About RIP enhancements

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Jan 14th, 2009
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Regarding the RIP enhancements:

- split horizon

- route poisoning

- poison reverse

- triggered updates


1) Are they valid for both RIP-v1 and RIP-v2 ?

2) Can each be enabled/disabled on Cisco routers? Or only either all enabled or all disabled ?

3) In normal situations we'll see in action only 'split horizon'?

4) In faulty situations 'split horizon' will be replaced and superseded by 'poison reverse' ?

5) Without 'route poisoning' the sender part will not advertise AT ALL the subnet that is down (i.e will not advertise that subnet with metric 16) ?

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Giuseppe Larosa Wed, 01/14/2009 - 08:17
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Hello Badalam,


1) yes they should be supported by both RIPv1 and RIPv2


2) no, only split-horizon can be disabled on a per interface basis


see


http://www.cisco.com/en/US/docs/ios/iproute/command/reference/irp_rip.html


3) 4) in normal conditions you see split horizon with poison reverse in action when

an healthy network is announced back with metric 16 so if a network fails convergence is faster.

if RB reiceves network A from RA it will alwasys send out network A in its updates to RA with metric 16.


You can verify this using the appropriate debug ip rip command.


5) it is the opposite see above: it is the downstream router (the more far router from network) to advertise a poisoned route so that the upstream router cannot take in consideration its neighbor as a potential path to a failed route.


without poison reverse a process called count to infinity can take place if three or more routers are involved.



poison reverse is indeed needed in some redundant topologies to assure that convergence is correct.


Hope to help

Giuseppe



badalam_nt Thu, 01/15/2009 - 07:04
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Thanks Giuseppe.

Just to clarify some points:

- what is the difference between route poisoning and poison reverse? Which one of them enters in action first and who sends which of them ? The router whose directly connected subnet fails will send nothing or will peform route poisoning or poison reverse? And the distant router will perform what?


My understanding is that Split horizon tells a router NOT to advertise a subnet received on one of its interface back on that interface, if the metric for that subnet is strictly higher than the metric received on that interface.

I said that poison reverse (or route poisoning???) superseded split horizon, as it allows the router to advertise back the subnet, but with a metric of 16.


Thanks for helping me clarify these points.

Giuseppe Larosa Thu, 01/15/2009 - 10:14
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Hello Petru,

your undestanding is correct I've reviewed some concepts


route poisoning :

happens first and it is the update sent from the upstream router (the one nearest to the failed subnet) with metric 16

it is sent in a triggered partial update


poison reverse:

the explicit sending back of an update with metric 16 as an answer to poisoning


happens later sent by downstream router (the router that is more far from failed subnet)


split horizon simply says don't advertise back a subnet out the interface where you have learned about it happens always not only in case or a route changing metric.

A route changing metric triggers a RIP timer the holddown timer that causes the changed route to be not installed immediately but to wait up to 180 seconds (waiting to route to stabilize).

It is poison reverse that overrides split horizon and allows to advertise an infinite metric back to the upstream router.


I had merged the route poisoning and poison reverse in my previous post sorry for that.


Hope to help

Giuseppe






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