSPF packets ........................ 171
A.2 The Options field .................................... 172 A.2 The Options field .................................... 173
A.3 OSPF Packet Formats .................................. 174 A.3 OSPF Packet Formats .................................. 175
A.3.1 The OSPF packet header ............................... 175 A.3.1 The OSPF packet header ............................... 176
A.3.2 The Hello packet ..................................... 177 A.3.2 The Hello packet ..................................... 178
A.3.3 The Database Description packet ...................... 179 A.3.3 The Database Description packet ...................... 180
A.3.4 The Link State Request packet ........................ 181 A.3.4 The Link State Request packet ........................ 182
A.3.5 The Link State Update packet ......................... 183 A.3.5 The Link State Update packet ......................... 184
A.3.6 The Link State Acknowledgment packet ................. 185 A.3.6 The Link State Acknowledgment packet ................. 186
A.4 LSA formats .......................................... 187 A.4 LSA formats .......................................... 188
A.4.1 The LSA header ....................................... 188 A.4.1 The LSA header ....................................... 189
A.4.2 Router-LSAs .......................................... 190 A.4.2 Router-LSAs .......................................... 191
A.4.3 Network-LSAs ......................................... 194 A.4.3 Network-LSAs ......................................... 195
A.4.4 Summary-LSAs ......................................... 195 A.4.4 Summary-LSAs ......................................... 196
A.4.5 AS-external-LSAs ..................................... 197 A.4.5 AS-external-LSAs ..................................... 198
B Architectural Constants .............................. 199 B Architectural Constants .............................. 200
C Configurable Constants ............................... 201 C Configurable Constants ............................... 202
C.1 Global parameters .................................... 201 C.1 Global parameters .................................... 202
C.2 Area parameters ...................................... 202 C.2 Area parameters ...................................... 203
C.3 Router interface parameters .......................... 203 C.3 Router interface parameters .......................... 204
C.4 Virtual link parameters .............................. 205 C.4 Virtual link parameters .............................. 206
C.5 NBMA network parameters .............................. 206 C.5 NBMA network parameters .............................. 207
C.6 Point-to-MultiPoint network parameters ............... 207 C.6 Point-to-MultiPoint network parameters ............... 208
C.7 Host route parameters ................................ 207 C.7 Host route parameters ................................ 208
D Authentication ....................................... 208 D Authentication ....................................... 209
D.1 Null authentication .................................. 208 D.1 Null authentication .................................. 209
D.2 Simple password authentication ....................... 208 D.2 Simple password authentication ....................... 209
D.3 Cryptographic authentication ......................... 209 D.3 Cryptographic authentication ......................... 210
D.4 Message generation ................................... 211 D.4 Message generation ................................... 212
D.4.1 Generating Null authentication ....................... 212 D.4.1 Generating Null authentication ....................... 213
D.4.2 Generating Simple password authentication ............ 212 D.4.2 Generating Simple password authentication ............ 213
D.4.3 Generating Cryptographic authentication .............. 212 D.4.3 Generating Cryptographic authentication .............. 213
D.5 Message verification ................................. 214 D.5 Message verification ................................. 215
D.5.1 Verifying Null authentication ........................ 214 D.5.1 Verifying Null authentication ........................ 215
D.5.2 Verifying Simple password authentication ............. 214 D.5.2 Verifying Simple password authentication ............. 215
D.5.3 Verifying Cryptographic authentication ............... 214 D.5.3 Verifying Cryptographic authentication ............... 215
E An algorithm for assigning Link State IDs ............ 216 E An algorithm for assigning Link State IDs ............ 217
F Multiple interfaces to the same network/subnet ....... 218 F Multiple interfaces to the same network/subnet ....... 219
G Differences from RFC 1583 ............................ 219 G Differences from RFC 1583 ............................ 220
G.1 Enhancements to OSPF authentication .................. 219 G.1 Enhancements to OSPF authentication .................. 220
G.2 Addition of Point-to-MultiPoint interface ............ 219 G.2 Addition of Point-to-MultiPoint interface ............ 220
G.3 Support for overlapping area ranges .................. 220 G.3 Support for overlapping area ranges .................. 221
G.4 A modification to the flooding algorithm ............. 221 G.4 A modification to the flooding algorithm ............. 222
G.5 Introduction of the MinLSArrival constant ............ 221 G.5 Introduction of the MinLSArrival constant ............ 222
G.6 Optionally advertising point-to-point links as subnets 222 G.6 Optionally advertising point-to-point links as subnets 223
G.7 Advertising same external route from multiple areas .. 222 G.7 Advertising same external route from multiple areas .. 223
G.8 Retransmission of initial Database Description packets 224 G.8 Retransmission of initial Database Description packets 225
G.9 Detecting interface MTU mismatches ................... 224 G.9 Detecting interface MTU mismatches ................... 225
Security Considerations .............................. 225 Security Considerations .............................. 226
Author's Address ..................................... 225 Author's Address ..................................... 226
1. Introduction 1. Introduction
This document is a specification of the Open Shortest Path First This document is a specification of the Open Shortest Path First
(OSPF) TCP/IP internet routing protocol. OSPF is classified as an (OSPF) TCP/IP internet routing protocol. OSPF is classified as an
Interior Gateway Protocol (IGP). This means that it distributes Interior Gateway Protocol (IGP). This means that it distributes
routing information between routers belonging to a single Autonomous routing information between routers belonging to a single Autonomous
System. The OSPF protocol is based on link-state or SPF technology. System. The OSPF protocol is based on link-state or SPF technology.
This is a departure from the Bellman-Ford base used by traditional This is a departure from the Bellman-Ford base used by traditional
TCP/IP internet routing protocols. TCP/IP internet routing protocols.
skipping to change at page 22, line 18 skipping to change at page 22, line 18
N3 RT3 7 N3 RT3 7
N4 RT3 8 N4 RT3 8
Ib * 7 Ib * 7
Ia RT10 12 Ia RT10 12
N6 RT10 8 N6 RT10 8
N7 RT10 12 N7 RT10 12
N8 RT10 10 N8 RT10 10
N9 RT10 11 N9 RT10 11
N10 RT10 13 N10 RT10 13
N11 RT10 14 N11 RT10 14
H1 RT10 21
__________________________________ __________________________________
RT5 RT5 6 RT5 RT5 6
RT7 RT10 8 RT7 RT10 8
Table 2: The portion of Router RT6's routing table listing local Table 2: The portion of Router RT6's routing table listing local
destinations. destinations.
Routes to networks belonging to other AS'es (such as N12) appear 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 as dashed lines on the shortest path tree in Figure 5. Use of
this externally derived routing information is considered in the this externally derived routing information is considered in the
skipping to change at page 28, line 49 skipping to change at page 29, line 4
Figure 7 shows the resulting link-state database for the Area 1. Figure 7 shows the resulting link-state database for the Area 1.
The figure completely describes that area's intra-area routing. The figure completely describes that area's intra-area routing.
It also shows the complete view of the internet for the two 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 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 routers, RT3 and RT4, to advertise into Area 1 the distances to
all destinations external to the area. These are indicated in all destinations external to the area. These are indicated in
Figure 7 by the dashed stub routes. Also, RT3 and RT4 must Figure 7 by the dashed stub routes. Also, RT3 and RT4 must
advertise into Area 1 the location of the AS boundary routers advertise into Area 1 the location of the AS boundary routers
RT5 and RT7. Finally, AS-external-LSAs from RT5 and RT7 are RT5 and RT7. Finally, AS-external-LSAs from RT5 and RT7 are
flooded throughout the entire AS, and in particular throughout 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.
........................... ...........................
. + . . + .
. | 3+---+ . N12 N14 . | 3+---+ . N12 N14
. N1|--|RT1|\ 1 . \ N13 / . N1|--|RT1|\ 1 . \ N13 /
. | +---+ \ . 8\ |8/8 . | +---+ \ . 8\ |8/8
. + \ ____ . \|/ . + \ ____ . \|/
. / \ 1+---+8 8+---+6 . / \ 1+---+8 8+---+6
. * N3 *---|RT4|------|RT5|--------+ . * N3 *---|RT4|------|RT5|--------+
. \____/ +---+ +---+ | . \____/ +---+ +---+ |
. + / \ . |7 | . + / \ . |7 |
skipping to change at page 30, line 4 skipping to change at page 30, line 4
. +--+SLIP +----+ . . +---+ . . +--+SLIP +----+ . . +---+ .
. |2 . . |4 . . |2 . . |4 .
. | . . | . . | . . | .
. +---------+ . . +--------+ . . +---------+ . . +--------+ .
. N10 . . N7 . . N10 . . N7 .
. . .Area 2 . . . .Area 2 .
.Area 3 . ................................ .Area 3 . ................................
.......................... ..........................
Figure 6: A sample OSPF area configuration Figure 6: A sample OSPF area configuration
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 Routers RT3 and RT4 must also summarize Area 1's topology for
distribution to the backbone. Their backbone LSAs are shown in distribution to the backbone. Their backbone LSAs are shown in
Table 4. These summaries show which networks are contained in Table 4. These summaries show which networks are contained in
Area 1 (i.e., Networks N1-N4), and the distance to these Area 1 (i.e., Networks N1-N4), and the distance to these
networks from the routers RT3 and RT4 respectively. networks from the routers RT3 and RT4 respectively.
The link-state database for the backbone is shown in Figure 8. The link-state database for the backbone is shown in Figure 8.
The set of routers pictured are the backbone routers. Router The set of routers pictured are the backbone routers. Router
RT11 is a backbone router because it belongs to two areas. In RT11 is a backbone router because it belongs to two areas. In
order to make the backbone connected, a virtual link has been order to make the backbone connected, a virtual link has been
skipping to change at page 30, line 35 skipping to change at page 30, line 38
The backbone enables the exchange of summary information between The backbone enables the exchange of summary information between
area border routers. Every area border router hears the area area border routers. Every area border router hears the area
summaries from all other area border routers. It then forms a summaries from all other area border routers. It then forms a
picture of the distance to all networks outside of its area by picture of the distance to all networks outside of its area by
examining the collected LSAs, and adding in the backbone examining the collected LSAs, and adding in the backbone
distance to each advertising router. distance to each advertising router.
Again using Routers RT3 and RT4 as an example, the procedure Again using Routers RT3 and RT4 as an example, the procedure
goes as follows: They first calculate the SPF tree for the 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.
Network RT3 adv. RT4 adv. Network RT3 adv. RT4 adv.
_____________________________ _____________________________
N1 4 4 N1 4 4
N2 4 4 N2 4 4
N3 1 1 N3 1 1
N4 2 3 N4 2 3
Table 4: Networks advertised to the backbone Table 4: Networks advertised to the backbone
by Routers RT3 and RT4. by Routers RT3 and RT4.
skipping to change at page 32, line 39 skipping to change at page 32, line 39
N14| | |8 | | | | | N14| | |8 | | | | |
N15| | | | |9 | | | N15| | | | |9 | | |
Figure 8: The backbone's database. Figure 8: The backbone's database.
Networks and routers are represented by vertices. Networks and routers are represented by vertices.
An edge of cost X connects Vertex A to Vertex B iff An edge of cost X connects Vertex A to Vertex B iff
the intersection of Column A and Row B is marked the intersection of Column A and Row B is marked
with an X. with an X.
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 Next, by looking at the area summaries from these area border
routers, RT3 and RT4 can determine the distance to all networks routers, RT3 and RT4 can determine the distance to all networks
outside their area. These distances are then advertised outside their area. These distances are then advertised
internally to the area by RT3 and RT4. The advertisements that 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. 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 Note that Table 6 assumes that an area range has been configured
for the backbone which groups Ia and Ib into a single LSA. 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
dist from dist from dist from dist from
RT3 RT4 RT3 RT4
__________________________________ __________________________________
to RT3 * 21 to RT3 * 21
to RT4 22 * to RT4 22 *
to RT7 20 14 to RT7 20 14
to RT10 15 22 to RT10 15 22
to RT11 18 25
__________________________________ __________________________________
to Ia 20 27 to Ia 20 27
to Ib 15 22 to Ib 15 22
__________________________________ __________________________________
to RT5 14 8 to RT5 14 8
to RT7 20 14 to RT7 20 14
Table 5: Backbone distances calculated Table 5: Backbone distances calculated
by Routers RT3 and RT4. by Routers RT3 and RT4.
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.
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).
Destination RT3 adv. RT4 adv.
_________________________________ _________________________________
Ia,Ib 20 27 Ia,Ib 20 27
N6 16 15 N6 16 15
N7 20 19 N7 20 19
N8 18 18 N8 18 18
N9-N11,H1 29 36 N9-N11,H1 29 36
_________________________________ _________________________________
RT5 14 8 RT5 14 8
RT7 20 14 RT7 20 14
Table 6: Destinations advertised into Area 1 Table 6: Destinations advertised into Area 1
by Routers RT3 and RT4. by Routers RT3 and RT4.
load share between the two for traffic to Network N8.
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 Note that a failure of the line between Routers RT6 and RT10
will cause the backbone to become disconnected. Configuring a will cause the backbone to become disconnected. Configuring a
virtual link between Routers RT7 and RT10 will give the backbone virtual link between Routers RT7 and RT10 will give the backbone
more connectivity and more resistance to such failures. more connectivity and more resistance to such failures.
3.5. IP subnetting support 3.5. IP subnetting support
OSPF attaches an IP address mask to each advertised route. The OSPF attaches an IP address mask to each advertised route. The
mask indicates the range of addresses being described by the mask indicates the range of addresses being described by the
particular route. For example, a summary-LSA for the particular route. For example, a summary-LSA for the
skipping to change at page 41, line 13 skipping to change at page 41, line 13
multicast address. multicast address.
LS LSA LSA description LS LSA LSA description
type name type name
________________________________________________________ ________________________________________________________
1 Router-LSAs Originated by all routers. 1 Router-LSAs Originated by all routers.
This LSA describes This LSA describes
the collected states of the the collected states of the
router's interfaces to an router's interfaces to an
area. Flooded throughout a area. Flooded throughout a
single area only.
________________________________________________________ ________________________________________________________
2 Network-LSAs Originated for broadcast 2 Network-LSAs Originated for broadcast
and NBMA networks by and NBMA networks by
the Designated Router. This the Designated Router. This
LSA contains the LSA contains the
list of routers connected list of routers connected
to the network. Flooded to the network. Flooded
throughout a single area only. throughout a single area only.
________________________________________________________ ________________________________________________________
3,4 Summary-LSAs Originated by area border 3,4 Summary-LSAs Originated by area border
routers, and flooded through- routers, and flooded through-
out the LSA's associated out the LSA's associated
area. Each summary-LSA area. Each summary-LSA
describes a route to a describes a route to a
destination outside the area, destination outside the area,
yet still inside the AS yet still inside the AS
(i.e., an inter-area route). (i.e., an inter-area route).
Type 3 summary-LSAs describe Type 3 summary-LSAs describe
routes to networks. Type 4 routes to networks. Type 4
summary-LSAs describe summary-LSAs describe
routes to AS boundary routers.
________________________________________________________ ________________________________________________________
5 AS-external-LSAs Originated by AS boundary 5 AS-external-LSAs Originated by AS boundary
routers, and flooded through- routers, and flooded through-
out the AS. Each out the AS. Each
AS-external-LSA describes AS-external-LSA describes
a route to a destination in a route to a destination in
another Autonomous System. another Autonomous System.
Default routes for the AS can Default routes for the AS can
also be described by also be described by
AS-external-LSAs. AS-external-LSAs.
skipping to change at page 46, line 44 skipping to change at page 47, line 5
The OSPF backbone is the special OSPF area responsible for The OSPF backbone is the special OSPF area responsible for
disseminating inter-area routing information. disseminating inter-area routing information.
The area link-state database consists of the collection of router- The area link-state database consists of the collection of router-
LSAs, network-LSAs and summary-LSAs that have originated from the LSAs, network-LSAs and summary-LSAs that have originated from the
area's routers. This information is flooded throughout a single area's routers. This information is flooded throughout a single
area only. The list of AS-external-LSAs (see Section 5) is also area only. The list of AS-external-LSAs (see Section 5) is also
considered to be part of each area's link-state database. considered to be part of each area's link-state database.
Area ID
A 32-bit number identifying the area. The Area ID of 0.0.0.0 is
reserved for the backbone.
List of area address ranges
In order to aggregate routing information at area boundaries,
+----+ +----+
|RT10|------+ |RT10|------+
+----+ \+-------------+ +----+ \+-------------+
/ \ |Routing Table| / \ |Routing Table|
/ \ +-------------+ / \ +-------------+
/ \ / \
+------+ / \ +--------+ +------+ / \ +--------+
|Area 2|---+ +---|Backbone| |Area 2|---+ +---|Backbone|
+------+***********+ +--------+ +------+***********+ +--------+
/ \ * / \ / \ * / \
skipping to change at page 47, line 32 skipping to change at page 47, line 33
|Neighbor| |Neighbor| | |Neighbor RT11| |Neighbor RT6| |Neighbor| |Neighbor| | |Neighbor RT11| |Neighbor RT6|
| RT8 | | RT7 | | +-------------+ +------------+ | RT8 | | RT7 | | +-------------+ +------------+
+--------+ +--------+ | +--------+ +--------+ |
| |
+-------------+ +-------------+
|Neighbor RT11| |Neighbor RT11|
+-------------+ +-------------+
Figure 9: Router RT10's Data structures Figure 9: Router RT10's Data structures
Area ID
A 32-bit number identifying the area. The Area ID of 0.0.0.0 is
reserved for the backbone.
List of area address ranges
In order to aggregate routing information at area boundaries,
area address ranges can be employed. Each address range is area address ranges can be employed. Each address range is
specified by an [address,mask] pair and a status indication of specified by an [address,mask] pair and a status indication of
either Advertise or DoNotAdvertise (see Section 12.4.3). either Advertise or DoNotAdvertise (see Section 12.4.3).
Associated router interfaces Associated router interfaces
This router's interfaces connecting to the area. A router This router's interfaces connecting to the area. A router
interface belongs to one and only one area (or the backbone). interface belongs to one and only one area (or the backbone).
For the backbone area this list includes all the virtual links. For the backbone area this list includes all the virtual links.
A virtual link is identified by the Router ID of its other A virtual link is identified by the Router ID of its other
endpoint; its cost is the cost of the shortest intra-area path endpoint; its cost is the cost of the shortest intra-area path
skipping to change at page 53, line 27 skipping to change at page 53, line 32
Two graphs are possible, depending on whether a Designated Two graphs are possible, depending on whether a Designated
Router is elected for the network. On physical point-to-point Router is elected for the network. On physical point-to-point
networks, Point-to-MultiPoint networks and virtual links, networks, Point-to-MultiPoint networks and virtual links,
neighboring routers become adjacent whenever they can neighboring routers become adjacent whenever they can
communicate directly. In contrast, on broadcast and NBMA communicate directly. In contrast, on broadcast and NBMA
networks only the Designated Router and the Backup Designated networks only the Designated Router and the Backup Designated
Router become adjacent to all other routers attached to the Router become adjacent to all other routers attached to the
network. network.
These graphs are shown in Figure 10. It is assumed that Router
RT7 has become the Designated Router, and Router RT3 the Backup
Designated Router, for the Network N2. The Backup Designated
Router performs a lesser function during the flooding procedure
than the Designated Router (see Section 13.3). This is the
reason for the dashed lines connecting the Backup Designated
Router RT3.
8. Protocol Packet Processing
This section discusses the general processing of OSPF routing
protocol packets. It is very important that the router link-state
databases remain synchronized. For this reason, routing protocol
packets should get preferential treatment over ordinary data
packets, both in sending and receiving.
Routing protocol packets are sent along adjacencies only (with the
exception of Hello packets, which are used to discover the
adjacencies). This means that all routing protocol packets travel a
+---+ +---+ +---+ +---+
|RT1|------------|RT2| o---------------o |RT1|------------|RT2| o---------------o
+---+ N1 +---+ RT1 RT2 +---+ N1 +---+ RT1 RT2
RT7 RT7
o---------+ o---------+
+---+ +---+ +---+ /|\ | +---+ +---+ +---+ /|\ |
|RT7| |RT3| |RT4| / | \ | |RT7| |RT3| |RT4| / | \ |
+---+ +---+ +---+ / | \ | +---+ +---+ +---+ / | \ |
| | | / | \ | | | | / | \ |
+-----------------------+ RT5o RT6o oRT4 | +-----------------------+ RT5o RT6o oRT4 |
| | N2 * * * | | | N2 * * * |
+---+ +---+ * * * | +---+ +---+ * * * |
|RT5| |RT6| * * * | |RT5| |RT6| * * * |
+---+ +---+ *** | +---+ +---+ *** |
o---------+ o---------+
RT3 RT3
Figure 10: The graph of adjacencies Figure 10: The graph of adjacencies
These graphs are shown in Figure 10. It is assumed that Router
RT7 has become the Designated Router, and Router RT3 the Backup
Designated Router, for the Network N2. The Backup Designated
Router performs a lesser function during the flooding procedure
than the Designated Router (see Section 13.3). This is the
reason for the dashed lines connecting the Backup Designated
Router RT3.
8. Protocol Packet Processing
This section discusses the general processing of OSPF routing
protocol packets. It is very important that the router link-state
databases remain synchronized. For this reason, routing protocol
packets should get preferential treatment over ordinary data
packets, both in sending and receiving.
Routing protocol packets are sent along adjacencies only (with the
exception of Hello packets, which are used to discover the
adjacencies). This means that all routing protocol packets travel a
single IP hop, except those sent over virtual links. single IP hop, except those sent over virtual links.
All routing protocol packets begin with a standard header. The All routing protocol packets begin with a standard header. The
sections below provide details on how to fill in and verify this sections below provide details on how to fill in and verify this
standard header. Then, for each packet type, the section giving standard header. Then, for each packet type, the section giving
more details on that particular packet type's processing is listed. more details on that particular packet type's processing is listed.
8.1. Sending protocol packets 8.1. Sending protocol packets
When a router sends a routing protocol packet, it fills in the When a router sends a routing protocol packet, it fills in the
skipping to change at page 102, line 22 skipping to change at page 102, line 22
N Ib 0 intra-area 7 * * N Ib 0 intra-area 7 * *
N Ia 0 intra-area 12 RT10 * N Ia 0 intra-area 12 RT10 *
N N6 0 intra-area 8 RT10 * N N6 0 intra-area 8 RT10 *
N N7 0 intra-area 12 RT10 * N N7 0 intra-area 12 RT10 *
N N8 0 intra-area 10 RT10 * N N8 0 intra-area 10 RT10 *
N N9 0 intra-area 11 RT10 * N N9 0 intra-area 11 RT10 *
N N10 0 intra-area 13 RT10 * N N10 0 intra-area 13 RT10 *
N N11 0 intra-area 14 RT10 * N N11 0 intra-area 14 RT10 *
N H1 0 intra-area 21 RT10 * N H1 0 intra-area 21 RT10 *
R RT5 0 intra-area 6 RT5 * R RT5 0 intra-area 6 RT5 *
R RT7 0 intra-area 8 RT10 *
____________________________________________________________ ____________________________________________________________
N N12 * type 1 ext. 10 RT10 RT7 N N12 * type 1 ext. 10 RT10 RT7
N N13 * type 1 ext. 14 RT5 RT5 N N13 * type 1 ext. 14 RT5 RT5
N N14 * type 1 ext. 14 RT5 RT5 N N14 * type 1 ext. 14 RT5 RT5
N N15 * type 1 ext. 17 RT10 RT7 N N15 * type 1 ext. 17 RT10 RT7
Table 12: The routing table for Router RT6 Table 12: The routing table for Router RT6
(no configured areas). (no configured areas).
11.3. Sample routing table, with areas 11.3. Sample routing table, with areas
skipping to change at page 104, line 12 skipping to change at page 104, line 12
Each LSA begins with a standard 20-byte header. This LSA header is Each LSA begins with a standard 20-byte header. This LSA header is
discussed below. discussed below.
Type Dest Area Path Type Cost Next Adv. Type Dest Area Path Type Cost Next Adv.
Hops(s) Router(s) Hops(s) Router(s)
__________________________________________________________________ __________________________________________________________________
N N1 1 intra-area 4 RT1 * N N1 1 intra-area 4 RT1 *
N N2 1 intra-area 4 RT2 * N N2 1 intra-area 4 RT2 *
N N3 1 intra-area 1 * * N N3 1 intra-area 1 * *
N N4 1 intra-area 3 RT3 * N N4 1 intra-area 3 RT3 *
R RT3 1 intra-area 1 * *
__________________________________________________________________ __________________________________________________________________
N Ib 0 intra-area 22 RT5 * N Ib 0 intra-area 22 RT5 *
N Ia 0 intra-area 27 RT5 * N Ia 0 intra-area 27 RT5 *
R RT3 0 intra-area 21 RT5 * R RT3 0 intra-area 21 RT5 *
R RT5 0 intra-area 8 * * R RT5 0 intra-area 8 * *
R RT7 0 intra-area 14 RT5 * R RT7 0 intra-area 14 RT5 *
R RT10 0 intra-area 22 RT5 * R RT10 0 intra-area 22 RT5 *
R RT11 0 intra-area 25 RT5 * R RT11 0 intra-area 25 RT5 *
__________________________________________________________________ __________________________________________________________________
N N6 0 inter-area 15 RT5 RT7 N N6 0 inter-area 15 RT5 RT7
N N7 0 inter-area 19 RT5 RT7 N N7 0 inter-area 19 RT5 RT7
N N8 0 inter-area 18 RT5 RT7 N N8 0 inter-area 18 RT5 RT7
N N9-N11,H1 0 inter-area 36 RT5 RT11
__________________________________________________________________ __________________________________________________________________
N N12 * type 1 ext. 16 RT5 RT5,RT7 N N12 * type 1 ext. 16 RT5 RT5,RT7
N N13 * type 1 ext. 16 RT5 RT5 N N13 * type 1 ext. 16 RT5 RT5
N N14 * type 1 ext. 16 RT5 RT5 N N14 * type 1 ext. 16 RT5 RT5
N N15 * type 1 ext. 23 RT5 RT7 N N15 * type 1 ext. 23 RT5 RT7
Table 13: Router RT4's routing table Table 13: Router RT4's routing table
in the presence of areas. in the presence of areas.
Type Dest Area Path Type Cost Next Adv. Type Dest Area Path Type Cost Next Adv.
Hop(s) Router(s) Hop(s) Router(s)
________________________________________________________________ ________________________________________________________________
N Ib 0 intra-area 16 RT3 * N Ib 0 intra-area 16 RT3 *
N Ia 0 intra-area 21 RT3 * N Ia 0 intra-area 21 RT3 *
R RT3 0 intra-area 1 * * R RT3 0 intra-area 1 * *
R RT10 0 intra-area 16 RT3 * R RT10 0 intra-area 16 RT3 *
R RT11 0 intra-area 19 RT3 *
________________________________________________________________ ________________________________________________________________
N N9-N11,H1 0 inter-area 30 RT3 RT11 N N9-N11,H1 0 inter-area 30 RT3 RT11
Table 14: Changes resulting from an Table 14: Changes resulting from an
additional virtual link. additional virtual link.
12.1. The LSA Header 12.1. The LSA Header
The LSA header contains the LS type, Link State ID and The LSA header contains the LS type, Link State ID and
Advertising Router fields. The combination of these three Advertising Router fields. The combination of these three
skipping to change at page 106, line 47 skipping to change at page 107, line 16
The LS type field dictates the format and function of the The LS type field dictates the format and function of the
LSA. LSAs of different types have different names (e.g., LSA. LSAs of different types have different names (e.g.,
router-LSAs or network-LSAs). All LSA types defined by this router-LSAs or network-LSAs). All LSA types defined by this
memo, except the AS-external-LSAs (LS type = 5), are flooded memo, except the AS-external-LSAs (LS type = 5), are flooded
throughout a single area only. AS-external-LSAs are flooded throughout a single area only. AS-external-LSAs are flooded
throughout the entire Autonomous System, excepting stub throughout the entire Autonomous System, excepting stub
areas (see Section 3.6). Each separate LSA type is briefly areas (see Section 3.6). Each separate LSA type is briefly
described below in Table 15. described below in Table 15.
12.1.4. Link State ID
This field identifies the piece of the routing domain that
is being described by the LSA. Depending on the LSA's LS
type, the Link State ID takes on the values listed in Table
LS Type LSA description LS Type LSA description
________________________________________________ ________________________________________________
1 These are the router-LSAs. 1 These are the router-LSAs.
They describe the collected They describe the collected
states of the router's states of the router's
interfaces. For more information, interfaces. For more information,
consult Section 12.4.1.
________________________________________________ ________________________________________________
2 These are the network-LSAs. 2 These are the network-LSAs.
They describe the set of routers They describe the set of routers
attached to the network. For attached to the network. For
more information, consult more information, consult
Section 12.4.2. Section 12.4.2.
________________________________________________ ________________________________________________
3 or 4 These are the summary-LSAs. 3 or 4 These are the summary-LSAs.
They describe inter-area routes, They describe inter-area routes,
and enable the condensation of and enable the condensation of
routing information at area routing information at area
borders. Originated by area border borders. Originated by area border
routers, the Type 3 summary-LSAs routers, the Type 3 summary-LSAs
describe routes to networks while the describe routes to networks while the
Type 4 summary-LSAs describe routes to Type 4 summary-LSAs describe routes to
AS boundary routers.
________________________________________________ ________________________________________________
5 These are the AS-external-LSAs. 5 These are the AS-external-LSAs.
Originated by AS boundary routers, Originated by AS boundary routers,
they describe routes they describe routes
to destinations external to the to destinations external to the
Autonomous System. A default route for Autonomous System. A default route for
the Autonomous System can also be the Autonomous System can also be
described by an AS-external-LSA. described by an AS-external-LSA.
Table 15: OSPF link state advertisements (LSAs). Table 15: OSPF link state advertisements (LSAs).
12.1.4. Link State ID
This field identifies the piece of the routing domain that
is being described by the LSA. Depending on the LSA's LS
type, the Link State ID takes on the values listed in Table
16. 16.
Actually, for Type 3 summary-LSAs (LS type = 3) and AS- Actually, for Type 3 summary-LSAs (LS type = 3) and AS-
external-LSAs (LS type = 5), the Link State ID may external-LSAs (LS type = 5), the Link State ID may
additionally have one or more of the destination network's additionally have one or more of the destination network's
"host" bits set. For example, when originating an AS- "host" bits set. For example, when originating an AS-
external-LSA for the network 10.0.0.0 with mask of external-LSA for the network 10.0.0.0 with mask of
255.0.0.0, the Link State ID can be set to anything in the 255.0.0.0, the Link State ID can be set to anything in the
range 10.0.0.0 through 10.255.255.255 inclusive (although range 10.0.0.0 through 10.255.255.255 inclusive (although
10.0.0.0 should be used whenever possible). The freedom to 10.0.0.0 should be used whenever possible). The freedom to
set certain host bits allows a router to originate separate set certain host bits allows a router to originate separate
LSAs for two networks having the same address but different LSAs for two networks having the same address but different
LS Type Link State ID
_______________________________________________
1 The originating router's Router ID.
2 The IP interface address of the
network's Designated Router.
3 The destination network's IP address.
4 The Router ID of the described AS
boundary router.
5 The destination network's IP address.
Table 16: The LSA's Link State ID.
masks. See Appendix E for details. masks. See Appendix E for details.
When the LSA is describing a network (LS type = 2, 3 or 5), When the LSA is describing a network (LS type = 2, 3 or 5),
the network's IP address is easily derived by masking the the network's IP address is easily derived by masking the
Link State ID with the network/subnet mask contained in the Link State ID with the network/subnet mask contained in the
body of the LSA. When the LSA is describing a router (LS body of the LSA. When the LSA is describing a router (LS
type = 1 or 4), the Link State ID is always the described type = 1 or 4), the Link State ID is always the described
router's OSPF Router ID. router's OSPF Router ID.
When an AS-external-LSA (LS Type = 5) is describing a When an AS-external-LSA (LS Type = 5) is describing a
default route, its Link State ID is set to default route, its Link State ID is set to
DefaultDestination (0.0.0.0). DefaultDestination (0.0.0.0).
LS Type Link State ID
_______________________________________________
1 The originating router's Router ID.
2 The IP interface address of the
network's Designated Router.
3 The destination network's IP address.
4 The Router ID of the described AS
boundary router.
5 The destination network's IP address.
Table 16: The LSA's Link State ID.
12.1.5. Advertising Router 12.1.5. Advertising Router
This field specifies the OSPF Router ID of the LSA's This field specifies the OSPF Router ID of the LSA's
originator. For router-LSAs, this field is identical to the originator. For router-LSAs, this field is identical to the
Link State ID field. Network-LSAs are originated by the Link State ID field. Network-LSAs are originated by the
network's Designated Router. Summary-LSAs originated by network's Designated Router. Summary-LSAs originated by
area border routers. AS-external-LSAs are originated by AS area border routers. AS-external-LSAs are originated by AS
boundary routers. boundary routers.
12.1.6. LS sequence number 12.1.6. LS sequence number
skipping to change at page 115, line 5 skipping to change at page 115, line 27
AS-external-LSAs that it had previously originated. These AS-external-LSAs that it had previously originated. These
LSAs can be flushed via the premature aging procedure LSAs can be flushed via the premature aging procedure
specified in Section 14.1. specified in Section 14.1.
The construction of each type of LSA is explained in detail The construction of each type of LSA is explained in detail
below. In general, these sections describe the contents of the below. In general, these sections describe the contents of the
LSA body (i.e., the part coming after the 20-byte LSA header). LSA body (i.e., the part coming after the 20-byte LSA header).
For information concerning the building of the LSA header, see For information concerning the building of the LSA header, see
Section 12.1. Section 12.1.
12.4.1. Router-LSAs
A router originates a router-LSA for each area that it
belongs to. Such an LSA describes the collected states of
the router's links to the area. The LSA is flooded
throughout the particular area, and no further.
The format of a router-LSA is shown in Appendix A (Section
A.4.2). The first 20 bytes of the LSA consist of the
generic LSA header that was discussed in Section 12.1.
router-LSAs have LS type = 1. The router indicates whether
it is willing to calculate separate routes for each IP TOS
by setting (or resetting) the T-bit of the LSA's Options
field.
A router also indicates whether it is an area border router,
or an AS boundary router, by setting the appropriate bits
(bit B and bit E, respectively) in its router-LSAs. This
enables paths to those types of routers to be saved in the
routing table, for later processing of summary-LSAs and AS-
external-LSAs. Bit B should be set whenever the router is
actively attached to two or more areas, even if the router
.................................... ....................................
. 192.1.2 Area 1 . . 192.1.2 Area 1 .
. + . . + .
. | . . | .
. | 3+---+1 . . | 3+---+1 .
. N1 |--|RT1|-----+ . . N1 |--|RT1|-----+ .
. | +---+ \ . . | +---+ \ .
. | \ _______N3 . . | \ _______N3 .
. + \/ \ . 1+---+ . + \/ \ . 1+---+
. * 192.1.1 *------|RT4| . * 192.1.1 *------|RT4|
skipping to change at page 115, line 30 skipping to change at page 116, line 29
. | |RT3|----------------|RT6| . | |RT3|----------------|RT6|
. + +---+ . +---+ . + +---+ . +---+
. 192.1.3 |2 . 18.10.0.6|7 . 192.1.3 |2 . 18.10.0.6|7
. | . | . | . |
. +------------+ . . +------------+ .
. 192.1.4 (N4) . . 192.1.4 (N4) .
.................................... ....................................
Figure 15: Area 1 with IP addresses shown Figure 15: Area 1 with IP addresses shown
12.4.1. Router-LSAs
A router originates a router-LSA for each area that it
belongs to. Such an LSA describes the collected states of
the router's links to the area. The LSA is flooded
throughout the particular area, and no further.
The format of a router-LSA is shown in Appendix A (Section
A.4.2). The first 20 bytes of the LSA consist of the
generic LSA header that was discussed in Section 12.1.
router-LSAs have LS type = 1. The router indicates whether
it is willing to calculate separate routes for each IP TOS
by setting (or resetting) the T-bit of the LSA's Options
field.
A router also indicates whether it is an area border router,
or an AS boundary router, by setting the appropriate bits
(bit B and bit E, respectively) in its router-LSAs. This
enables paths to those types of routers to be saved in the
routing table, for later processing of summary-LSAs and AS-
external-LSAs. Bit B should be set whenever the router is
actively attached to two or more areas, even if the router
is not currently attached to the OSPF backbone area. Bit E is not currently attached to the OSPF backbone area. Bit E
should never be set in a router-LSA for a stub area (stub should never be set in a router-LSA for a stub area (stub
areas cannot contain AS boundary routers). areas cannot contain AS boundary routers).
In addition, the router sets bit V in its router-LSA for In addition, the router sets bit V in its router-LSA for
Area A if and only if the router is the endpoint of one or Area A if and only if the router is the endpoint of one or
more fully adjacent virtual links having Area A as their more fully adjacent virtual links having Area A as their
Transit area. The setting of bit V enables other routers in Transit area. The setting of bit V enables other routers in
Area A to discover whether the area supports transit traffic Area A to discover whether the area supports transit traffic
(see TransitCapability in Section 6). (see TransitCapability in Section 6).
skipping to change at page 130, line 26 skipping to change at page 131, line 4
floods each LSA one hop further from its point of origination. To floods each LSA one hop further from its point of origination. To
make the flooding procedure reliable, each LSA must be acknowledged make the flooding procedure reliable, each LSA must be acknowledged
separately. Acknowledgments are transmitted in Link State separately. Acknowledgments are transmitted in Link State
Acknowledgment packets. Many separate acknowledgments can also be Acknowledgment packets. Many separate acknowledgments can also be
grouped together into a single packet. grouped together into a single packet.
The flooding procedure starts when a Link State Update packet has The flooding procedure starts when a Link State Update packet has
been received. Many consistency checks have been made on the been received. Many consistency checks have been made on the
received packet before being handed to the flooding procedure (see received packet before being handed to the flooding procedure (see
Section 8.2). In particular, the Link State Update packet has been Section 8.2). In particular, the Link State Update packet has been
associated with a particular neighbor, and a particular area. If
the neighbor is in a lesser state than Exchange, the packet should
be dropped without further processing.
All types of LSAs, other than AS-external-LSAs, are associated with
+ +
| |
+---+.....|.BGP +---+.....|.BGP
|RTA|-----|.....+---+ |RTA|-----|.....+---+
+---+ |-----|RTX| +---+ |-----|RTX|
| +---+ | +---+
+---+ | +---+ |
|RTB|-----| |RTB|-----|
+---+ | +---+ |
| |
+---+ | +---+ |
|RTC|-----| |RTC|-----|
+---+ | +---+ |
| |
+ +
Figure 16: Forwarding address example Figure 16: Forwarding address example
associated with a particular neighbor, and a particular area. If
the neighbor is in a lesser state than Exchange, the packet should
be dropped without further processing.
All types of LSAs, other than AS-external-LSAs, are associated with
a specific area. However, LSAs do not contain an area field. An a specific area. However, LSAs do not contain an area field. An
LSA's area must be deduced from the Link State Update packet header. LSA's area must be deduced from the Link State Update packet header.
For each LSA contained in a Link State Update packet, the following For each LSA contained in a Link State Update packet, the following
steps are taken: steps are taken:
(1) Validate the LSA's LS checksum. If the checksum turns out to be (1) Validate the LSA's LS checksum. If the checksum turns out to be
invalid, discard the LSA and get the next one from the Link invalid, discard the LSA and get the next one from the Link
State Update packet. State Update packet.
skipping to change at page 139, line 31 skipping to change at page 140, line 4
packets is described in Table 19. The circumstances surrounding packets is described in Table 19. The circumstances surrounding
the receipt of the LSA are listed in the left column. The the receipt of the LSA are listed in the left column. The
acknowledgment action then taken is listed in one of the two acknowledgment action then taken is listed in one of the two
right columns. This action depends on the state of the right columns. This action depends on the state of the
concerned interface; interfaces in state Backup behave concerned interface; interfaces in state Backup behave
differently from interfaces in all other states. Delayed differently from interfaces in all other states. Delayed
acknowledgments must be delivered to all adjacent routers acknowledgments must be delivered to all adjacent routers
associated with the interface. On broadcast networks, this is associated with the interface. On broadcast networks, this is
accomplished by sending the delayed Link State Acknowledgment accomplished by sending the delayed Link State Acknowledgment
packets as multicasts. The Destination IP address used depends packets as multicasts. The Destination IP address used depends
on the state of the interface. If the interface state is DR or
Backup, the destination AllSPFRouters is used. In all other
states, the destination AllDRouters is used. On non-broadcast
networks, delayed Link State Acknowledgment packets must be
unicast separately over each adjacency (i.e., neighbor whose
state is >= Exchange).
The reasoning behind sending the above packets as multicasts is
best explained by an example. Consider the network
configuration depicted in Figure 15. Suppose RT4 has been
elected as Designated Router, and RT3 as Backup Designated
Router for the network N3. When Router RT4 floods a new LSA to
Network N3, it is received by routers RT1, RT2, and RT3. These
routers will not flood the LSA back onto net N3, but they still
must ensure that their link-state databases remain synchronized
with their adjacent neighbors. So RT1, RT2, and RT4 are waiting
to see an acknowledgment from RT3. Likewise, RT4 and RT3 are
both waiting to see acknowledgments from RT1 and RT2. This is
best achieved by sending the acknowledgments as multicasts.
The reason that the acknowledgment logic for Backup DRs is
slightly different is because they perform differently during
Action taken in state Action taken in state
Circumstances Backup All other states Circumstances Backup All other states
_______________________________________________________________ _______________________________________________________________
LSA has No acknowledgment No acknowledgment LSA has No acknowledgment No acknowledgment
been flooded back sent. sent. been flooded back sent. sent.
out receiving in- out receiving in-
terface (see Sec- terface (see Sec-
tion 13, step 5b).
_______________________________________________________________ _______________________________________________________________
LSA is Delayed acknowledg- Delayed ack- LSA is Delayed acknowledg- Delayed ack-
more recent than ment sent if adver- nowledgment sent. more recent than ment sent if adver- nowledgment sent.
database copy, but tisement received database copy, but tisement received
was not flooded from Designated was not flooded from Designated
back out receiving Router, otherwise back out receiving Router, otherwise
interface do nothing interface do nothing
_______________________________________________________________ _______________________________________________________________
LSA is a Delayed acknowledg- No acknowledgment LSA is a Delayed acknowledg- No acknowledgment
duplicate, and was ment sent if adver- sent. duplicate, and was ment sent if adver- sent.
treated as an im- tisement received treated as an im- tisement received
plied acknowledg- from Designated plied acknowledg- from Designated
ment (see Section Router, otherwise ment (see Section Router, otherwise
13, step 7a). do nothing
_______________________________________________________________ _______________________________________________________________
LSA is a Direct acknowledg- Direct acknowledg- LSA is a Direct acknowledg- Direct acknowledg-
duplicate, and was ment sent. ment sent. duplicate, and was ment sent. ment sent.
not treated as an not treated as an
implied ack- implied ack-
nowledgment. nowledgment.
_______________________________________________________________ _______________________________________________________________
LSA's LS Direct acknowledg- Direct acknowledg- LSA's LS Direct acknowledg- Direct acknowledg-
age is equal to ment sent. ment sent. age is equal to ment sent. ment sent.
MaxAge, and there is MaxAge, and there is
no current instance no current instance
of the LSA of the LSA
in the link state in the link state
database (see database (see
Section 13, step 4). Section 13, step 4).
Table 19: Sending link state acknowledgements. Table 19: Sending link state acknowledgements.
on the state of the interface. If the interface state is DR or
Backup, the destination AllSPFRouters is used. In all other
states, the destination AllDRouters is used. On non-broadcast
networks, delayed Link State Acknowledgment packets must be
unicast separately over each adjacency (i.e., neighbor whose
state is >= Exchange).
The reasoning behind sending the above packets as multicasts is
best explained by an example. Consider the network
configuration depicted in Figure 15. Suppose RT4 has been
elected as Designated Router, and RT3 as Backup Designated
Router for the network N3. When Router RT4 floods a new LSA to
Network N3, it is received by routers RT1, RT2, and RT3. These
routers will not flood the LSA back onto net N3, but they still
must ensure that their link-state databases remain synchronized
with their adjacent neighbors. So RT1, RT2, and RT4 are waiting
to see an acknowledgment from RT3. Likewise, RT4 and RT3 are
both waiting to see acknowledgments from RT1 and RT2. This is
best achieved by sending the acknowledgments as multicasts.
The reason that the acknowledgment logic for Backup DRs is
slightly different is because they perform differently during
the flooding of LSAs (see Section 13.3, step 4). the flooding of LSAs (see Section 13.3, step 4).
13.6. Retransmitting LSAs 13.6. Retransmitting LSAs
LSAs flooded out an adjacency are placed on the adjacency's Link LSAs flooded out an adjacency are placed on the adjacency's Link
state retransmission list. In order to ensure that flooding is state retransmission list. In order to ensure that flooding is
reliable, these LSAs are retransmitted until they are reliable, these LSAs are retransmitted until they are
acknowledged. The length of time between retransmissions is a acknowledged. The length of time between retransmissions is a
configurable per-interface value, RxmtInterval. If this is set configurable per-interface value, RxmtInterval. If this is set
too low for an interface, needless retransmissions will ensue. too low for an interface, needless retransmissions will ensue.
skipping to change at page 225, line 43 skipping to change at page 226, line 43
John Moy John Moy
Cascade Communications Corp. Cascade Communications Corp.
5 Carlisle Road 5 Carlisle Road
Westford, MA 01886 Westford, MA 01886
Phone: 508-952-1367 Phone: 508-952-1367
Fax: 508-692-9214 Fax: 508-692-9214
Email: jmoy@casc.com Email: jmoy@casc.com
This document expires in July 1997. This document expires in August 1997.
 End of changes. 

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