Routing loops mitigated - Distance Vector Protocols

To keep your data from floating around in never-ending loops, there are a few methods used by Distance-Vector protocols to avoid looping. They are:

Hop Counts
When a routing protocol sends its routing table to its neighbors, it increases teh hop counts on connected routes by one each time. In the case of a network that is unavailable or looped, this could go on forever (a scenario known as "counting to infinity"). Maximum hop counts are the most basic of the mitigation methods for DV protocols. This way, once a hop count metric reaches the max value, the route is assumed as unavailable.

Maximum hop counts are:

RIP v1 and v2 15
EIGRP 224

Split-horizon
This one, simply stated is to not advertise a route out the same interface in which it came. If I tell you that I am connected to network 192.168.1.0/24, why would I tell you that 192.168.1.0/24 is one router away from me? First, you already know where it is, and because I increment the hop count, you would now think that 192.168.1.0/24 is *two* hops away from me.

Route Poisoning
When a link fails, the router to which it is attached sends out a routing update with a hop count that exceeds the maximum value for that protocol. So, if I have a router connected to 192.168.2.0/24, and the interface connected to that network fails, I send an update to my neighbors incrementing the hop count for that network to 16. Then all my neighbors know it is unavailable. Cool.

Poison Reverse
Poison reverse overrides the split-horizon rule and sends the route update back out the interface on which it came. This provides an acknowledgement that the device received the change to the topology.

Hold-down Timers
Simply stated, hold-down timers store an update for a given amount of time before adding the change to their routing table. The idea is that, should a link start going up and down rapidly (otherwise known as "flapping"), the link will come up before the timer expires and there is no need to change the routing table. If the timer expires and the router doesn't hear of a route with a lower metric, it adds the route to its table.

Triggered Updates
To speed convergence of the network, a triggered update allows a router to send an update in the event of a failed link instead of waiting for the regular update. This allows the network to react more quickly to topology changes.

Invalid/Dead Timers
When a device is removed from the network but doesn't fail, a triggered update isn't sent, so the remaining routers may think the device still exists. To combat this, when a router stops receiving updates from another router, after a certain amount of time the router is considered "dead" and the routes are invalid. The routes are then removed from the table. This method is also used in the case where a router stops receiving updates from a given routing protocol but is no longer. Perhaps we changed our topology and started to use EIGRP instead of RIP, and after that set amount of time the RIP-learned routes are considered invalid and removed.

Tomorrow, more routing protocols! I know you can hardly wait, Bear...