"Mohamed Heeba" <[EMAIL PROTECTED]> wrote in message
91B200CBBEC3D111992A00805F31E6CB88076A@MINAMAIL">news:91B200CBBEC3D111992A00805F31E6CB88076A@MINAMAIL...
> hi
> i guess wat u have said is partially right , the router will not send any
> update outside its serial because there is inconsistent mask
> but i dont think that this is due to split horizon since (as i know )
split
> horizon concerns receiving routing update on an interface then not to
> propagate any of this updates back from the same interface .
> one thing can clear this ...if we Disable the split horizon on the serial
> interface ..do u think it is going to propagate the route ??
>
> ill check and let u know !!!
Good idea to check. I will need to do a little research, but I think split
horizon not only covers routing updates received via an interface, but also
routes about that interface itself. To state it more generally, it won't
advertise routes out an interface that is has *learned* via that same
interface (no matter how they were learned - via routing updates or via
direct connection). It is possible that this doesn't technically fall under
"split horizon", but I am pretty sure that a router will not send a routing
update out an interface that includes details of that same interface, which
is what I believe is happening here (since the network is being
autosummarized). Doing so could create routing loops. My guess is that it
will send the updates with split horizon turned off, but I have been wrong
before (just a few times :-)). Even if it doesn't send them with split
horizon turned off, it could still be that the router just refuses to send
updates out an interface about that interface.
---JRE---
> Mohamed A.Heeba
> CCNP,CCDA,MCSE,ASE
>
>
>
>
>
> -----Original Message-----
> From: Jim Erickson [mailto:[EMAIL PROTECTED]]
> Sent: Monday, December 04, 2000 7:34 PM
> To: [EMAIL PROTECTED]
> Subject: Re: Rip problem , suppressing null update!
>
>
> I looked up my BSCN course material regarding this and I think split
horizon
> may be the issue, as I had guessed in an earlier post in another part of
> this thread. The BSCN material says (emphasis added):
>
> "When routes are exchanged with foreign networks (networks whose *network
> portion does not match ours*), subnetwork information from this network
> cannot be included because the routing mask of the other network is not
> know. As a result, the subnetwork information from this network must be
> *summarized to a classful boundary* using a default routing mask *prior to
> inclusion in the routng update*."
>
> So, in the original poster's question, because e0 and s0 have different
> masks, the network address for e0 will be summarized to 172.16.0.0/16
> *before* it is included in a routing update. Notice that this places s0 in
> the same network, so we now have only one network that we could possibly
> want to advertise.
>
> Now, split horizon does not say that you will not accept an update because
> you are already advertising the same route. It says you will not advertise
a
> route out the same interface from which you learned it (subtle but
important
> distinction). In this example, this means that R1 will not advertise
routes
> to the 172.16.0.0/16 network out its s0 interface, because it would be
> advertising the route to the same interface it learned it on (due to the
> fact that the interface is within that network.) Thus, R1 has no networks
to
> advertise, and the null update is suppressed.
>
> As the router sees it, this would be equivalent to having two ethernet
> interfaces on the same segment (since s0 and e0 are within the same
> network). So, for example, say I have two ethernet interfaces on the same
> 192.168.1.0/24 network. I will not advertise out one interface a route
about
> the second interface on the same network because I could create a routing
> loop by doing so.
>
> Now, back to the original poster's example, when he changed the mask of s0
> on R1 to match that of e0 on R1, no classful summarization had to take
place
> since the network portions of the addresses on the interfaces were the
same.
> Notice that, even with the mask change, e0 and s0 are still in *different*
> subnets. This means that RIP *will* advertise the 172.16.1.0/27 network
out
> s0, but it should not advertise the 171.16.1.32/27 network.
>
> ---JRE---
>
> "Brian" <[EMAIL PROTECTED]> wrote in message
> [EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
> > On Tue, 5 Dec 2000, Mohamed Heeba wrote:
> >
> > > i believe that i shouldnt use different masks on Ripv1 router , i was
> trying
> > > to check if the mask will be changed on the other end of the
connection
> > > (which is right :)) , although now this doesnt have any meaning since
> the
> > > serial link should aslo have the same mask of the ethernet subnet of
the
> > > source router (which will force me to make consistant mask for the
> entire
> > > network to make everything work fine )
> > >
> > >
> > > i dont think split horizon has anything here ...hasnt it ??
> >
> > your right. What I was thinking (without reading) was that sometimes
when
> > using VLSM, you'll create a situation where you have overlapping
subnets,
> > and that can lead to a problem where a router won't accept the update
> > because it thinks it already is announcing that same route..........I
was
> > to quick with the reply
> >
> > brian
> >
> >
> > >
> > > thx for the help
> > > > -----Original Message-----
> > > > From: Brian [SMTP:[EMAIL PROTECTED]]
> > > > Sent: Monday, December 04, 2000 3:50 PM
> > > > To: Mohamed Heeba
> > > > Cc: '[EMAIL PROTECTED]'
> > > > Subject: Re: Rip problem , suppressing null update!
> > > >
> > > > On Mon, 4 Dec 2000, Mohamed Heeba wrote:
> > > >
> > > > > guys !
> > > > > i have two routers connected through their serial , im configuring
> rip
> > > > on
> > > > > them
> > > > >
> > > > > R1 e0 ip add is 172.16.1.1/27
> > > > > ser 0 ip add is 172.16.1.33/28
> > > > >
> > > > > R2 e0 ip add is 172.16.1.49/29
> > > > > ser 0 ip add is 172.16.1.34/28
> > > > >
> > > > > there is no rip updates between the two routers ..im getting only
> > > > > "suppressing null updates "
> > > >
> > > > are you using rip v1? you need to use something that understands
> VLSM.
> > > > Otherwise split horizon will get you in the above case.
> > > >
> > > > >
> > > > > also when i change the mask of ser0 on R1 to /27 , the updates
pass
> > > > normally
> > > > > and take the mask of the
> > > > > /28 on the other end
> > > >
> > > > yes they use the mask of the interface they have that subnet
> on..........
> > > >
> > > > >
> > > > > my question is
> > > > > Does this mean that if the mask is inconsistent on RIP router , it
> will
> > > > not
> > > > > send updates ?????
> > > >
> > > > you can't use inconsistant masks for a network running a FLSM
protocol
> > > > like RIPv1. You have to either use like masks, or use a VLSM
capible
> > > > protocol.
> > > >
> > > > > this means when R1 mask is consistent (/27) for both interfaces ,
> the R1
> > > > > will send updates but it will not receive anything because R2
masks
> are
> > > > not
> > > > > consistent ??
> > > > >
> > > >
> > > > brian
> > > >
> > > > >
> > > > > somebody to confirm please !!!!!
> > > > >
> > > > > _________________________________
> > > > > FAQ, list archives, and subscription info:
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> > > > >
> > > >
> > > > -----------------------------------------------
> > > > Brian Feeny, CCNP+ATM, CCDP [EMAIL PROTECTED]
> > > > Network Administrator
> > > > ShreveNet Inc. (ASN 11881)
> > >
> >
> > -----------------------------------------------
> > Brian Feeny, CCNP+ATM, CCDP [EMAIL PROTECTED]
> > Network Administrator
> > ShreveNet Inc. (ASN 11881)
> >
> > _________________________________
> > FAQ, list archives, and subscription info:
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> >
>
>
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