Annotation of embedaddon/quagga/doc/ospf_fundamentals.texi, revision 1.1
1.1 ! misho 1: @c Copyright 2006 Sun Microsystems, Inc. All Rights Reserved.
! 2: @cindex OSPF Fundamentals
! 3: @node OSPF Fundamentals
! 4: @section OSPF Fundamentals
! 5:
! 6: @cindex Link-state routing protocol
! 7: @cindex Distance-vector routing protocol
! 8: @acronym{OSPF} is, mostly, a link-state routing protocol. In contrast
! 9: to @dfn{distance-vector} protocols, such as @acronym{RIP} or
! 10: @acronym{BGP}, where routers describe available @dfn{paths} (i.e@. routes)
! 11: to each other, in @dfn{link-state} protocols routers instead
! 12: describe the state of their links to their immediate neighbouring
! 13: routers.
! 14:
! 15: @cindex Link State Announcement
! 16: @cindex Link State Advertisement
! 17: @cindex LSA flooding
! 18: @cindex Link State DataBase
! 19: Each router describes their link-state information in a message known
! 20: as an @acronym{LSA,Link State Advertisement}, which is then propogated
! 21: through to all other routers in a link-state routing domain, by a
! 22: process called @dfn{flooding}. Each router thus builds up an
! 23: @acronym{LSDB,Link State Database} of all the link-state messages. From
! 24: this collection of LSAs in the LSDB, each router can then calculate the
! 25: shortest path to any other router, based on some common metric, by
! 26: using an algorithm such as @url{http://www.cs.utexas.edu/users/EWD/,
! 27: Edgser Dijkstra}'s @acronym{SPF,Shortest Path First}.
! 28:
! 29: @cindex Link-state routing protocol advantages
! 30: By describing connectivity of a network in this way, in terms of
! 31: routers and links rather than in terms of the paths through a network,
! 32: a link-state protocol can use less bandwidth and converge more quickly
! 33: than other protocols. A link-state protocol need distribute only one
! 34: link-state message throughout the link-state domain when a link on any
! 35: single given router changes state, in order for all routers to
! 36: reconverge on the best paths through the network. In contrast, distance
! 37: vector protocols can require a progression of different path update
! 38: messages from a series of different routers in order to converge.
! 39:
! 40: @cindex Link-state routing protocol disadvantages
! 41: The disadvantage to a link-state protocol is that the process of
! 42: computing the best paths can be relatively intensive when compared to
! 43: distance-vector protocols, in which near to no computation need be done
! 44: other than (potentially) select between multiple routes. This overhead
! 45: is mostly negligible for modern embedded CPUs, even for networks with
! 46: thousands of nodes. The primary scaling overhead lies more in coping
! 47: with the ever greater frequency of LSA updates as the size of a
! 48: link-state area increases, in managing the @acronym{LSDB} and required
! 49: flooding.
! 50:
! 51: This section aims to give a distilled, but accurate, description of the
! 52: more important workings of @acronym{OSPF}@ which an administrator may need
! 53: to know to be able best configure and trouble-shoot @acronym{OSPF}@.
! 54:
! 55: @subsection OSPF Mechanisms
! 56:
! 57: @acronym{OSPF} defines a range of mechanisms, concerned with detecting,
! 58: describing and propogating state through a network. These mechanisms
! 59: will nearly all be covered in greater detail further on. They may be
! 60: broadly classed as:
! 61:
! 62: @table @dfn
! 63: @cindex OSPF Hello Protocol overview
! 64: @item The Hello Protocol
! 65:
! 66: @cindex OSPF Hello Protocol
! 67: The OSPF Hello protocol allows OSPF to quickly detect changes in
! 68: two-way reachability between routers on a link. OSPF can additionally
! 69: avail of other sources of reachability information, such as link-state
! 70: information provided by hardware, or through dedicated reachability
! 71: protocols such as @acronym{BFD,Bi-directional Forwarding Detection}.
! 72:
! 73: OSPF also uses the Hello protocol to propagate certain state between
! 74: routers sharing a link, for example:
! 75:
! 76: @itemize @bullet
! 77: @item Hello protocol configured state, such as the dead-interval.
! 78: @item Router priority, for DR/BDR election.
! 79: @item DR/BDR election results.
! 80: @item Any optional capabilities supported by each router.
! 81: @end itemize
! 82:
! 83: The Hello protocol is comparatively trivial and will not be explored in
! 84: greater detail than here.
! 85:
! 86: @cindex OSPF LSA overview
! 87: @item LSAs
! 88:
! 89: At the heart of @acronym{OSPF} are @acronym{LSA,Link State
! 90: Advertisement} messages. Despite the name, some @acronym{LSA}s do not,
! 91: strictly speaking, describe link-state information. Common
! 92: @acronym{LSA}s describe information such as:
! 93:
! 94: @itemize @bullet
! 95: @item
! 96: Routers, in terms of their links.
! 97: @item
! 98: Networks, in terms of attached routers.
! 99: @item
! 100: Routes, external to a link-state domain:
! 101:
! 102: @itemize @bullet
! 103: @item External Routes
! 104:
! 105: Routes entirely external to @acronym{OSPF}@. Routers originating such
! 106: routes are known as @acronym{ASBR,Autonomous-System Border Router}
! 107: routers.
! 108:
! 109: @item Summary Routes
! 110:
! 111: Routes which summarise routing information relating to OSPF areas
! 112: external to the OSPF link-state area at hand, originated by
! 113: @acronym{ABR,Area Boundary Router} routers.
! 114: @end itemize
! 115: @end itemize
! 116:
! 117: @item LSA Flooding
! 118: OSPF defines several related mechanisms, used to manage synchronisation of
! 119: @acronym{LSDB}s between neighbours as neighbours form adjacencies and
! 120: the propogation, or @dfn{flooding} of new or updated @acronym{LSA}s.
! 121:
! 122: @xref{OSPF Flooding}.
! 123:
! 124: @cindex OSPF Areas overview
! 125: @item Areas
! 126: OSPF provides for the protocol to be broken up into multiple smaller
! 127: and independent link-state areas. Each area must be connected to a
! 128: common backbone area by an @acronym{ABR,Area Boundary Router}. These
! 129: @acronym{ABR} routers are responsible for summarising the link-state
! 130: routing information of an area into @dfn{Summary LSAs}, possibly in a
! 131: condensed (i.e. aggregated) form, and then originating these summaries
! 132: into all other areas the @acronym{ABR} is connected to.
! 133:
! 134: Note that only summaries and external routes are passed between areas.
! 135: As these describe @emph{paths}, rather than any router link-states,
! 136: routing between areas hence is by @dfn{distance-vector}, @strong{not}
! 137: link-state.
! 138:
! 139: @xref{OSPF Areas}.
! 140: @end table
! 141:
! 142: @subsection OSPF LSAs
! 143:
! 144: @acronym{LSA}s are the core object in OSPF@. Everything else in OSPF
! 145: revolves around detecting what to describe in LSAs, when to update
! 146: them, how to flood them throughout a network and how to calculate
! 147: routes from them.
! 148:
! 149: There are a variety of different @acronym{LSA}s, for purposes such
! 150: as describing actual link-state information, describing paths (i.e.
! 151: routes), describing bandwidth usage of links for
! 152: @acronym{TE,Traffic Engineering} purposes, and even arbitrary data
! 153: by way of @emph{Opaque} @acronym{LSA}s.
! 154:
! 155: @subsubsection LSA Header
! 156: All LSAs share a common header with the following information:
! 157:
! 158: @itemize @bullet
! 159: @item Type
! 160:
! 161: Different types of @acronym{LSA}s describe different things in
! 162: @acronym{OSPF}@. Types include:
! 163:
! 164: @itemize @bullet
! 165: @item Router LSA
! 166: @item Network LSA
! 167: @item Network Summary LSA
! 168: @item Router Summary LSA
! 169: @item AS-External LSA
! 170: @end itemize
! 171:
! 172: The specifics of the different types of LSA are examined below.
! 173:
! 174: @item Advertising Router
! 175:
! 176: The Router ID of the router originating the LSA, see @ref{ospf router-id}.
! 177:
! 178: @item LSA ID
! 179:
! 180: The ID of the LSA, which is typically derived in some way from the
! 181: information the LSA describes, e.g. a Router LSA uses the Router ID as
! 182: the LSA ID, a Network LSA will have the IP address of the @acronym{DR}
! 183: as its LSA ID@.
! 184:
! 185: The combination of the Type, ID and Advertising Router ID must uniquely
! 186: identify the @acronym{LSA}@. There can however be multiple instances of
! 187: an LSA with the same Type, LSA ID and Advertising Router ID, see
! 188: @ref{OSPF LSA sequence number,,LSA Sequence Number}.
! 189:
! 190: @item Age
! 191:
! 192: A number to allow stale @acronym{LSA}s to, eventually, be purged by routers
! 193: from their @acronym{LSDB}s.
! 194:
! 195: The value nominally is one of seconds. An age of 3600, i.e. 1 hour, is
! 196: called the @dfn{MaxAge}. MaxAge LSAs are ignored in routing
! 197: calculations. LSAs must be periodically refreshed by their Advertising
! 198: Router before reaching MaxAge if they are to remain valid.
! 199:
! 200: Routers may deliberately flood LSAs with the age artificially set to
! 201: 3600 to indicate an LSA is no longer valid. This is called
! 202: @dfn{flushing} of an LSA@.
! 203:
! 204: It is not abnormal to see stale LSAs in the LSDB, this can occur where
! 205: a router has shutdown without flushing its LSA(s), e.g. where it has
! 206: become disconnected from the network. Such LSAs do little harm.
! 207:
! 208: @anchor{OSPF LSA sequence number}
! 209: @item Sequence Number
! 210:
! 211: A number used to distinguish newer instances of an LSA from older instances.
! 212: @end itemize
! 213:
! 214: @subsubsection Link-State LSAs
! 215: Of all the various kinds of @acronym{LSA}s, just two types comprise the
! 216: actual link-state part of @acronym{OSPF}, Router @acronym{LSA}s and
! 217: Network @acronym{LSA}s. These LSA types are absolutely core to the
! 218: protocol.
! 219:
! 220: Instances of these LSAs are specific to the link-state area in which
! 221: they are originated. Routes calculated from these two LSA types are
! 222: called @dfn{intra-area routes}.
! 223:
! 224: @itemize @bullet
! 225: @item Router LSA
! 226:
! 227: Each OSPF Router must originate a router @acronym{LSA} to describe
! 228: itself. In it, the router lists each of its @acronym{OSPF} enabled
! 229: interfaces, for the given link-state area, in terms of:
! 230:
! 231: @itemize @bullet
! 232: @item Cost
! 233:
! 234: The output cost of that interface, scaled inversely to some commonly known
! 235: reference value, @xref{OSPF auto-cost reference-bandwidth,,auto-cost
! 236: reference-bandwidth}.
! 237:
! 238: @item Link Type
! 239: @itemize @bullet
! 240: @item Transit Network
! 241:
! 242: A link to a multi-access network, on which the router has at least one
! 243: Full adjacency with another router.
! 244:
! 245: @item @acronym{PtP,Point-to-Point}
! 246:
! 247: A link to a single remote router, with a Full adjacency. No
! 248: @acronym{DR, Designated Router} is elected on such links; no network
! 249: LSA is originated for such a link.
! 250:
! 251: @item Stub
! 252:
! 253: A link with no adjacent neighbours, or a host route.
! 254: @end itemize
! 255:
! 256: @item Link ID and Data
! 257:
! 258: These values depend on the Link Type:
! 259:
! 260: @multitable @columnfractions .18 .32 .32
! 261: @headitem Link Type @tab Link ID @tab Link Data
! 262:
! 263: @item Transit
! 264: @tab Link IP address of the @acronym{DR}
! 265: @tab Interface IP address
! 266:
! 267: @item Point-to-Point
! 268: @tab Router ID of the remote router
! 269: @tab Local interface IP address,
! 270: or the @acronym{ifindex,MIB-II interface index}
! 271: for unnumbered links
! 272:
! 273: @item Stub
! 274: @tab IP address
! 275: @tab Subnet Mask
! 276:
! 277: @end multitable
! 278: @end itemize
! 279:
! 280: Links on a router may be listed multiple times in the Router LSA, e.g.
! 281: a @acronym{PtP} interface on which OSPF is enabled must @emph{always}
! 282: be described by a Stub link in the Router @acronym{LSA}, in addition to
! 283: being listed as PtP link in the Router @acronym{LSA} if the adjacency
! 284: with the remote router is Full.
! 285:
! 286: Stub links may also be used as a way to describe links on which OSPF is
! 287: @emph{not} spoken, known as @dfn{passive interfaces}, see @ref{OSPF
! 288: passive-interface,,passive-interface}.
! 289:
! 290: @item Network LSA
! 291:
! 292: On multi-access links (e.g. ethernets, certain kinds of ATM and X@.25
! 293: configurations), routers elect a @acronym{DR}@. The @acronym{DR} is
! 294: responsible for originating a Network @acronym{LSA}, which helps reduce
! 295: the information needed to describe multi-access networks with multiple
! 296: routers attached. The @acronym{DR} also acts as a hub for the flooding of
! 297: @acronym{LSA}s on that link, thus reducing flooding overheads.
! 298:
! 299: The contents of the Network LSA describes the:
! 300:
! 301: @itemize @bullet
! 302: @item Subnet Mask
! 303:
! 304: As the @acronym{LSA} ID of a Network LSA must be the IP address of the
! 305: @acronym{DR}, the Subnet Mask together with the @acronym{LSA} ID gives
! 306: you the network address.
! 307:
! 308: @item Attached Routers
! 309:
! 310: Each router fully-adjacent with the @acronym{DR} is listed in the LSA,
! 311: by their Router-ID. This allows the corresponding Router @acronym{LSA}s to be
! 312: easily retrieved from the @acronym{LSDB}@.
! 313: @end itemize
! 314: @end itemize
! 315:
! 316: Summary of Link State LSAs:
! 317:
! 318: @multitable @columnfractions .18 .32 .40
! 319: @headitem LSA Type @tab LSA ID Describes @tab LSA Data Describes
! 320:
! 321: @item Router LSA
! 322: @tab The Router ID
! 323: @tab The @acronym{OSPF} enabled links of the router, within
! 324: a specific link-state area.
! 325:
! 326: @item Network LSA
! 327: @tab The IP address of the @acronym{DR} for the network
! 328: @tab The Subnet Mask of the network, and the Router IDs of all routers
! 329: on the network.
! 330: @end multitable
! 331:
! 332: With an LSDB composed of just these two types of @acronym{LSA}, it is
! 333: possible to construct a directed graph of the connectivity between all
! 334: routers and networks in a given OSPF link-state area. So, not
! 335: surprisingly, when OSPF routers build updated routing tables, the first
! 336: stage of @acronym{SPF} calculation concerns itself only with these two
! 337: LSA types.
! 338:
! 339: @subsubsection Link-State LSA Examples
! 340:
! 341: The example below (@pxref{OSPF Link-State LSA Example}) shows two
! 342: @acronym{LSA}s, both originated by the same router (Router ID
! 343: 192.168.0.49) and with the same @acronym{LSA} ID (192.168.0.49), but of
! 344: different LSA types.
! 345:
! 346: The first LSA being the router LSA describing 192.168.0.49's links: 2 links
! 347: to multi-access networks with fully-adjacent neighbours (i.e. Transit
! 348: links) and 1 being a Stub link (no adjacent neighbours).
! 349:
! 350: The second LSA being a Network LSA, for which 192.168.0.49 is the
! 351: @acronym{DR}, listing the Router IDs of 4 routers on that network which
! 352: are fully adjacent with 192.168.0.49.
! 353:
! 354: @anchor{OSPF Link-State LSA Example}
! 355: @example
! 356: # show ip ospf database router 192.168.0.49
! 357:
! 358: OSPF Router with ID (192.168.0.53)
! 359:
! 360:
! 361: Router Link States (Area 0.0.0.0)
! 362:
! 363: LS age: 38
! 364: Options: 0x2 : *|-|-|-|-|-|E|*
! 365: LS Flags: 0x6
! 366: Flags: 0x2 : ASBR
! 367: LS Type: router-LSA
! 368: Link State ID: 192.168.0.49
! 369: Advertising Router: 192.168.0.49
! 370: LS Seq Number: 80000f90
! 371: Checksum: 0x518b
! 372: Length: 60
! 373: Number of Links: 3
! 374:
! 375: Link connected to: a Transit Network
! 376: (Link ID) Designated Router address: 192.168.1.3
! 377: (Link Data) Router Interface address: 192.168.1.3
! 378: Number of TOS metrics: 0
! 379: TOS 0 Metric: 10
! 380:
! 381: Link connected to: a Transit Network
! 382: (Link ID) Designated Router address: 192.168.0.49
! 383: (Link Data) Router Interface address: 192.168.0.49
! 384: Number of TOS metrics: 0
! 385: TOS 0 Metric: 10
! 386:
! 387: Link connected to: Stub Network
! 388: (Link ID) Net: 192.168.3.190
! 389: (Link Data) Network Mask: 255.255.255.255
! 390: Number of TOS metrics: 0
! 391: TOS 0 Metric: 39063
! 392: # show ip ospf database network 192.168.0.49
! 393:
! 394: OSPF Router with ID (192.168.0.53)
! 395:
! 396:
! 397: Net Link States (Area 0.0.0.0)
! 398:
! 399: LS age: 285
! 400: Options: 0x2 : *|-|-|-|-|-|E|*
! 401: LS Flags: 0x6
! 402: LS Type: network-LSA
! 403: Link State ID: 192.168.0.49 (address of Designated Router)
! 404: Advertising Router: 192.168.0.49
! 405: LS Seq Number: 80000074
! 406: Checksum: 0x0103
! 407: Length: 40
! 408: Network Mask: /29
! 409: Attached Router: 192.168.0.49
! 410: Attached Router: 192.168.0.52
! 411: Attached Router: 192.168.0.53
! 412: Attached Router: 192.168.0.54
! 413: @end example
! 414:
! 415: Note that from one LSA, you can find the other. E.g. Given the
! 416: Network-LSA you have a list of Router IDs on that network, from which
! 417: you can then look up, in the local @acronym{LSDB}, the matching Router
! 418: LSA@. From that Router-LSA you may (potentially) find links to other
! 419: Transit networks and Routers IDs which can be used to lookup the
! 420: corresponding Router or Network LSA@. And in that fashion, one can find
! 421: all the Routers and Networks reachable from that starting @acronym{LSA}@.
! 422:
! 423: Given the Router LSA instead, you have the IP address of the
! 424: @acronym{DR} of any attached transit links. Network LSAs will have that IP
! 425: as their LSA ID, so you can then look up that Network LSA and from that
! 426: find all the attached routers on that link, leading potentially to more
! 427: links and Network and Router LSAs, etc. etc.
! 428:
! 429: From just the above two @acronym{LSA}s, one can already see the
! 430: following partial topology:
! 431: @example
! 432: @group
! 433:
! 434:
! 435: --------------------- Network: ......
! 436: | Designated Router IP: 192.168.1.3
! 437: |
! 438: IP: 192.168.1.3
! 439: (transit link)
! 440: (cost: 10)
! 441: Router ID: 192.168.0.49(stub)---------- IP: 192.168.3.190/32
! 442: (cost: 10) (cost: 39063)
! 443: (transit link)
! 444: IP: 192.168.0.49
! 445: |
! 446: |
! 447: ------------------------------ Network: 192.168.0.48/29
! 448: | | | Designated Router IP: 192.168.0.49
! 449: | | |
! 450: | | Router ID: 192.168.0.54
! 451: | |
! 452: | Router ID: 192.168.0.53
! 453: |
! 454: Router ID: 192.168.0.52
! 455: @end group
! 456: @end example
! 457:
! 458: Note the Router IDs, though they look like IP addresses and often are
! 459: IP addresses, are not strictly speaking IP addresses, nor need they be
! 460: reachable addresses (though, OSPF will calculate routes to Router IDs).
! 461:
! 462: @subsubsection External LSAs
! 463:
! 464: External, or "Type 5", @acronym{LSA}s describe routing information which is
! 465: entirely external to @acronym{OSPF}, and is "injected" into
! 466: @acronym{OSPF}@. Such routing information may have come from another
! 467: routing protocol, such as RIP or BGP, they may represent static routes
! 468: or they may represent a default route.
! 469:
! 470: An @acronym{OSPF} router which originates External @acronym{LSA}s is known as an
! 471: @acronym{ASBR,AS Boundary Router}. Unlike the link-state @acronym{LSA}s, and
! 472: most other @acronym{LSA}s, which are flooded only within the area in
! 473: which they originate, External @acronym{LSA}s are flooded through-out
! 474: the @acronym{OSPF} network to all areas capable of carrying External
! 475: @acronym{LSA}s (@pxref{OSPF Areas}).
! 476:
! 477: Routes internal to OSPF (intra-area or inter-area) are always preferred
! 478: over external routes.
! 479:
! 480: The External @acronym{LSA} describes the following:
! 481:
! 482: @itemize @bullet
! 483: @item IP Network number
! 484:
! 485: The IP Network number of the route is described by the @acronym{LSA} ID
! 486: field.
! 487:
! 488: @item IP Network Mask
! 489:
! 490: The body of the External LSA describes the IP Network Mask of the
! 491: route. This, together with the @acronym{LSA} ID, describes the prefix
! 492: of the IP route concerned.
! 493:
! 494: @item Metric
! 495:
! 496: The cost of the External Route. This cost may be an OSPF cost (also
! 497: known as a "Type 1" metric), i.e. equivalent to the normal OSPF costs,
! 498: or an externally derived cost ("Type 2" metric) which is not comparable
! 499: to OSPF costs and always considered larger than any OSPF cost. Where
! 500: there are both Type 1 and 2 External routes for a route, the Type 1 is
! 501: always preferred.
! 502:
! 503: @item Forwarding Address
! 504:
! 505: The address of the router to forward packets to for the route. This may
! 506: be, and usually is, left as 0 to specify that the ASBR originating the
! 507: External @acronym{LSA} should be used. There must be an internal OSPF
! 508: route to the forwarding address, for the forwarding address to be
! 509: useable.
! 510:
! 511: @item Tag
! 512:
! 513: An arbitrary 4-bytes of data, not interpreted by OSPF, which may
! 514: carry whatever information about the route which OSPF speakers desire.
! 515: @end itemize
! 516:
! 517: @subsubsection AS External LSA Example
! 518:
! 519: To illustrate, below is an example of an External @acronym{LSA} in the
! 520: @acronym{LSDB} of an OSPF router. It describes a route to the IP prefix
! 521: of 192.168.165.0/24, originated by the ASBR with Router-ID
! 522: 192.168.0.49. The metric of 20 is external to OSPF. The forwarding
! 523: address is 0, so the route should forward to the originating ASBR if
! 524: selected.
! 525:
! 526: @example
! 527: @group
! 528: # show ip ospf database external 192.168.165.0
! 529: LS age: 995
! 530: Options: 0x2 : *|-|-|-|-|-|E|*
! 531: LS Flags: 0x9
! 532: LS Type: AS-external-LSA
! 533: Link State ID: 192.168.165.0 (External Network Number)
! 534: Advertising Router: 192.168.0.49
! 535: LS Seq Number: 800001d8
! 536: Checksum: 0xea27
! 537: Length: 36
! 538: Network Mask: /24
! 539: Metric Type: 2 (Larger than any link state path)
! 540: TOS: 0
! 541: Metric: 20
! 542: Forward Address: 0.0.0.0
! 543: External Route Tag: 0
! 544: @end group
! 545: @end example
! 546:
! 547: We can add this to our partial topology from above, which now looks
! 548: like:
! 549: @example
! 550: @group
! 551: --------------------- Network: ......
! 552: | Designated Router IP: 192.168.1.3
! 553: |
! 554: IP: 192.168.1.3 /---- External route: 192.168.165.0/24
! 555: (transit link) / Cost: 20 (External metric)
! 556: (cost: 10) /
! 557: Router ID: 192.168.0.49(stub)---------- IP: 192.168.3.190/32
! 558: (cost: 10) (cost: 39063)
! 559: (transit link)
! 560: IP: 192.168.0.49
! 561: |
! 562: |
! 563: ------------------------------ Network: 192.168.0.48/29
! 564: | | | Designated Router IP: 192.168.0.49
! 565: | | |
! 566: | | Router ID: 192.168.0.54
! 567: | |
! 568: | Router ID: 192.168.0.53
! 569: |
! 570: Router ID: 192.168.0.52
! 571: @end group
! 572: @end example
! 573:
! 574: @subsubsection Summary LSAs
! 575:
! 576: Summary LSAs are created by @acronym{ABR}s to summarise the destinations available within one area to other areas. These LSAs may describe IP networks, potentially in aggregated form, or @acronym{ASBR} routers.
! 577:
! 578: @anchor{OSPF Flooding}
! 579: @subsection OSPF Flooding
! 580:
! 581: @anchor{OSPF Areas}
! 582: @subsection OSPF Areas
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