Information Technology Reference
In-Depth Information
Poison Reverse
Poison reverse is a route update sent out an interface with an infinite metric for routes
learned (received) from the same interface. Poison reverse simply indicates that the learned
route is unreachable. It is more reliable than split horizon alone. Examine Figure 10-7. In-
stead of suppressing the route for Network D, Router 2 sends that route in the routing
table marked as unreachable. In RIP, the poison-reverse route is marked with a metric of
16 (infinite) to prevent that path from being used.
Net A = metric
Net B = metric
Net D = unr
eachable
Router 1
Router 2
Router 3
Network A
Network B
Network C
Network D
With Poison Reverse, Router 2 sends Net A
and Net B routes to Router 3; also, a
route for Net D with an infinite metric.
Figure 10-7
Poison Reverse
Counting to Infinity
Some routing protocols keep track of router hops as the packet travels through the net-
work. In large networks where a routing loop might be present because of a network out-
age, routers might forward a packet without its reaching its destination.
Counting to infinity is a loop-prevention technique in which the router discards a packet
when it reaches a maximum limit. It assumes that the network diameter is smaller than the
maximum allowed hops. RIP has a maximum of 16 hops. And EIGRP has a maximum of
100 hops by default. These values are considered infinity.
Tr i g g e re d U p d a te s
Another loop-prevention and fast-convergence technique used by routing protocols is
triggered updates. When a router interface changes state (up or down), the router is re-
quired to send an update message, even if it is not time for the periodic update message.
Immediate notification about a network outage is key to maintaining valid routing entries
within all routers in the network by allowing faster convergence. Some distance-vector
protocols, including RIP, specify a small time delay to avoid having triggered updates gen-
erate excessive network traffic. The time delay is variable for each router.
Summarization
Another characteristic of routing protocols is the ability to summarize routes. Protocols
that support VLSMs can perform summarization outside of IP class boundaries. By sum-
updates on the network occur.
