RFC1582 - Extensions to RIP to Support Demand Circuits( 六 )

the aging timer expires the entry MUST be set to unreachable and
the hold down timer started. If the hold down timer expires the
entry disappears from the routing database.
o If the interface goes down, the circuit manager should indicate
that all circuits on that interface have gone down.
Database timer values are covered in section 7.
2.7 New Packet Types
To support triggered updates, three new packet types MUST be
supported:
TRIGGERED REQUEST
A request to the responding system to send all
appropriate elements in its routing database.
A triggered request is retransmitted at periodic
intervals until a triggered response is received.
Routing requests are transmitted in the following
circumstances:
o Firstly when the router is powered on.
o Secondly when the circuit manager indicates a
destination has been in an unreachable (circuit down)
state for an extended period and changes to a
reachable (circuit up) state.
o Thirdly in the event of all routing update fragments
failing to arrive within a set period.
o It may also send triggered requests at other times to
compensate for discarding non-optimal routing
information.
TRIGGERED RESPONSE
A message containing all appropriate elements of the
routing database. An appropriate element is an entry
NOT learned from the interface to which the routing
information is being sent out. This is known as "split
horizon".
Stability is improved by adding "poisoned reverse" on
routes learned from a destination. This consists of also
including some routes learned from a destination in
routing updates sent back to that destination, but
setting the routes as unreachable. A route is only
poisoned if it is the best route (rather than an inferior
alternative route) in the database.
A triggered response message may be sent in response to a
triggered request, or it may be an update message issued
because of a change in the routing database.
A triggered response message MUST be sent in response to
a triggered request message even if there are no routes
to propagate. This would be the case for a host which
had a WAN interface only, but which wished to run the
triggered update protocol.
A triggered response is retransmitted at periodic
intervals until a triggered acknowledgement is received.
TRIGGERED ACKNOWLEDGEMENT
A message sent in response to every triggered response
packet received.
Triggered response and triggered acknowledgement packets MUST contain
additional fields for a sequence number, fragment number and number
of fragments.
If a triggered request or response is not acknowledged after 10
retransmissions, routes to the destination should be marked as
unreachable for the duration of a hold down timer before being
deleted.
The destination should then be polled at a lower frequency using
triggered request packets. When a triggered response is received,
the router should prime the next hop router my sending its routing
database through triggered response packets.
Strictly speaking polling should occur indefinitely to guarantee
database integrity. However the administrator MAY wish the router to
cease polling after a few attempts believing that the lack of
response is due to a mis-configuration of the next hop router. The
destination should be marked as NOT supporting the mechanism and no
further routing messages should be sent to that destination.
Before marking the destination as not supporting the mechanism, at
least 5 triggered request polls (without acknowledgement) should be
sent.
If a destination marked as not supporting the mechanism, subsequently