CCNP - eigrp->topology change->split horizon temporarily disabled

Author

eigrp->topology change->split horizon temporarily disabled

2000-12-19, 1:54 pm

http://www.cisco.com/warp/public/103/eigrp2.html#2

"In this case, Router Two turns off split horizon for this route, and advertises Network A as unreachable"

QUESTION:
why should router TWO needs to disable split horizon to be able to send an update to the NON-ADVERTISING routers, routers THREE and ONE?

thanks for your comments.

frans

[This message has been edited by emo francois (edited 12-19-2000).]

2000-12-19, 4:31 pm

I ain't eigrp guru, but here's what I think.

If I understand well the diagram, three is advertising the route to Network A on the ethernet (shared with one and two). Since two receives the update from three on its interface to the ethernet, it's bound not to send updates regarding Network A on that interface (because of the split horizon rule). So it needs to disable split horizon in order to send the poisoned update to one. Does it make sense?!

Cheers!

2000-12-20, 7:26 am

dmaftei,
to clarify my question, i would have expected something as simple as that:

INITIAL SITUATION:
there are 2 paths to Network A from router TWO:
- 1st advertised by router THREE
- 2nd advertised by router FOUR

TOPOLOGY CHANGE:
the 2nd path becomes "unreachable".

UPDATE SENT:
router TWO sends an update THREE and ONE stating the path as "unreachable"

BASIC MECHANISM
i just apply here the basic mechanism:
topology change detected on directly connected interface->update sent on all other interfaces

but actually, what i describe under "BASIC MECHANISM" may be false:
- router TWO gets a feasible successor that is router THREE then maybe it doesn't send any update at all...

what do you think?
frans

[This message has been edited by emo francois (edited 12-20-2000).]

2000-12-20, 1:12 pm

FROM COM.DCOM.SYS.CISCO

When a link fails, the router turn off split horizon and turn on poison
reverse.
Turning off split-horizon causes problems if you've expected link failures
on interfaces such as multipoint serials. So poison reverse updates are sent
out of multipoint interface announcing possible networks as unreachable.
Turing off split-horizon permits you to send routes back to the interfaces
from which they came.
Bye
------

DEFINITIONS
http://www.cisco.com/warp/public/103/eigrp2.html

-Split horizon
"Never advertise a route out of the interface through which you learned it. "

-Poison reverse:
"Once you learn of a route through an interface, advertise it as unreachable back through that same interface. "

QUESTION:
"When a link fails, the router turn off split horizon and turn on poison"

->let's say the router learned the route through the interface connected to the failed link:
-what is the point to disable split horizon on that interface?
-furthermore, what is the point to poison the route back to the failed link?

"i'm sinking in the quicksand of my thought"
(bowie)

thanks for your help
frans

Sponsored Links