rfc1793.txt

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        Time T0: Router RTB comes up.

            Assume RTB supports the demand circuit OSPF modifications.
            When Router RTB comes up and establishes links to Routers
            RTC and RTD, it will flood the same information over both.
            However, LSAs sent over the demand circuit (to Router RTC)
            will have the DoNotAge bit set, while those sent over the
            leased line to Router RTD will not. Because the DoNotAge bit
            is not taken into account when comparing LSA instances, the
            routers on the right side of RTB (RTC, RTE and RTD) may or
            may not have the DoNotAge bit set in their database copies
            of RTA's and RTB's router-LSAs.  This depends on whether the
            LSAs sent over the demand link reach the routers before
            those sent over the leased line. One possibility is pictured
            in Table 2.






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RFC 1793               OSPF over Demand Circuits              April 1995


                                          LS age
            LSA                in RTC        in RTD   in RTE
            ________________________________________________
            RTA's Router-LSA   DoNotAge+20   21       21
            RTB's Router-LSA   DoNotAge+5    6        6


              Table 2: After Time T0 in Example 2, LS age
                fields on the right side of Router RTB.



                                          LS age
            LSA                in RTC       in RTD   in RTE
            _______________________________________________
            RTA's Router-LSA   5            6        6
            RTB's Router-LSA   DoNotAge+5   1785     1785


              Table 3: After Time T2 in Example 2, LS age
                fields on the right side of Router RTB.



                                          LS age
        LSA                in RTC       in RTD       in RTE
        _______________________________________________________
        RTA's Router-LSA   325          326          326
        RTB's Router-LSA   DoNotAge+5   DoNotAge+6   DoNotAge+6


              Table 4: After Time T3 in Example 2, LS age
                fields on the right side of Router RTB.



                                          LS age
        LSA                in RTC       in RTD       in RTE
        _______________________________________________________
        RTA's Router-LSA   DoNotAge+7   DoNotAge+8   DoNotAge+8
        RTB's Router-LSA   DoNotAge+5   DoNotAge+6   DoNotAge+6


              Table 5: After Time T4 in Example 2, LS age
                fields on the right side of Router RTB.






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RFC 1793               OSPF over Demand Circuits              April 1995


        Time T1: Underlying data-link connection is torn down.

            All application traffic is flowing over the leased line
            connecting Routers RTB and RTD instead of the demand
            circuit, due to the leased line's lesser OSPF cost. After
            some period of inactivity, the data-link connection
            underlying the demand circuit will be torn down. This does
            not affect the OSPF database or the routers' routing tables.

        Time T2: Router RTA refreshes its router-LSA.

            When Router RTA refreshes its router-LSA (as all routers do
            every LSRefreshInterval), Router RTB floods the refreshed
            LSA over the leased line but not over the demand circuit,
            because the contents of the LSA have not changed. This new
            LSA will not have the DoNotAge bit set, and will replace the
            old instances (whether or not they have the DoNotAge bit
            set) by virtue of its higher LS Sequence number. This is
            pictured in Table 3.

        Time T3: Leased line becomes inoperational.

            When the leased line becomes inoperational, the data-link
            connection underlying the demand circuit will be reopened,
            in order to flood a new (and changed) router-LSA for RTB and
            also to carry the application traffic between Hosts H1 and
            H2. After flooding the new LSA, all routers on the right
            side of the demand circuit will have DoNotAge set in their
            copy of RTB's router-LSA and DoNotAge clear in their copy of
            RTA's router-LSA (see Table 4).

        Time T4: In Router RTE, Router RTA's router-LSA times out.

            Refreshes of Router RTA's router-LSA are not being flooded
            over the demand circuit. However, RTA's router-LSA is aging
            in all of the routers to the right of the demand circuit.
            For this reason, the router-LSA will eventually be aged out
            and reflooded (by router RTE in our example).  Because this
            aged out LSA constitutes a real change (see Section 3.3), it
            is flooded over the demand circuit from Router RTC to RTB.
            There are then two possible scenarios. First, the LS
            Sequence number for RTA's router-LSA may be larger on RTB's
            side of the demand link. In this case, when router RTB
            receives the flushed LSA it will respond by flooding back
            the more recent instance (see Section 2.4). If instead the
            LS sequence numbers are the same, the flushed LSA will be
            flooded all the way back to Router RTA, which will then be
            forced to reoriginate the LSA.



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RFC 1793               OSPF over Demand Circuits              April 1995


            In any case, after a small period all the routers on the
            right side of the demand link will have the DoNotAge bit set
            in their copy of RTA's router-LSA (see Table 5). In the
            small interval between the flushing and waiting for a new
            instance of the LSA, there will be a temporary loss of
            connectivity between Hosts H1 and H2.

        Time T5: A non-supporting router joins.

            Suppose Router RTY now becomes operational, and does not
            support the demand circuit OSPF extensions. Router RTY's
            router-LSA then will not have the DC-bit set in its Options
            field, and as the router-LSA is flooded throughout the
            internetwork it flushes all LSAs having the DoNotAge bit set
            and causes the flooding behavior over the demand circuit to
            revert back to the normal flooding behavior defined in [1].
            However, although all LSAs will now be flooded over the
            demand circuit, regardless of whether their contents have
            really changed, Hellos will still continue to be suppressed
            on the demand circuit (see Section 3.2.2).

   4.3.  Example 3: Operation when oversubscribed

      The following example shows the behavior of the demand circuit
      extensions in the presence of oversubscribed interfaces. Note that
      the example's topology excludes the possibility of alternative
      paths. The combination of oversubscription and redundant topology
      (i.e., alternative paths) poses special problems for the demand
      circuit extensions. These problems are discussed later in Section
      7.

      Figure 4 shows a single Router (RT1) connected via demand circuits
      to three other routers (RT2-RT4). Assume that RT1 can only have
      two out of three underlying data-link connections open at once.
      This may be due to one of the following reasons: Router RT1 may be
      using a single Basic Rate ISDN interface (2 B channels) to support
      all three demand circuits, or, RT1 may be connected to a data-link
      switch (e.g., an X.25 or Frame relay switch) that is only capable
      of so many simultaneous data-link connections.

      The following events may transpire, starting with Router RT1
      coming up.









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RFC 1793               OSPF over Demand Circuits              April 1995


        Time T0: Router RT1 comes up.

            Router RT1 attempts to establish neighbor connections and
            synchronize OSPF databases with routers RT2-RT4. But,


                                                 +  +--+
                                          +---+  |--|H2|
                                +---------|RT2|--|  +--+
                               /          +---+  |
                              / ODL              +
                +--+  +      /
                |H1|--|     /                    +
                +--+  |  +---+    ODL     +---+  |  +--+
                      |--|RT1|------------|RT3|--|--|H3|
                      |  +---+            +---+  |  +--+
                      |      \                   +
                      +       \ODL
                               \                 +  +--+
                                \         +---+  |--|H4|
                                 +--------|RT4|--|  +--+
                                          +---+  |
                                                 +


                     Figure 4: Example 3's internetwork.



            because it cannot have data-link connections open to all
            three at once, it will synchronize with RT2 and RT3, while
            Hellos sent to RT4 will be discarded (see Section 1).

        Time T1: Data-link connection to RT2 closed due to inactivity.

            Assuming that no application traffic is being sent to/from
            Host H2, the underlying data-link connection to RT2 will
            eventually close due to inactivity. This will allow RT1 to
            finally synchronize with RT4; the next Hello that RT1
            attempts to send to RT4 will cause that data-link connection
            to open and synchronization with RT4 will ensue. Note that,
            until this time, H4 will have been considered unreachable by
            OSPF routing. However, data traffic would not have been
            deliverable to H4 until now in any case.







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RFC 1793               OSPF over Demand Circuits              April 1995


        Time T2: RT2's LAN interface becomes inoperational

            This causes RT2 to reissue its router-LSA. However, it may
            be unable to flood it to RT1 if RT1 already has data-link
            connections open to RT3 and RT4. While the data-link
            connection from RT2 to RT1 cannot be opened due to resource
            shortages, the new router-LSA will be continually
            retransmitted (and dropped by RT2's ISDN interface; see
            Section 1). This means that the routers RT1, RT3 and RT4
            will not detect the unreachability of Host H2 until a data-
            link connection on RT1 becomes available.

5.  Topology recommendations

   Because LSAs indicating topology changes are still flooded over
   demand circuits, it is still advantageous to design networks so that
   the demand circuits are isolated from as many topology changes as
   possible. In OSPF, this is done by encasing the demand circuits
   within OSPF stub areas or within NSSAs (see [3]). In both cases, this
   isolates the demand circuits from AS external routing changes, which
   in many networks are the most frequent (see [6]). Stub areas can even
   isolate the demand circuits from changes in other OSPF areas.

   Also, considering the interoperation of OSPF routers supporting
   demand circuits and those that do not (see Section 2