routing - once again

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Hello

Once again i have question about routing:
--network1---Router1----Router2---network2

RIP protocol used. Normally Router2 sends updates to
Router1: network2, metrics=1.

Case1: network2 is disconnected, i DO NOT use route poisoning, and i do  
not use split horizon. How does that update from Router2 to Router1 looks  
like ?
After that how does update from Router1 to Router2 looks like ?

Case2: network2 is disconnected, i DO use route poisoning, and i do not  
use split horizon. How does that update from Router2 to Router1 looks like  
?
After that how does update from Router1 to Router2 looks like ?

Thanx
Michal


Re: routing - once again


Route poisoning and split-horizon are used to prevent loops.  You don't
have any possible loop in your example so it doesn't matter!

Doan

On Tue, 24 May 2005, vertigo wrote:

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Re: routing - once again



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it does matter - because i still do not understeand what is the difference
between systems with and without router poisoning, what is the difference
between sending data with metric 16 and not sending data - they both can be
interpreted in the same way - network is down. So what route poisoning is
really for ? For me it only makes convergence faster...
If somebody could resolve these two Cases below, maybe i could
understeand it finally.

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Thanx for the patience and help :)


Re: routing - once again


On Wed, 25 May 2005, vertigo wrote:

Ok. My mistakes, under some circumstances routing loop can still
happenned.  Let's work this out and see what would happenned.

Let's start from the beginning.  At t1, the routing table of Router1 &
Router2 looks  like this:
R1:
network1 - connected
R2:
network2 - connected

Now, at t2 update, both routers exchange the whole routing table:
R1:
network1 - connected
network2 - Via R2 - 1 hop
R2:
network1 - Via R1 - 1 hop
network2 - connected

the network has converged at this point.

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Now, Normally R2 will send an update to R1 announcing network2 is down and
they both converged to:
R1:
network1 - connected
R2:
network1 - via R1 - 1 hop.

But what would happenned if, for some reasons, R1 didn't receive this
update?  R1 still has the old information:
R1:
network1 - connected
network2 - Via R2 - 1 hop
and at the next update, send this information back to R2 (no
split-horizon).  R2 now has:
R2:
network1 - Via R1 - 1 hop
network2 - Via R1 - 2 hops
and at the next update, R2 now send this information back to R1 (again
because of no split-horizon).  R1 now has:
R1:
network1 - connected
network2 - Via R2 - 3 hops
and at the next update, R1 now send this information back to R2.
R2:
network1 - Via R1 - 1 hop
network2 - Via R1 - 4 hops
.....
As you can see, without split-horizon, a routing loop has occurred. With
RIP, this stops when the hop-count for network2 reached 16 (counting to
infinity), which is another feature to stop routing loops.



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Route poisoning or poisoned reverse, R1 will advertise back to Router2
that it can reach network2 with hop-count of 16 (infinity) and stop
the loop faster.
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Hope this helps.

Doan




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