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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> > > > Report misconduct and Nondisclosure violations to
[EMAIL PROTECTED]
> > > >
> > >
> > > -----------------------------------------------
> > > 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)
>
> _________________________________
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