The key to this issue is HOW cisco looks up routes AND the effect of "(no)
ip classless (classful vs. classless lookups)." To begin, the router
determines the classful network of the route in question, then proceeds to
lookup a more lengthy match within these routes. If it does not find a
match it does one of two things depending upon whether or not "ip classless"
is configured. If it is not configured (ie. no ip classless) it will not
look for any other matching routes. If it is(ie. ip classless), however, it
will go back and do a classless lookup, which is always matched with the
0.0.0.0 route. In a sense it works its way back to a route that, at LEAST,
matches.
Another good question is how does the route 0.0.0.0 always match? More
precisely, what is done to the route in questions so as to arrive at a route
of 0.0.0.0?
Hope this helps.
>From: Francisco Muniz <[EMAIL PROTECTED]>
>Reply-To: Francisco Muniz <[EMAIL PROTECTED]>
>To: [EMAIL PROTECTED]
>Subject: Another interview question
>Date: Thu, 21 Sep 2000 14:50:24 -0400
>
>The router is thus configured:
>
>no ip classless
>ip route 167.216.128.0 /22 s1
>ip route 167.216.240.0 /24 s2
>ip route 0.0.0.0 /0 s0
>
>and packet to 167.216.241.3 comes in, what does the router do?
>
>Answer: it drops the packet
>
>Why? Why doesn't it go out the default gateway? Anyone?
>
>Francisco.
>
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