rfc2328.hastabs.txt

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    information	obtained from outside the area can be used.  This
    protects intra-area	routing	from the injection of bad routing
    information.  We discuss inter-area	routing	in Section 3.2.


    3.1.  The backbone of the Autonomous System

	The OSPF backbone is the special OSPF Area 0 (often written as
	Area 0.0.0.0, since OSPF Area ID's are typically formatted as IP
	addresses). The	OSPF backbone always contains all area border
	routers. The backbone is responsible for distributing routing
	information between non-backbone areas.	The backbone must be
	contiguous. However, it	need not be physically contiguous;
	backbone connectivity can be established/maintained through the
	configuration of virtual links.

	Virtual	links can be configured	between	any two	backbone routers
	that have an interface to a common non-backbone	area.  Virtual
	links belong to	the backbone.  The protocol treats two routers
	joined by a virtual link as if they were connected by an
	unnumbered point-to-point backbone network.  On	the graph of the
	backbone, two such routers are joined by arcs whose costs are
	the intra-area distances between the two routers.  The routing
	protocol traffic that flows along the virtual link uses	intra-
	area routing only.


    3.2.  Inter-area routing

	When routing a packet between two non-backbone areas the
	backbone is used.  The path that the packet will travel	can be
	broken up into three contiguous	pieces:	an intra-area path from
	the source to an area border router, a backbone	path between the
	source and destination areas, and then another intra-area path
	to the destination.  The algorithm finds the set of such paths
	that have the smallest cost.

	Looking	at this	another	way, inter-area	routing	can be pictured
	as forcing a star configuration	on the Autonomous System, with
	the backbone as	hub and	each of	the non-backbone areas as
	spokes.




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	The topology of	the backbone dictates the backbone paths used
	between	areas.	The topology of	the backbone can be enhanced by
	adding virtual links.  This gives the system administrator some
	control	over the routes	taken by inter-area traffic.

	The correct area border	router to use as the packet exits the
	source area is chosen in exactly the same way routers
	advertising external routes are	chosen.	 Each area border router
	in an area summarizes for the area its cost to all networks
	external to the	area.  After the SPF tree is calculated	for the
	area, routes to	all inter-area destinations are	calculated by
	examining the summaries	of the area border routers.


    3.3.  Classification of routers

	Before the introduction	of areas, the only OSPF	routers	having a
	specialized function were those	advertising external routing
	information, such as Router RT5	in Figure 2.  When the AS is
	split into OSPF	areas, the routers are further divided according
	to function into the following four overlapping	categories:


	Internal routers
	    A router with all directly connected networks belonging to
	    the	same area. These routers run a single copy of the basic
	    routing algorithm.

	Area border routers
	    A router that attaches to multiple areas.  Area border
	    routers run	multiple copies	of the basic algorithm,	one copy
	    for	each attached area. Area border	routers	condense the
	    topological	information of their attached areas for
	    distribution to the	backbone.  The backbone	in turn
	    distributes	the information	to the other areas.

	Backbone routers
	    A router that has an interface to the backbone area.  This
	    includes all routers that interface	to more	than one area
	    (i.e., area	border routers).  However, backbone routers do
	    not	have to	be area	border routers.	 Routers with all
	    interfaces connecting to the backbone area are supported.



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	AS boundary routers
	    A router that exchanges routing information	with routers
	    belonging to other Autonomous Systems.  Such a router
	    advertises AS external routing information throughout the
	    Autonomous System.	The paths to each AS boundary router are
	    known by every router in the AS.  This classification is
	    completely independent of the previous classifications: AS
	    boundary routers may be internal or	area border routers, and
	    may	or may not participate in the backbone.


    3.4.  A sample area	configuration

	Figure 6 shows a sample	area configuration.  The first area
	consists of networks N1-N4, along with their attached routers
	RT1-RT4.  The second area consists of networks N6-N8, along with
	their attached routers RT7, RT8, RT10 and RT11.	 The third area
	consists of networks N9-N11 and	Host H1, along with their
	attached routers RT9, RT11 and RT12.  The third	area has been
	configured so that networks N9-N11 and Host H1 will all	be
	grouped	into a single route, when advertised external to the
	area (see Section 3.5 for more details).

	In Figure 6, Routers RT1, RT2, RT5, RT6, RT8, RT9 and RT12 are
	internal routers.  Routers RT3,	RT4, RT7, RT10 and RT11	are area
	border routers.	 Finally, as before, Routers RT5 and RT7 are AS
	boundary routers.

	Figure 7 shows the resulting link-state	database for the Area 1.
	The figure completely describes	that area's intra-area routing.
	It also	shows the complete view	of the internet	for the	two
	internal routers RT1 and RT2.  It is the job of	the area border
	routers, RT3 and RT4, to advertise into	Area 1 the distances to
	all destinations external to the area.	These are indicated in
	Figure 7 by the	dashed stub routes.  Also, RT3 and RT4 must
	advertise into Area 1 the location of the AS boundary routers
	RT5 and	RT7.  Finally, AS-external-LSAs	from RT5 and RT7 are
	flooded	throughout the entire AS, and in particular throughout
	Area 1.	 These LSAs are	included in Area 1's database, and yield
	routes to Networks N12-N15.

	Routers	RT3 and	RT4 must also summarize	Area 1's topology for



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	     ...........................
	     .	 +		       .
	     .	 | 3+---+	       .      N12      N14
	     . N1|--|RT1|\ 1	       .	\ N13 /
	     .	 |  +---+ \	       .	8\ |8/8
	     .	 +	   \ ____      .	  \|/
	     .		    /	 \   1+---+8	8+---+6
	     .		   *  N3  *---|RT4|------|RT5|--------+
	     .		    \____/    +---+	 +---+	      |
	     .	  +	    /	   \   .	   |7	      |
	     .	  | 3+---+ /	    \  .	   |	      |
	     .	N2|--|RT2|/1	    1\ .	   |6	      |
	     .	  |  +---+	      +---+8	6+---+	      |
	     .	  +		      |RT3|------|RT6|	      |
	     .			      +---+	 +---+	      |
	     .			    2/ .	 Ia|7	      |
	     .			    /  .	   |	      |
	     .		   +---------+ .	   |	      |
	     .Area 1	       N4      .	   |	      |
	     ...........................	   |	      |
	  ..........................		   |	      |
	  .	       N11	   .		   |	      |
	  .	   +---------+	   .		   |	      |
	  .		|	   .		   |	      |	   N12
	  .		|3	   .		 Ib|5	      |6 2/
	  .	      +---+	   .		 +----+	    +---+/
	  .	      |RT9|	   .	.........|RT10|.....|RT7|---N15.
	  .	      +---+	   .	.	 +----+	    +---+ 9    .
	  .		|1	   .	.    +	/3    1\      |1       .
	  .	       _|__	   .	.    | /	\   __|_       .
	  .	      /	   \	  1+----+2   |/		 \ /	\      .
	  .	     *	N9  *------|RT11|----|		  *  N6	 *     .
	  .	      \____/	   +----+    |		   \____/      .
	  .		|	   .	.    |		      |	       .
	  .		|1	   .	.    +		      |1       .
	  .  +--+   10+----+	   .	.   N8		    +---+      .
	  .  |H1|-----|RT12|	   .	.		    |RT8|      .
	  .  +--+SLIP +----+	   .	.		    +---+      .
	  .		|2	   .	.		      |4       .
	  .		|	   .	.		      |	       .
	  .	   +---------+	   .	.		  +--------+   .



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	  .	       N10	   .	.		      N7       .
	  .			   .	.Area 2			       .
	  .Area	3		   .	................................
	  ..........................

		    Figure 6: A	sample OSPF area configuration

	distribution to	the backbone.  Their backbone LSAs are shown in
	Table 4.  These	summaries show which networks are contained in
	Area 1 (i.e., Networks N1-N4), and the distance	to these
	networks from the routers RT3 and RT4 respectively.


	The link-state database	for the	backbone is shown in Figure 8.
	The set	of routers pictured are	the backbone routers.  Router
	RT11 is	a backbone router because it belongs to	two areas.  In
	order to make the backbone connected, a	virtual	link has been
	configured between Routers R10 and R11.

	The area border	routers	RT3, RT4, RT7, RT10 and	RT11 condense
	the routing information	of their attached non-backbone areas for
	distribution via the backbone; these are the dashed stubs that
	appear in Figure 8.  Remember that the third area has been
	configured to condense Networks	N9-N11 and Host	H1 into	a single
	route.	This yields a single dashed line for networks N9-N11 and
	Host H1	in Figure 8.  Routers RT5 and RT7 are AS boundary
	routers; their externally derived information also appears on
	the graph in Figure 8 as stubs.



		     Network   RT3 adv.	  RT4 adv.
		     _____________________________
		     N1	       4	  4
		     N2	       4	  4
		     N3	       1	  1
		     N4	       2	  3

	      Table 4: Networks	advertised to the backbone
			by Routers RT3 and RT4.





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			       **FROM**

			  |RT|RT|RT|RT|RT|RT|
			  |1 |2	|3 |4 |5 |7 |N3|
		       ----- -------------------
		       RT1|  |	|  |  |	 |  |0 |
		       RT2|  |	|  |  |	 |  |0 |
		       RT3|  |	|  |  |	 |  |0 |
		   *   RT4|  |	|  |  |	 |  |0 |
		   *   RT5|  |	|14|8 |	 |  |  |
		   T   RT7|  |	|20|14|	 |  |  |
		   O	N1|3 |	|  |  |	 |  |  |
		   *	N2|  |3	|  |  |	 |  |  |
		   *	N3|1 |1	|1 |1 |	 |  |  |
			N4|  |	|2 |  |	 |  |  |
		     Ia,Ib|  |	|20|27|	 |  |  |
			N6|  |	|16|15|	 |  |  |
			N7|  |	|20|19|	 |  |  |
			N8|  |	|18|18|	 |  |  |
		 N9-N11,H1|  |	|29|36|	 |  |  |
		       N12|  |	|  |  |8 |2 |  |
		       N13|  |	|  |  |8 |  |  |
		       N14|  |	|  |  |8 |  |  |
		       N15|  |	|  |  |	 |9 |  |

		      Figure 7:	Area 1's Database.

	      Networks and routers are represented by vertices.
	      An edge of cost X	connects Vertex	A to Vertex B iff
	      the intersection of Column A and Row B is	marked
			       with an X.













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				  **FROM**

			    |RT|RT|RT|RT|RT|RT|RT
			    |3 |4 |5 |6	|7 |10|11|
			 ------------------------
			 RT3|  |  |  |6	|  |  |	 |
			 RT4|  |  |8 |	|  |  |	 |
			 RT5|  |8 |  |6	|6 |  |	 |
			 RT6|8 |  |7 |	|  |5 |	 |
			 RT7|  |  |6 |	|  |  |	 |
		     *	RT10|  |  |  |7	|  |  |2 |
		     *	RT11|  |  |  |	|  |3 |	 |
		     T	  N1|4 |4 |  |	|  |  |	 |
		     O	  N2|4 |4 |  |	|  |  |	 |
		     *	  N3|1 |1 |  |	|  |  |	 |
		     *	  N4|2 |3 |  |	|  |  |	 |
			  Ia|  |  |  |	|  |5 |	 |
			  Ib|  |  |  |7	|  |  |	 |
			  N6|  |  |  |	|1 |1 |3 |
			  N7|  |  |  |	|5 |5 |7 |
			  N8|  |  |  |	|4 |3 |2 |
		   N9-N11,H1|  |  |  |	|  |  |11|
			 N12|  |  |8 |	|2 |  |	 |