Rip Timing Intervals - HP 7102dl - ProCurve Secure Router Configuration Manual

IP Routing—Configuring RIP, OSPF, BGP, and PBR
Configuring RIP
Worse, the count to infinity interferes with convergence to an actual valid route.
For example, Router C in Figure 15-2 also connects to Network 1 through a
five-hop redundant route. Router C waits until the count to infinity for the
invalid route reaches 6 before it starts using and advertising the correct route.
Split horizon is one solution for convergence problems. Split horizon specifies
that an interface should not send updates about a route to the interface from
which it received the route. In other words, routers assume that the router
from which they originally received a route to a destination is more directly
connected to and up-to-date on that destination. Split horizon also minimizes
the number of packets sent during routine operations.
In the network shown in Figure 15-2, split horizon would cut short the count
to infinity. Consider the period during which Routers A and C believe that they
can reach Network 1 through each other. As dictated by split horizon, Router
A does not send an update for Network 1 to Router C because Router C is its
source for the route. However, Router C takes Router A as its source, and when
Router A does not send the route to Network 1, Router C eventually times out
the route.
The ProCurve Secure Router supports split horizon with poison reverse. This
variation of split horizon specifies that a router advertises a route to the
neighbor from which it received it, but with a metric of 16. When routers point
to each other as sources for a route, they immediately inform each other that
the route is unreachable instead of waiting for the route to time out.
The ProCurve Secure Router also supports triggered updates, which lets a router
immediately send an update when the status of one of its interfaces changes,
instead of waiting 30 seconds. Triggered updates speed convergence and also
decrease the chance that another router will have time to send an update that
includes an invalid route.

RIP Timing Intervals

RIP specifies certain intervals at which routers must send updates or remove
routes for which they have not received current information. Routers use RIP
timers to regulate these intervals.
Routers broadcast their routing table at the close of every update interval,
which determines the timing for normal, maintenance updates. In addition to
sending these normal updates, the ProCurve Secure Router also sends trig-
gered updates: it broadcasts an update immediately whenever it changes the
metric for a route. In practice, triggered updates are most important for
speeding convergence when a network connection goes down.
15-17