[Re: [[nsp] 127.0.0.0/8 unroutable?]]

[Shankar Vemulapalli]

In this case, the router will end-up in dropping the packet.
This is because, it is pointing to the default-route handler.

As you observe below, we don't really have 127.0.0.1 entry in the
CEF table.  When we do a longest match, we return 0.0.0.0/0 [as seen
below].

This is either a default route (if the configuration has a default route)
or the default route handler, which will end up in dropping the traffic.

louis#sh ip cef 127.0.0.1
0.0.0.0/0, version 0, epoch 0, attached, default route handler <<<<<<
0 packets, 0 bytes
  via 0.0.0.0, 0 dependencies
    valid null adjacency

louis#sh ip cef 0.0.0.0 0.0.0.0
0.0.0.0/0, version 0, epoch 0, attached, default route handler <<<<<
0 packets, 0 bytes
  via 0.0.0.0, 0 dependencies
    valid null adjacency

louis#sh ip cef
Prefix        Next Hop    Interface
0.0.0.0/0     drop        Null0 (default route handler entry) <<<<<

Hope it helps,


/Shankar

At 1:29pm 11/11/03 -0500, joshua sahala wrote:
> Hank Nussbacher <hank at att.net.il> wrote:
>
> >
> > Ok.  I'll bite: and if a crafted packet comes in with source 127.x.x.x
> > or even better - destined to 127.x.x.x and there is no route entry, no
> > cef entry, nada - what does IOS do with the packet?
> >
> > -Hank
>
> there is a cef entry (from a 7513 running 12.0(25)S1):
>
> tricksy_router#sho ip route 127.0.0.1
> % Network not in table
> tricksy_router#sho ip cef 127.0.0.1
> 0.0.0.0/0, version 0, epoch 0, attached, default route handler
> 0 packets, 0 bytes
>   via 0.0.0.0, 0 dependencies
>     valid null adjacency
> tricksy_router#
>
> as to what the router does with it, i'm not sure, my bogon filters block
> it before it gets to the router (don't yours? ;) )
>
> it is an interesting question, will the router (lacking appropriate bogon
> and spoofing filters) pass a packet source from 127.x.x.x to a valid
> (or invalid) address on the other side of the router - sounds like
> something to play with....
>
> /joshua
>
>
>
> "Walk with me through the Universe,
>  And along the way see how all of us are Connected.
>  Feast the eyes of your Soul,
>  On the Love that abounds.
>  In all places at once, seemingly endless,
>  Like your own existence."
>      - Stephen Hawking -
>
>
>
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