Cisco Systems CCNA 2 X

Route poisoning is another process used by routers to prevent routing loops. Briefly review the

fact that routing loops are typically the result of slow convergence. The loops are interrupted

when maximum hop counts are defined so that packets that are caught in loops are eventually

dropped. Route poisoning is when the distance or hop count of a route is changed to 16, or 1

higher than the maximum number allowed, which makes it unreachable from the perspective

of the routers. This process of route poisoning results in an update about the poisoned route

that is sent out to neighboring routers before the routing update time has been reached.

Reference the graphic in this section. When Router A determines that Network X is down, it

poisons the route in its table. To do this, it sets the hop count to Network X to one more than

the maximum. It then sends a poison update to Router B regardless of the time schedule for

routing updates. This does not send the entire table. It only sends the route poisoning. This

single change, which indicates that Network X is now unreachable, is quickly propagated

through the network. This speeds convergence and reduces the likelihood that a loop will

develop.

Routing table updates are automatically sent out at specific time intervals by distance vector

routing protocols. As discussed earlier, slow convergence can create a scenario in which

routers incorrectly think a route to a network is available, which results in a routing loop.

Triggered updates such as route poisoning help prevent these routing loops by sending out

updates when topology changes occur without waiting for the update time to be reached. This

speeds up convergence in relationship to network topology changes.

Reference the graphic in this section. A triggered update would occur if Network X went down.

Router C would detect the change, update its routing table, and then send out an update to

Router B even though its update timer is set at 18. IP RIP would send out table updates at 30

seconds and IGRP would send them out at 90 seconds. This triggered update would poison

the route until the holddown timer, which is discussed in the next section, has expired.

Make sure students understand that a triggered update is generated by the router that detects

a topology change and sends the update to its neighbors.

Router B Router A