Hi Bob,
I think my reply to vtam's post actually belongs to Clayton Price's question
a few posts previous...
anyway:
When you create a interface summary address (or classfull auto summary) then
the router will create a null0 route to this summary address. As you
probably know this is to prevent routing loops.
The null0 route has an admin distance of 5, so its fairly hard to overide,
unless (of course) there is a more specific route - like you mentioned. So
the only way traffic gets routed to Null0 is if the router with the null 0
route cant find a more specific route to the destination within the summary
address range.
However you cant actually advertise a summary route from an interface unless
you have at least one more specific route for the summarised network in the
routers routing table.
Lets say you have network 10.0.0.0/16, 10.1.0.0/16, 10.2.0.0/16 connected
downstream to your router. You want to advertise a non-classful summary to
your upstream so you type:
int s0
ip summary-address eigrp 10.0.0.0 255.252.0.0
But this summary address may also attract traffic destined to 10.3.0.0/16,
and traffic to this subnet will be routed to Null 0 as there is not a more
specific route in the routers routing table. The router will not forward
10.3.0.0 to anyone else, or the default for that matter as that would create
a routing loop. That is the whole purpose of the null route.
Ivan Pepelnjak has written a great book (not too long..) " EIGRP Network
Design Solutions ". It has a whole load of interesting stuff on
summarisation which you might be interested in
Cheers
Simon
"Bob Hunter" <bobjhunter@^yahoo^.com> wrote in message
8v3m9l$qrp$[EMAIL PROTECTED]">news:8v3m9l$qrp$[EMAIL PROTECTED]...
> Simon,
>
> Since you seem to be very knowledgeable on this topic, I'm wondering if
you would help
> clear this issue. Now that the summary-address is applied to the
interface,
> and the software Null0 interface is in the routing table. Would you know
of any
> circumstances that would cause the Null0 interface to prevent the router
from trying
> to forward traffic to other routers in search of a more precise match?
>
> Thanks!
>
>
>
> Simon Hope wrote:
>
> > Eigrp will summarise to classfull boundry if you leave the default "auto
> > summary" running,
> >
> > to summarise your networks on a non clasfull boundry you need to use an
> > interface specific summarisation statement, but first turn off auto
> > summarisation on the EIGRP process
> >
> > example:
> >
> > router eigrp [AS]
> > no auto
> >
> > int s0.1
> > ip summary-address eigrp 10.4.0.0 255.255.0.0
> >
> > As for the mask command on the 3600,
> >
> > In version 12.0(4) T Cisco introduced the ability to mask off some
classful
> > networks from the eigrp process. This feature makes EIGRP a truly
classless
> > protocol. Check your IOS versions again - is one of them the "T"
version??
> >
> > Before 12.04T, any subnet in the network defined in your eigrp "network"
> > statement would go into the topology table. If you didn't want neighbors
to
> > see some of these subnets you would have to filter them
> >
> > Cheers
> >
> > SImon
> >
> > ""vtam"" <[EMAIL PROTECTED]> wrote in message
> > 8v28os$s86$[EMAIL PROTECTED]">news:8v28os$s86$[EMAIL PROTECTED]...
> > > Now i have two route sent to a eigrp neigbor, there are 23.125.0.0/19
and
> > > 23.125.0.0/16, i want the receiver just selete the 23.125.0.0/16
route. I
> > > cannot do summary in the output way, how can i do filter in the
receiver?
> > > Thanks.
> > >
> > >
> > > _________________________________
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