rfc2328.hastabs.txt

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						  **FROM**

					   *	  |RT1|RT2|
		+---+Ia	   +---+	   *   ------------
		|RT1|------|RT2|	   T   RT1|   |	X |
		+---+	 Ib+---+	   O   RT2| X |	  |
					   *	Ia|   |	X |
					   *	Ib| X |	  |

		     Physical point-to-point networks


						  **FROM**
		      +---+		   *
		      |RT7|		   *	  |RT7|	N3|
		      +---+		   T   ------------
			|		   O   RT7|   |	  |
	    +----------------------+	   *	N3| X |	  |
		       N3		   *

			      Stub networks

						  **FROM**
		+---+	   +---+
		|RT3|	   |RT4|	      |RT3|RT4|RT5|RT6|N2 |
		+---+	   +---+	*  ------------------------
		  |    N2    |		*  RT3|	  |   |	  |   |	X |
	    +----------------------+	T  RT4|	  |   |	  |   |	X |
		  |	     |		O  RT5|	  |   |	  |   |	X |
		+---+	   +---+	*  RT6|	  |   |	  |   |	X |
		|RT5|	   |RT6|	*   N2|	X | X |	X | X |	  |
		+---+	   +---+

			  Broadcast or NBMA networks



		    Figure 1a: Network map components




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



	The top	of Figure 1a shows two routers connected by a point-to-
	point link. In the resulting link-state	database graph,	the two
	router vertices	are directly connected by a pair of edges, one
	in each	direction. Interfaces to point-to-point	networks need
	not be assigned	IP addresses.  When interface addresses	are
	assigned, they are modelled as stub links, with	each router
	advertising a stub connection to the other router's interface
	address. Optionally, an	IP subnet can be assigned to the point-
	to-point network. In this case,	both routers advertise a stub
	link to	the IP subnet, instead of advertising each others' IP
	interface addresses.

	The middle of Figure 1a	shows a	network	with only one attached
	router (i.e., a	stub network). In this case, the network appears
	on the end of a	stub connection	in the link-state database's
	graph.

	When multiple routers are attached to a	broadcast network, the
	link-state database graph shows	all routers bidirectionally
	connected to the network vertex. This is pictured at the bottom
	of Figure 1a.

	Each network (stub or transit) in the graph has	an IP address
	and associated network mask.  The mask indicates the number of
	nodes on the network.  Hosts attached directly to routers
	(referred to as	host routes) appear on the graph as stub
	networks.  The network mask for	a host route is	always
	0xffffffff, which indicates the	presence of a single node.


	2.1.1.	Representation of non-broadcast	networks

	    As mentioned previously, OSPF can run over non-broadcast
	    networks in	one of two modes: NBMA or Point-to-MultiPoint.
	    The	choice of mode determines the way that the Hello



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	    protocol and flooding work over the	non-broadcast network,
	    and	the way	that the network is represented	in the link-
	    state database.

	    In NBMA mode, OSPF emulates	operation over a broadcast
	    network: a Designated Router is elected for	the NBMA
	    network, and the Designated	Router originates an LSA for the
	    network. The graph representation for broadcast networks and
	    NBMA networks is identical.	This representation is pictured
	    in the middle of Figure 1a.

	    NBMA mode is the most efficient way	to run OSPF over non-
	    broadcast networks,	both in	terms of link-state database
	    size and in	terms of the amount of routing protocol	traffic.
	    However, it	has one	significant restriction: it requires all
	    routers attached to	the NBMA network to be able to
	    communicate	directly. This restriction may be met on some
	    non-broadcast networks, such as an ATM subnet utilizing
	    SVCs. But it is often not met on other non-broadcast
	    networks, such as PVC-only Frame Relay networks. On	non-
	    broadcast networks where not all routers can communicate
	    directly you can break the non-broadcast network into
	    logical subnets, with the routers on each subnet being able
	    to communicate directly, and then run each separate	subnet
	    as an NBMA network (see [Ref15]). This however requires
	    quite a bit	of administrative overhead, and	is prone to
	    misconfiguration. It is probably better to run such	a non-
	    broadcast network in Point-to-Multipoint mode.

	    In Point-to-MultiPoint mode, OSPF treats all router-to-
	    router connections over the	non-broadcast network as if they
	    were point-to-point	links. No Designated Router is elected
	    for	the network, nor is there an LSA generated for the
	    network. In	fact, a	vertex for the Point-to-MultiPoint
	    network does not appear in the graph of the	link-state
	    database.

	    Figure 1b illustrates the link-state database representation
	    of a Point-to-MultiPoint network. On the left side of the
	    figure, a Point-to-MultiPoint network is pictured. It is
	    assumed that all routers can communicate directly, except
	    for	routers	RT4 and	RT5. I3	though I6 indicate the routers'



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	    IP interface addresses on the Point-to-MultiPoint network.
	    In the graphical representation of the link-state database,
	    routers that can communicate directly over the Point-to-
	    MultiPoint network are joined by bidirectional edges, and
	    each router	also has a stub	connection to its own IP
	    interface address (which is	in contrast to the
	    representation of real point-to-point links; see Figure 1a).

	    On some non-broadcast networks, use	of Point-to-MultiPoint
	    mode and data-link protocols such as Inverse ARP (see
	    [Ref14]) will allow	autodiscovery of OSPF neighbors	even
	    though broadcast support is	not available.






						  **FROM**
		+---+	   +---+
		|RT3|	   |RT4|	      |RT3|RT4|RT5|RT6|
		+---+	   +---+	*  --------------------
		I3|    N2    |I4	*  RT3|	  | X |	X | X |
	    +----------------------+	T  RT4|	X |   |	  | X |
		I5|	     |I6	O  RT5|	X |   |	  | X |
		+---+	   +---+	*  RT6|	X | X |	X |   |
		|RT5|	   |RT6|	*   I3|	X |   |	  |   |
		+---+	   +---+	    I4|	  | X |	  |   |
					    I5|	  |   |	X |   |
					    I6|	  |   |	  | X |



		    Figure 1b: Network map components
		       Point-to-MultiPoint networks

	     All routers can communicate directly over N2, except
		routers	RT4 and	RT5. I3	through	I6 indicate IP
			   interface addresses






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	2.1.2.	An example link-state database

	    Figure 2 shows a sample map	of an Autonomous System.  The
	    rectangle labelled H1 indicates a host, which has a	SLIP
	    connection to Router RT12.	Router RT12 is therefore
	    advertising	a host route.  Lines between routers indicate
	    physical point-to-point networks.  The only	point-to-point
	    network that has been assigned interface addresses is the
	    one	joining	Routers	RT6 and	RT10.  Routers RT5 and RT7 have
	    BGP	connections to other Autonomous	Systems.  A set	of BGP-
	    learned routes have	been displayed for both	of these
	    routers.

	    A cost is associated with the output side of each router
	    interface.	This cost is configurable by the system
	    administrator.  The	lower the cost,	the more likely	the
	    interface is to be used to forward data traffic.  Costs are
	    also associated with the externally	derived	routing	data
	    (e.g., the BGP-learned routes).

	    The	directed graph resulting from the map in Figure	2 is
	    depicted in	Figure 3.  Arcs	are labelled with the cost of
	    the	corresponding router output interface.	Arcs having no
	    labelled cost have a cost of 0.  Note that arcs leading from
	    networks to	routers	always have cost 0; they are significant
	    nonetheless.  Note also that the externally	derived	routing
	    data appears on the	graph as stubs.

	    The	link-state database is pieced together from LSAs
	    generated by the routers.  In the associated graphical
	    representation, the	neighborhood of	each router or transit
	    network is represented in a	single,	separate LSA.  Figure 4
	    shows these	LSAs graphically. Router RT12 has an interface
	    to two broadcast networks and a SLIP line to a host.
	    Network N6 is a broadcast network with three attached
	    routers.  The cost of all links from Network N6 to its
	    attached routers is	0.  Note that the LSA for Network N6 is
	    actually generated by one of the network's attached	routers:
	    the	router that has	been elected Designated	Router for the
	    network.





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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	      |
				|		   |	      |
			   +---------+		   |	      |
			       N4		   |	      |
						   |	      |
						   |	      |
		       N11			   |	      |
		   +---------+			   |	      |
			|			   |	      |	   N12
			|3			   |	      |6 2/
		      +---+			   |	    +---+/
		      |RT9|			   |	    |RT7|---N15
		      +---+			   |	    +---+ 9
			|1		     +	   |	      |1
		       _|__		     |	 Ib|5	    __|_
		      /	   \	  1+----+2   |	3+----+1   /	\
		     *	N9  *------|RT11|----|---|RT10|---*  N6	 *
		      \____/	   +----+    |	 +----+	   \____/
			|		     |		      |
			|1		     +		      |1
	     +--+   10+----+		    N8		    +---+
	     |H1|-----|RT12|				    |RT8|
	     +--+SLIP +----+				    +---+
			|2				      |4
			|				      |
		   +---------+				  +--------+
		       N10				      N7



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RFC 2328		     OSPF Version 2		      April 1998


		    Figure 2: A	sample Autonomous System

				**FROM**

		 |RT|RT|RT|RT|RT|RT|RT|RT|RT|RT|RT|RT|
		 |1 |2 |3 |4 |5	|6 |7 |8 |9 |10|11|12|N3|N6|N8|N9|
	      ----- ---------------------------------------------
	      RT1|  |  |  |  |	|  |  |	 |  |  |  |  |0	|  |  |	 |
	      RT2|  |  |  |  |	|  |  |	 |  |  |  |  |0	|  |  |	 |
	      RT3|  |  |  |  |	|6 |  |	 |  |  |  |  |0	|  |  |	 |
	      RT4|  |  |  |  |8	|  |  |	 |  |  |  |  |0	|  |  |	 |
	      RT5|  |  |  |8 |	|6 |6 |	 |  |  |  |  |	|  |  |	 |
	      RT6|  |  |8 |  |7	|  |  |	 |  |5 |  |  |	|  |  |	 |
	      RT7|  |  |  |  |6	|  |  |	 |  |  |  |  |	|0 |  |	 |
	  *   RT8|  |  |  |  |	|  |  |	 |  |  |  |  |	|0 |  |	 |
	  *   RT9|  |  |  |  |	|  |  |	 |  |  |  |  |	|  |  |0 |
	  T  RT10|  |  |  |  |	|7 |  |	 |  |  |  |  |	|0 |0 |	 |
	  O  RT11|  |  |  |  |	|  |  |	 |  |  |  |  |	|  |0 |0 |
	  *  RT12|  |  |  |  |	|  |  |	 |  |  |  |  |	|  |  |0 |
	  *    N1|3 |  |  |  |	|  |  |	 |  |  |  |  |	|  |  |	 |
	       N2|  |3 |  |  |	|  |  |	 |  |  |  |  |	|  |  |	 |
	       N3|1 |1 |1 |1 |	|  |  |	 |  |  |  |  |	|  |  |	 |
	       N4|  |  |2 |  |	|  |  |	 |  |  |  |  |	|  |  |	 |
	       N6|  |  |  |  |	|  |1 |1 |  |1 |  |  |	|  |  |	 |