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Re: Router forwarding directed broadcasts [7:57780] posted 11/21/2002
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Paul Williamson wrote:
> 
> Think i figured it out
> The router will forward the subnet broadcast regardless of
> whether the no ip
> directed-broadcast command is configured on the ingress/egress
> interface or
> not - IF the destination subnet isn't locally attached to the
> router.

That makes sense. The router can't know for sure if a packet for a non-local
destination even is a broadcast. It may not know the prefix boundary (subnet
mask) for non-local networks. It just knows this for its own interfaces
(because it's configured on its own interfaces).

> Guess the router just does a route lookup and forwards it
> 
> If the destination network is directly attached and ip forward
> directed-broadcasts is disabled then the router replys on
> behalf of the
> subnet but does not forward the broadcast out onto the subnet
> 
> The replies i was seeing were from subnets that were locally
> attached to non
> cisco firewalls

Ah. That explains it. Thanks for letting us know. 

Whew. I didn't have much hope for this thread! I'm glad it worked out. :-)

Priscilla

> 
> Thanks for your help
> 
> -----Original Message-----
> From: Priscilla Oppenheimer 
> To: cisco@xxxxxxxxxxxxxx 
> Date: Thursday, November 21, 2002 6:36 PM
> Subject: Re: Router forwarding directed broadcasts [7:57780]
> 
> 
> >zen7110@xxxxxxxxx wrote:
> >>
> >> I'm sure they will, but my routers still forwarding subnet
> >> broadcasts
> >> even with this line in a sh ip int output:-
> >> Directed broadcast forwarding is disabled
> >
> >Why don't you send us a config and some discussion of the
> situation and
> your
> >methods of testing.
> >
> >This group can be helpful, despite the numerous silly answers,
> but we can't
> >output a solution to your problem with no useful input.
> Troublehsooting
> >requires data. If you can give us data, perhaps we can help
> you. The end
> >result could be that everyone benefits.
> >
> >Also, please use a meaningful title on your messages. Thanks
> >
> >_______________________________
> >
> >Priscilla Oppenheimer
> >www.troubleshootingnetworks.com
> >www.priscilla.com
> >
> >>
> >> Thanks
> >> -P
> >>
> >> > 5 games of cricket Between Australia and England have just
> >> commenced...
> >> > Australia won the first game very convincingly
> >> >
> >> > Australia should go a clean sweep
> >> >
> >> > --
> >> > Regards,
> >> >
> >> > Peter Kingston
> >> > Telstra BigPond Direct
> >> > Freecall 1800 066 594
> >> > ""Priscilla Oppenheimer""  wrote in message
> >> > news:200211210005.AAA15404@xxxxxxxxxxxxxxxxx
> >> > > Well you better explain this to us Yankees. Our baseball
> >> season is over
> >> > > unfortunatley, and now all we have is football (ugh).
> Well
> >> we have hockey
> >> > > and basketball too, I guess, and they're a litte better!
> :-)
> >> > >
> >> > > Priscilla
> >> > >
> >> > > Peter Kingston wrote:
> >> > > >
> >> > > > I just as a little bit of friendly rivalry,
> >> > > >
> >> > > > I believe there is more than yourself confused in
> London,
> >> > > > naming your
> >> > > > cricketers.... 5 zips looks like a fair chance
> >> > > >
> >> > > > --
> >> > > > Regards,
> >> > > >
> >> > > > Peter Kingston
> >> > > > Telstra BigPond Direct
> >> > > > Freecall 1800 066 594
> >> > > >  wrote in message
> >> > > > news:200211201626.QAA10539@xxxxxxxxxxxxxxxxx
> >> > > > > Someone asked me a question which confused me:-
> >> > > > > If i ping a network broadcast from a host on a
> different
> >> > > > network, which
> >> > > > > passes through a cisco router why do i get replies
> from
> >> > > > certain devices.
> >> > > > >
> >> > > > > The router has directed broadcast forwarding
> disabled.
> >> > > > > I thought the router would therefore drop the packet
> >> > > > >
> >> > > > > Any thoughts
> >> > > > > Thanks
> >> > > > > -P
> 
> 




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