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12 Release Note
Software Release 2.3.1
C613-10325-00 REV B
OSPF on Demand
OSPF on demand circuits allow data link connections to be closed when not
carrying application traffic. A new parameter, DEMAND, has been added to
the following commands to support this feature:
ADD OSPF INTERFACE [DEMAND={ON|OFF|YES|NO|TRUE|FALSE}]
SET OSPF INTERFACE [DEMAND={ON|OFF|YES|NO|TRUE|FALSE}]
For example, to set the OSPF interface ppp0 to a demand circuit over the point-
to-point link, use the command:
SET OSPF INTERFACE=PPP0 DEMAND=ON
The DEMAND parameter specifies whether or not the interface connects to a
demand circuit. Two routers connecting to the same common network segment
need not agree on that segment’s demand circuit status. This means that
configuring one router does not require configuring other routers which
connect to the same common network segment. If only one router has been
configured, and the common network is a broadcast or non-broadcast multi-
access (NBMA) network, the behaviour (e.g. sending, receiving hello packets)
of the network will remain the same, as if the interface has not been configured
as a demand circuit. If only one router has been configured and the common
network segment is a point-to-point link, the router on the other end may agree
to treat the link as a demand circuit and the point-to-point network receives the
full benefit. When broadcast and non-broadcast multi-access (NBMA)
networks are declared as demand circuits (i.e. more than one router has the
network configured as a demand circuit), routing update traffic is reduced but
the periodic sending of Hellos is not, which in effect still requires that the data
link connection remain constantly open. The values ON, YES and TRUE are
equivalent. The values OFF, NO and FALSE are equivalent. The default is OFF.
OSPF on demand is used on cost-conscious networks, such as ISDN, X.25 and
dial-up networks. If there is no traffic crossing the network, (either routing
protocol traffic or application traffic), the data link connection is closed. When
there is traffic to send, the data link connection is established, the data is sent
and the connection stays open until the link has been idle for a specified period
of time. At this point the data link connection is closed to conserve cost and
resources. Figure 3 and Figure 4 illustrate before and after OSPF on demand
scenarios.
OSPF on demand is defined in RFC 1793, “Extending OSPF to Support Demand
Circuits”. All routers in the network must support at least Part II of the RFC.
Routers attached to on-demand links must support Part III of the RFC.
To enable OSPF to be used for routing between Router B to Router A and
beyond, OSPF on demand is required. Turning OSPF on demand on at either
end of the ISDN link will disable static and RIP routes.
The command ADD OSPF INTERFACE=interface VIRTUALLINK=router-id will
ignore the setting of the parameter DEMAND. This means that if DEMAND is set to
OFF, the virtual link is still treated as a demand circuit.