Thank you very much Simon, for the very clear explanation. I'll forward this along to
the Dallas Cisco users group, we appreciate the help.
Simon Hope wrote:
> 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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