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New Member

split horizon


I still have doubt on this even after reading 100 times. Hold down timer is intiated when the router recives an update with metric higher than the current metric it has.

(1) Split horizon with route poison sends back few updates with metric 16. Upon receiving this update, what the local router does with it? Will it drop?

(2) If for some reason, hold down timer is initiated, local router still advertise that route in its regular updates?




Re: split horizon

Split horizon: router will not advertise the route out of the interface

on which it was received from neighbor.

Poison reverse: router will advertise the route out the interface on

which it was received, with a metric of infinity.

Both Split horizon & Poison reverse are mechanisms for loop avoidance.

Hold-down timer: A function that prevents a router from being updated

for a specified period in order to give other nodes some time to

reconfigure and prevent a routing loop. When a router is notified of a

route failure, it starts the holddown timer. In the meantime, if a

notification of a route is received from its neighbor with equal or

better metrics than the route that failed, the router stops the timer

and updates its routing table. If the new route metrics are inferior,

it keeps the timer running and does not update.

New Member

Re: split horizon


I think my question is not clear.

|R1| - |R2|

(1) R2 sent a route Eg: to R1. R1 will receive that route and sent back the same route to R2 with metric of 16(because of split horizon with poison reverse). What is the advantage of sending back this route? After recieving this route, what R2 does? Will it ignore?

(2) If for some reason, hold down timer for route is inititaed in R2. while hold down timer is active for this route, R2 still advertise this route to R1? or it waits for some time?

If still I am not clear, please let me know.


Re: split horizon

R2 is sending the update to R1. R1 will not send the route back to R2 because of split-horizon. When the route fails in R2, R2 will tell R1 that the route has failed and R1 will initiate split-horizon with poison-reverse. In that event, R1 will send R2 an infinity metric. Just correct me guys if I'm wrong.

Hall of Fame Super Bronze

Re: split horizon


Split horizons should prevent routing loops between adjacent routers, but poison-reverse updates are necessary to defeat larger routing loops.

Increases in routing metrics generally indicate routing loops.

Poison-reverse updates then are sent to remove the route and place it in holddown.

On Distance-Vector protocols, poison-reverse updates are sent if a route metric has increased by a factor of 1.1 or greater.


2. Let's take EIGRP for example:

EIGRP maintains a number of timers and variables containing time intervals.

These include an update timer, an invalid timer, a hold-time period, and a flush timer.

- The update timer specifies how frequently routing update messages should be sent. The EIGRP default for this variable is 90 seconds.

- The invalid timer specifies how long a router should wait in the absence of routing-update messages about a specific route before declaring that route invalid. The EIGRP default for this variable is three times the update period.

- The hold-time variable specifies the holddown period. The EIGRP default for this variable is three times the update timer period plus 10 seconds.

- Finally, the flush timer indicates how much time should pass before a route should be flushed from the routing table. The EIGRP default is seven times the routing update period.

To answer your question, if the route is received within the interval on the timers mentioned above, the process is reset and the route is installed back into the routing table

New Member

Re: split horizon

Hi Edison,

1. I read that "split horizon with poison reverse" sends the routes back to the sender with metric of 16. Isnt it?

2. Let us assume that R2 did not receive route with better metric with in the hold down time. During this hold down time, will it still advertise the route to R1? or not?

Re: split horizon

The route will not be advertised but it will be in the routing table till the expiration of hold-down timer.