TCP/IP Tutorial and Technical Overview - IBM Redbooks

The limitation to this rule is that each node must wait for the route to the
unreachable destination to time out before the route is removed from the
distance vector table. In RIP environments, this timeout is at least three minutes
after the initial outage. During that time, the device continues to provide
erroneous information to other nodes about the unreachable destination. This
propagates routing loops and other routing anomalies.
Split horizon with poison reverse
Poison reverse is an enhancement to the standard split horizon implementation.
It is supported in RFC 1058. With poison reverse, all known networks are
advertised in each routing update. However, those networks learned through a
specific interface are advertised as unreachable in the routing announcements
sent out to that interface.
This drastically improves convergence time in complex, highly-redundant
environments. With poison reverse, when a routing update indicates that a
network is unreachable, routes are immediately removed from the routing table.
This breaks erroneous, looping routes before they can propagate through the
network. This approach differs from the basic split horizon rule where routes are
eliminated through timeouts.
Poison reverse has no benefit in networks with no redundancy (single path
networks).
One disadvantage to poison reverse is that it might significantly increase the size
of routing annoucements exchanged between neighbors. This is because all
routes in the distance vector table are included in each announcement. Although
this is generally not an issue on local area networks, it can cause periods of
increased utilization on lower-capacity WAN connections.
Triggered updates
Like split horizon with poison reverse, algorithms implementing triggered updates
are designed to reduce network convergence time. With triggered updates,
whenever a router changes the cost of a route, it immediately sends the modified
distance vector table to neighboring devices. This mechanism ensures that
topology change notifications are propagated quickly, rather than at the normal
periodic interval.
Triggered updates are supported in RFC 1058.
188 TCP/IP Tutorial and Technical Overview