And Triggered Updates - HP ProCurve Secure Router 7203 dl Advanced Management And Configuration Manual

Speeding Convergence: Split Horizon, Poison Reverse, and
Triggered Updates
One shortcoming of RIP is its relatively slow convergence in some network
environments. Routers send updates every 30 seconds. In a large network,
a router may not receive accurate and up-to-date information on a route for
several minutes.
Another problem with slow convergence is that it can trigger a network-
clogging count to infinity when a connection fails. For example, examine the
network in Figure 13-1 and consider the updates that each router receives for
Network 1 when routers run simple RIP without split horizon or poison
reverse.
Router A
Figure 13-1. Network That Does Not Use Split Horizon or Poison Reverse
Router B is directly connected to Network 1, so it advertises a route to it with
a metric of 1. Routers A and C receive this route from Router B. They both
store the route to Network 1 with B as the next-hop address and a metric of 2.
Routers A and C then begin advertising this route. Router C receives the route
from Router A. It does not alter its routing table to indicate that Router A is
the next hop, because the metric (2) is higher than that advertised by Router
B. Router B also receives the route from Router A. Nothing in the update
Router B receives from Router A indicates that this route is ultimately through
Router B itself. Router B simply rejects the route for the same reason Router
C did: the metric is higher than the route it already has.
IP Routing—Configuring RIP, OSPF, BGP, and PBR
Router B
Router C
If no connection fails
Destination Next Hop Metric
* 1 B 2
1 A 3
1 D 5
Configuring RIP
Network 1
4 hops
Router D
13-15