Understanding the network.pdf - Back to Home

All routers on the network keep track of their update times and the
ages ofthe routes in their tables. If a router has not received an
update message from a particular network after 180 seconds, the
route is marked as invalid. After the route has been marked as invalid,
it is in a hold-down state. While the route is in this state, the router
continues to forward traffic using that route. During this period, the
router is also trying to find a route with a higher metric to use as an
alternative. If a higher metric alternative is not found, and there still
has not been an update from the router, the route is flushed from the
table and users are notified by Internet Control Message Protocol
(ICMP) that their datagrams cannot be delivered.
Route Poisoning
To prevent routing the routing loops that are endemic to DVPs, RIP
employs route poisoning. Route poisoning is a method of inflating
route metrics contained in the update messages that are sent to
neighbor routers. This is done so the routes that are exchanged
between locally connected routers are not interpreted as alternative
routes. If this were allowed to happen, routing loops would occur in
the event of a router failure. RIP uses two types of route poisoning:
• Split horizon
• Poison reverse
In Figure 8.9, the Fast Ethernet hub that Router E is attached to has
just died. Router E has the route to 192.124.39.0 in hold-down mode
and is looking for an alternative route. Router C has just sent its
update and says that it can reach 192.124.39.0 with a hop count of 2.
Router E is happy; it changes its routing table and starts sending from
the 192.124.38.0 network that is destined for the 192.124.39.0 to
Router C, and Router C starts sending it back to Router E. This starts
the beginning of a routing loop that continues until both routers have
raised the hop count for the route to 16, known as the Count to
Infinity.