  CONFIGURATION EDITOR
  IP ROUTING

# Poison Reverse For IP Routing

Settings for Frame Relay Ports

  This table enables you to specify which ports to poison. You should include only the ports you want to poison for return updates.
  Range: Ports: 1 -- 8, Enable Poison Reverse: Yes or No

More About Poison Reverse

  If a segment fails, the routers continue to send updates to one another. The router attached to the failed segment marks the routes on that segment as unreachable with a metric of 16.
  This is fine until another router sends this router an update containing the entire routing table. This table contains routing information for the failed segment. Therefore, the router that had originally marked that segment as unreachable now determines that the other router has an alternate route to that segment, even thought that segment is down.
  When the other router is updated with the original router's table, it then determines the original router has an alternate route to the failed segment. Thus, both routers are pointing to one another for a route that doesn't exist.
  Each time the routers send these updates, the metric count for the failed segment increases by one until it eventually reaches 16, meaning the route is down. This problem is know as "count to infinity".
  To solve this problem, the unit uses poison reverse. This feature poisons routing information before it returns to the original route. Thus, if a router receives RIP updates from another network, it forces the metric for those paths to 16 in any updates it broadcasts on the network.
  If a segment fails, and poison reverse is enabled, other routers acknowledge that failed segment.